Color

!-ULTIDISCIPLINARY!PPROACH

Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

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Preface

Preface This book is based on a series of color-related cornerstones in my life. The first of these is my four decades in color chemistry: one as a research chemist in industrial dye chemistry and three, first at the University of Basle, then at the Swiss Federal Institute of Technology in Zurich (ETH),in teaching and research in organic chemical technology, with a particular accent on color and textile chemistry. In industry, I was already struck by the fact that color chemists were in possession of a wealth of empirical observations which would also be of considerable interest to investigators in other branches of chemistry. Yet, only a very few of those chemists were aware of these treasures. Since that time, I have, therefore, devoted practically all of my own research activities to the strengthening of such potential crosslinks; first mainly with physical organic chemistry, later also with physical chemistry, and via colorimetry, physics. In teaching, I tried to follow the maxim of Tadeusz Reichstein (1897–1996,discoverer of cortisone,Nobel Prize 1950), the principal mentor of my ‘Habilitation’ at the University of Basle, that one should enjoy one’s profession by being inquisitive and pursuing the unexpected. Through colorimetry, I developed an interest in color vision and, in the context of my interest in languages, color naming. In a literal sense,but in a highly complex manner,color terms may be called a psychological response function of color perception and cognition in the brain (see Chapt. 6), as well as a link to color in art and other human cultural activities (Chapt. 7). The multidisciplinary approach of this book cannot be comprehensive at all. In every chapter, I try first to describe some facts, experiments, examples, and observations, and then to give a personal interpretation which may lead to greater understanding or – as indicated briefly for brain research and consciousness in the epilogue (Chapt. 8) – to the recognition that there are color phenomena which we just cannot fully understand. This is actually the deeper reason for using the word ‘approach’ in the title of this book. In addition, ‘approach’ also relates to the principal method used in every chapter, namely the application of subjective points of emphasis as guidelines. This sometimes results in the inclusion of unexpected apparent details in the science chapters, such as Orgel’s explanation of the color of the inorganic crystal ruby, or the Hans Kuhn free-electron model for organic dyes – a historically important and better understandable method than more recent ones used now by specialists. AnaloV

Preface

gous guidelines lie behind the discussion of color in the Japanese language and culture, as well as that of animal color vision. In my opinion, these are interesting color phenomena partially different to perceptions of Western Man. Color as a factor in culture, particularly in art or in psychology, is discussed also on a similar pattern. For example, I discuss the work of only a few artists – even in such an important and many-faceted field as painting in France since the mid-nineteenth century. In summary, I hope that my approach is helpful for readers interested in the potential interplay of various aspects of color. It is, therefore, written as a book to be read rather than as a reference work to be dipped into for specific questions or for not yet matured developments, such as the socalled postmodern art. I used this type of approach also in teaching color chemistry,and I remember the reaction of students with pleasure. I fully agree with a statement of the physicist Richard P. Feynman (1918–1988, Nobel Prize 1965) mentioned in his autobiography Surely you’re joking, Mr. Feynman (1985): ‘I find that teaching and the students keep life going, and I would never accept any position in which somebody has invented a happy situation for me where I don’t have to teach’1. Readers will realize also that I do not advocate the creation of a third culture, but that I follow the prediction of the novelist and spectroscopist Charles P. Snow (1905–1980) in the second edition of The Two Cultures (1963) that better communication between the two cultures, humanities and natural sciences, will develop of its own accord. Interest outside one’s areas of specialization is a condition against sterile expertism and for receiving new insights. At least to a certain degree, it also helps for disentangling complex webs of information and making connections between apparently unconnected data or ideas. I hope, therefore, that it will be stimulating to read this book on color disciplines not obviously related to one another, but which seem interesting subjects for cross-cultural discussion. Such an attitude is, of course, not new. For example, chemist and writer Elias Canetti said in the volume Die Fackel im Ohr (English edition: The Torch in my Ear, A. Deutsch, London 1982, p. 254) of his autobiography (1980, p. 238–239): ‘Die Verbindungen 1 I am glad to acknowledge that I heard that statement in a lecture by Olaf Kübler, President of the ETH, in November 1998.

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zwischen Dingen, die weit abseits voneinander lagen […],blieben mir lange verhüllt, was sein Gutes hatte, denn sie traten dann Jahre später mit umso grösserer Kraft und Sicherheit zutage. Ich bin nicht der Meinung, dass es von Gefahr ist, sich zu weit anzulegen. Verengungen […] kann man […] aufhalten und [ihnen] entgegenwirken, indem man sich möglichst weit ansiedelt.’2. My wholehearted thanks go to my friend Dr. M. Volkan Kisakürek, Managing Director of Helvetica Chimica Acta Publishers, for encouraging me to write a book of this nature on color. His commitment aroused in me much more enthusiasm and interest than I would have thought possible at the beginning. In its later stages, he made many welcome suggestions with respect to content, presentation, and lay-out. Although the body of work was initially completed and the manuscript submitted in Autumn 1997, he gave me the opportunity to continuously add new text in the light of new research results, up until Spring 1999. Dr. Andrew Beard (Newbury, England) improved my English in a comprehensive and diligent way which made the text more readable and understandable.In addition,his broad knowledge of science and humanities was the source for a number of additions and changes. I have never had before the privilege to work with such an excellent English-language editor. I am also very grateful to several colleagues who answered my questions in discussions and by correspondence.Two of my colleagues at ETH Zurich deserve special mention: Klaus Hepp, who discussed the chapter on color vision with me, and Konrad Osterwalder, now Rector of the ETH, who gave me encouragement for some ideas in the epilogue. John Mollon of Cambridge University also read the chapter on color vision and discussed it with me. I learned much on color in art from the book Colour and Culture (1993) by John Gage,and I was very pleased to make his acquaintance when I stayed at Cambridge University. Many discussions during my very long 2

‘The connections between things that were remote from one another […] remained concealed from me for a long time, which was a good thing, for they then emerged years later, all the more strongly and surely. I do not feel that it is dangerous to make plans that are too all-encompassing. A narrowing can at least [be] […] hold up and [one can] work against it by spreading out as far as possible’. Canetti was born in Bulgaria.As a child he came to Vienna and went later to the Gymnasium (high school) of the State of Zurich. He graduated in chemistry at the University of Vienna, and later found recognition as a writer in German. From the 1930s onwards he lived in Paris, London, and Zurich, where he died in 1994. He received the Nobel Prize for Literature in 1981.

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friendships with Don Hoffner, former Director of the Bezalel Academy of Art in Jerusalem and still practicing artist, and with Earl Peters, Executive Director of the Chemistry Department of Cornell University in Ithaca, New York, were helpful for various chapters of this book. Herbert Deinert, also of Cornell University, was kind enough to translate Goethe’s color poem on page 215 into literary English. I met two of the artists whose paintings I discuss in this book. I visited Augusto Giacometti in his studio when I was a boy. Richard Paul Lohse and I gave a joint lecture course on color at the University of Zurich in the 1970s. More recently, my knowledge of these two painters was further improved by very enjoyable contacts with some of their relatives: namely Fernando and Marta Giacometti-Dolfi in Stampa (Bregaglia Valley, Switzerland) and Johanna and Bryn James-Lohse in Zurich. My ETH colleague Alfred Roth provided me with useful and interesting information on Piet Mondrian with whom he was in close contact in Paris during the 1930s. I am deeply grateful to all these persons. My interest in Japanese culture, reflected in the chapter on color-term linguistics and in the Japanese art section in the chapter on color in art, is mainly the result of contacts with Japanese co-workers at ETH and of visiting professorships at Japanese universities. My foremost ‘sensei’ (teacher) was Toshiro Iijima, formerly at the Tokyo Institute of Technology, now President of Jissen Women’s University in Tokyo. I am indebted to another former co-worker,Toshikazu Saito,for providing copyrights for reproductions of Japanese art works. I am very thankful also to many other colleagues and friends who answered questions. Space prevents me from mentioning them all. Finally, I thank Mrs. M. Kalt for transcribing my manuscript and my former co-worker Peter Skrabal who helped me in proofreading. I am very grateful to my wife Heidi for her understanding during the preparation of this book. Küsnacht, Zürich, April 1999 Heinrich Zollinger

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Contents

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. What Do We Mean by Color? . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Historical Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 3

2. Physics of Light and Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.1. The Nature of Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2. Color by Refraction: Newton’s Experiments . . . . . . . . . . . . . . 18 2.3. The Rainbow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.4. Peacock’s Colors: A Phenomenon of Interference . . . . . . . . . . 28 2.5. How Many Causes of Color Do We Know? . . . . . . . . . . . . . . . . 36 3. Chemistry of Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.1. History of Colorants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.2. Inorganic Pigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.3. Organic Colorants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3.4. Correlations between Chemical Structure and Color . . . . . . . 56 4. Colorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.1. Color Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.2. Color: Harmony or Contrasts? . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5. How Do We See Colors? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Perception and Cognition of Color . . . . . . . . . . . . . . . . . . . . . . 5.2. Anatomy of the Human Eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Photochemistry in the Retina . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. What the Eye ‘Tells’ the Brain . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Psychophysical Investigations into Color Vision . . . . . . . . . . . 5.6. Color Vision in Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

79 79 81 87 93 103 112

6. How Do We Name Colors? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. From Color Chemistry to Color Linguistics . . . . . . . . . . . . . . 6.2. The Phenomenon of ‘Human Language’ . . . . . . . . . . . . . . . . . 6.3. Categorization of the Color Space by Color Naming . . . . . . . 6.4. Color and Phonological Universals . . . . . . . . . . . . . . . . . . . . . . 6.5. Influence of Culture on Color Naming . . . . . . . . . . . . . . . . . . .

123 123 125 127 136 141

7. Color in Art and in Other Cultural Activities . . . . . . . . . . . . . . . . . 161 7.1. Color in European Art from Antiquity to Gothic . . . . . . . . . . 161 7.2. From Renaissance to Neo-Impressionism . . . . . . . . . . . . . . . . 175 IX

Contents

7.3. Color in Twentieth-Century Art . . . . . . . . . . . . . . . . . . . . . . . . 7.4. Color in the Art of Non-European Cultures: The Case of Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5. Color in Psychology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6. Goethe’s Zur Farbenlehre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7. Sound – Color Synesthesia . . . . . . . . . . . . . . . . . . . . . . . . . . . .

188 197 207 212 220

8. Epilogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 Name Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249

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Introduction

1. Introduction 1.1. What Do We Mean by Color? The world of color. Who is not attracted, fascinated, and even enchanted by it? Obviously painters,and artists in general. Many scientists too: among them biologists, interested in colors in the living world, mineralogists in the inorganic world, and the physicists and chemists who investigate color’s scientific basis. In one of current-day scientific research’s most exciting fields, molecular biologists, physiologists, neuroscientists, and ophthalmologists cooperate in unraveling the sensation of color vision; the processing of color stimuli in the eye and in the brain, as well as in psychological reactions. Culturally conditioned behavioral patterns, such as color naming, are of great interest to linguists, psychologists, anthropologists, and artists. And amateur enthusiasts and hobbyists also find themselves attracted in considerable numbers to one or more of these branches of the world of color. Color, therefore, is a highly multi-faceted phenomenon in nature, biology, and culture. This is already evident in the term ‘color’. ‘Colours speak all languages’, wrote essayist Joseph Addison in 1712, while in our own time (1968) James Gibson commented that ‘the meaning of the term color is one of the worst muddles in the history of science’. Webster’s Encyclopedic Unabridged Dictionary (1994) lists 24 different meanings for the noun ‘color’ and five for the verb. There are meanings related to physics (light, emission, absorption, spectrum, coloration etc.) and to our perceptual response to such physical effects. Yet ‘color’ is also used in the context of many phenomena bearing no relationship to the physics of color, primarily for perceptual effects of other senses in non-visual human cultural activities such as music, poetry, and fiction. The range of meanings of the term color is, therefore, much larger than would appear at first sight. It is not the same in all cultures, however. In English and German, it is ambivalent with respect to the achromatic colors black, white, and gray. There is indeed a physical difference between chromatic and achromatic colors (see Fig. 4.1), but both, viewed physically, are colors. Yet linguistic tests in these languages demonstrated that some science students who served as subjects did not include black, white, and gray in their color vocabulary ‘because they are not colors’. 1 Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

Introduction

The Japanese view the situation differently, as the following episode demonstrates.When color television was introduced in Japan,the Japanese did not translate that expression literally (iro no terebi), but used the wording tennenshoku terebi (‘natural television’).Black and white television is considered by the Japanese to be a two-color process, rather than a colorless one.Tennenshoku terebi has in recent years,however,more and more come to be replaced by the internationalized term color terebi1. An analogous development took place for the translation of color photographs (tennenshoku shashin). There are also some languages, such as the Austronesian language Mbula, which have terms for specific colors, but no word for color itself. There is an enormous variety of meanings for terms related to color, particularly for terms of specific colors, hues, and shades. Reasons of space preclude any comprehensive discussion of the realm of ‘colorfulness’ (but see Chapts. 6 and 7 for specific aspects). By coincidence, two books with almost identical titles were published in New York within eighteen months of one another. The first was The Primary Colors2 by Alexander Theroux,followed by the anonymously penned Primary Colors3. ‘Primary colors’ is a well-established term in color science and in art (see Sects 4.1, 4.2, 7.2, and 7.3, and also Chapt. 8). It means a set of colors from which all other colors may be obtained by mixing; normally red, yellow, and blue for subtractive mixing and red, green, and blue for additive mixing (see Sect. 4.1). Theroux’s book does fit this definition, consisting of three essays on cultural and other aspects of blue, yellow, and red. The title is also appropriate for the other book, a novel in which every person and every situation looks like someone and somewhere in the real world. A thinly disguised retelling of President Bill Clinton’s 1992 primary election campaign, the book has as its hero Jack Stanton, governor of an unnamed small southern state. The author, political reporter Joe Klein, who covered Clinton’s campaign for New York magazine, was able to preserve his anonymity for half a year after publication. Primary Colors is, of course, a much more attractive and succinct title than, say, the more precise but less colorful Colors in the Primary Elections would make. These works aside, however, there are only a few relatively recent books about color, covering several aspects of the topic4–9. 2

Introduction

Returning to Theroux’s Primary Colors, a British reviewer wrote: ‘It could act as a handy whetstone on which to hone the intellect and imagination. Perceptive, provocative, evocative, nostalgic, idiosyncratic, enigmatic, riveting, maddening, entertaining … all this and inexpensive too. What more can one ask?’ Theroux’s writing style achieves this partly through unexpected or illogical use of color terms, in ways whose significance might not be immediately evident. Let me give you two examples: 1) The IBM supercomputer which was able to beat chess champion Kasparov in 1997 in some – but not all! – games, was called ‘Deep Blue’.Why this name? It was derived from the nickname of the IBM company,‘Big Blue’. 2) In the field of subatomic physics, new types of mass-energy found in the search for elementary particles were christened R, G, and B, or colored quarks. In his book (1985, see Ref. 3 in Chapt. 2) for non-physicists, Richard Feynman (1918 – 1988, Nobel Prize 1965 in Physics) expressed his distaste for the naming practices adopted by the experts.‘These idiot physicists were unable to come up with any wonderful Greek words anymore…’.

1.2. Historical Survey The history of human endeavor with color can be traced back to 30,000 years ago to pictorial representations of animals in caves like Altamira in Spain and Grotte Chauvet in southern France where the oldest known paintings are currently under investigation10. In the classical era, when thinking on color was largely based on the hypothesis of Aristotle (384–322 B. C.) that all colors are mixtures of black and white, Greece and Roman Italy both strongly favored polychrome sculpture and buildings11. At about the same time, the development of very intricate techniques for the production of Ancient Purple by the Phoenicians marked the beginning of the long tradition of colorant production technology.Islamic architecture,art, and craft, like the Mosque of Omar (Dome of the Rock) in Jerusalem or Persian carpets, bear witness to the development of a highly colored abstract art in Islamic culture, in which religion did not allow pictorial representation of human figures. In Europe, color once more became an important part of art in late medieval times, first in Italy with Giotto di Bondone (1266–1337). The development culminated in the work of the Venetian Tiziano Vecellio (Titian, b. between 1476 and 1490, d. 1576), who is considered by many experts to 3

Introduction

be the greatest of all colorists. For this reason one of his masterpieces,Bacchus and Ariadne (1523; Fig. 1.1), is included here. It is discussed in depth in Sect. 7.2 in the context of developments in European painting. The revival of European scientists’ interest in color came later than that of artists.The foundations of modern color research were laid by Isaac Newton (1642–1727). It was known before Newton that sunlight was split by a glass prism into the colors of the rainbow. He found, however, that an inverted prism positioned after the first would recombine these colors

Fig. 1.1. Bacchus and Ariadne (Titian; reproduced by courtesy of the Trustees,The National Gallery London)

4

Introduction

into achromatic light. The result of this experiment was clearly incompatible with Aristotle’s hypothesis, mentioned above. Newton also observed that a second, non-inverted prism was not able to split any of the colored components obtained after the first prism any further. These two experiments are fairly well-known to non-physicists. For this introduction, however, some of his revolutionary conclusions are even more important, but not so well-known. When we speak of colors, we generally specify the color of objects: ‘the apple is red’, ‘the leaves are green’; certain things are even given an obviously incorrect color (‘white wine’) or one constant color, even if it often varies greatly (‘the blue sea’). In this way, we convey the impression that color is a property that these things really possess, that it represents an objective fact. We do not acknowledge that colors are sensed and experienced by our egos (in a very broad sense) and are not an objective property of the environment. This is one of the most serious barriers to fully comprehending them. We experience colors through an extremely complex path of physical, chemical, neurological, and mental processes. From his experiments, Newton recognized the relationship between light and color, and also color’s non-objectivity. This is obvious from the following quotations from his first publication (1672) on color (see Fig. 1.2) and from his book Opticks (1704, see also Refs. 8 and 9 in Chapt. 2): ‘I shall conclude with this general remark, that the Colours of all natural Bodies have no other origin than this, that they are variously qualified to reflect one sort of light in greater plenty than another’(1672). ‘I speak here of Colours so far as they arise from Light. For they appear sometimes from other Causes, as when by the power of Fantasy we see Colours in a Dream’ (1704). ‘Indeed, rays, properly expressed, are not coloured. There is nothing else in them but a certain power […] to produce in us the sensation of this or that colour’ (1704). ‘Rays […] are not coloured’. In deference to Newton’s affirmation, we shall only rarely use expressions such as ‘white, red, colorless […] light’ in this book. ‘Achromatic, chromatic, monochromatic […] light’ will be used whenever possible. 5

Introduction

Fig. 1.2. Title page of the Philosophical Transactions [of the Royal Society] 1 6 7 2,No.80, which contains the first work by Isaac Newton on colors

6

Introduction

Newton’s statement is very close to a saying of Democritus (b. ca. 460 B. C.) two millennia earlier:‘Sweet and bitter, cold and warm, as well as colors, all that exists only as an idea, but not in reality; what really exists, are stable elementary particles and their movement in the vacuum’. A century after Newton, Johann Wolfgang von Goethe (1749–1832) vehemently repudiated Newton’s theory. Goethe’s book Zur Farbenlehre (Theory of Color, 1810, see Ref. 60 in Chapt. 7) is the most voluminous book he ever published. According to Goethe, it is inconceivable that white could possibly be a combination of all spectral colors. Even today, most people would probably agree intuitively with Goethe, though without questioning the validity of Newton’s observations. For almost two centuries there was no convergence of these disparate conclusions.We shall discuss that dichotomy in Sect. 7.6 and demonstrate how it arises from two different stages in the neural processing of light stimuli in the eye and the brain. In continuation of Newton’s work, Thomas Young (1773–1829), an ingenious medical doctor with a wide range of interests in the sciences and humanities (see Sects. 2.1, 2.3, 5.3, and 5.5) suggested in 1802 that all conceivable colors can be obtained by mixing together a small number, probably three, of colors; these having their origin in the retina of the eye and not in the physics of light. The situation was clarified later by the early work of James Clerk Maxwell (1831–1879), whose major contribution to physics was the electromagnetic field theory, and by the investigations of Hermann von Helmholtz (1821–1894), who studied color vision among many other subjects in physics and physiology. Contemporaneously, but independently, color was becoming a very important subject for research in another branch of science.Color chemistry started in 1856, with the 18 year old chemist William Henry Perkin’s serendipitous discovery of the first synthetic dye, called Mauve. This was the birth of the colorant industry, developing initially in Great Britain, Germany, Switzerland, and other European countries, and, in the twentieth century, in the United States, Japan, and other far-eastern nations. The colorant industry was also to be the cradle of other research-based chemical industries, the first pharmaceuticals and plastics being manufactured in dyestuff-producing enterprises in Germany. Color chemists developed an enormous number of commercial dyes and pigments (several tens of thousands of compounds since 1856) with better and better properties (brilliance, fastness, ease of application). Color chemistry is the subject of Chapt. 3. 7

Introduction

Technological progress was to come not only from color chemistry, but also from Young’s,Maxwell’s,and Helmholtz’s previously mentioned investigations. Their interpretation of color vision, the so-called trichromatic or tristimulus theory, was the seedbed for several modern technologies: notably color printing, color photography, and color television.As early as 1861, Maxwell demonstrated that color photography was possible, using black and white transparencies of a multicolored ribbon and projecting these photographs in superposition with red, green, and blue filters during a Royal Institution lecture. These color photographs were taken ten years before chemical sensitization of silver emulsion was discovered. Trichromatic theory is also the basis of the colorimetric system of color classification introduced by the International Commission on Illumination (CIE, Commission Internationale de l’Éclairage) in 1931. It is discussed in Sect. 4.1. Light entering the eye is absorbed in the retina by two types of photoreceptors: the rod cells and the cone cells. Rods are extremely sensitive to light, enabling people to see in very dim conditions, but cannot differentiate colors.Hence the saying ‘At night, all cats are gray’.Cones are less sensitive. They are responsible for daylight vision and sensitive to color. Dramatic supporting evidence for the trichromatic theory was to come in 1964,with two independent microspectrophotometric investigations of individual cone cells. W. B. Marks, W. H. Dobelle, and E. F. MacNichol recorded the spectra of ten primate cones, and Brown and Wald12, 13 those of four human ones. They found three types of cones with distinctly different peak sensitivities, corresponding to three receptors, which they originally named blue-,green-,and red-sensitive cones.The three sensitivity ranges in the visible spectrum overlap, however, and it has more recently been established that the vision system in the eye and brain is able to distinguish light of different wavelengths only by evaluation of the sensation intensity differences of two cone types: In recent years the three types have therefore been renamed short-, middle-, and long-wavelength cones. Traditionally, we assume that vision is something that takes place in the eye. Over the last few decades, however, neuroscientists have established that the process of visual cognition, although of course making use of information received from the retina in the eye, takes place in the cortex of the brain. As implied above, the specialization into rod and cone cells in the eye results in partial separation of color information from form and movement there. Several areas of the cortex also show evidence of 8

Introduction

this separate processing, but elsewhere the form, color, and movement input gets at least partially recombined. Brain research has made enormous progress in recent years (see Sect. 5.4), but several open questions still remain, some of which are intrinsically unanswerable. The epilogue of this book (Chapt. 8) contains some thoughts on that problem. Is it possible to find a correlation between color in physics, chemistry, and neurobiology on one hand, and color in psychology, art, and other human cultural activities on the other? A decisive factor in this question may be the names different people give to different colors. This is for two reasons: first, because color naming can be tested systematically with test subjects, and second,because it can be considered as a psychological response function of seeing colored objects, i.e., as a very literal response to the question ‘What color do you call this object?’. Today, color terms are probably the most intensely investigated words in linguistics, having been studied in over three hundred of the world’s languages. As shown in Chapt. 6, this approach shows promise, although the ‘direct’ link is always masked by psychological, social, cultural, and technological differences. Color is, obviously, not only important in language, but in most other cultural activities of mankind. In Chapt. 7, color in the history of visual arts is discussed, first for western art from antiquity to the present day. A case study on Japanese art (Sect. 7.4) highlights similarities and differences between arts at the two ends of Eurasia. In sections on psychology and synesthesia, it is shown how color perception influences human activities not directly associated with color, such as alchemy, dreams, and music. This introduction shows how the phenomenon of color pervades almost every discipline, scientific and otherwise. As a scientist, I cannot but be acutely aware of Snow’s Two Cultures generally held to inhibit communication between scientists and laypeople14. This book’s title has been carefully chosen with that in mind. I am optimistic, however, that in the case of the phenomenon that is color (as well as in others),it is possible to adopt a ‘multidisciplinary approach’ suited to both scientist and non-scientist. I take heart in this from comments made by two of the greatest physicists of all time. The first, by Niels Bohr (1885 – 1962), was reported by Werner Heisenberg (1901 – 1976), from a conversation he had with Bohr and Wolfgang Pauli (1900 – 1957) during an evening promenade on the pier of Copenhagen Harbor: ‘As far as science is concerned, however, Niels is certainly right to underwrite the demands of pragmatists and positivists for meticulous attention to detail and for semantic clarity.It is only in respect to its taboos that we can object to positivism, for if we may no longer speak 9

Introduction

or even think about the wider connections, we are without a compass and hence in danger of losing our way’. Albert Einstein (1879–1955) said:‘The most beautiful experience we can have is the mysterious. It is the fundamental emotion that stands at the cradle of true art and true science. Whoever does not know it and can no longer wonder, is as good as dead’. References and Notes 1. For the broader meaning of ‘iro’ in Japanese, see also Table 6.2 and remarks on Japanese black/white art in Sect. 7.4. 2. A. Theroux, The Primary Colors. Three Essays, Henry Holt, New York, 1994. 3. Anonymous (later J. Klein), Primary Colors , Random House, New York, 1996. 4. J. Gage, Colour and Culture: Practice and Meaning from Antiquity to Abstraction, Thames and Hudson, London, 1993. German edition: Kunstgeschichte der Farbe, Otto Maier, Ravensburg, 1994. 5. S. J. Williamson, H. Z. Cummins, Light and Color in Nature and Art, John Wiley and Sons, New York, 1983. 6. H.Rossotti,Colour: Why the World Isn’t Grey,Princeton University Press,Princeton N. J., 1983 – 1985. Also available in paperback. 7. M. Minnaert, Light and Color in the Outdoors, Springer-Verlag Berlin, 1992. 8. T. Lamb, J. Bourriau (Eds.), Colour: Art & Science, Cambridge University Press, Cambridge, 1995. This book is an excellent introduction for nonspecialists. It is preferable to the more recent book Color Vision: Perspectives from Different Disciplines, Eds. W. G. K. Backhaus, R. Kliegel, and J. S. Werner, Walter de Gruyter, Berlin, 1998. 9. K. Nassau (Ed.), Color for Science, Art and Technology, Elsevier Science,Amsterdam, 1998. 10. Reproduction of paintings discovered in Grotte Chauvet were published recently: M. Balter,‘New Light on the Oldest Art’, Science 1999, 283, 920 – 922. 11. See reproductions in Ref. 4, p. 20, 21, 24. 12. W. B. Marks, W. H. Dobelle, E. F. MacNichol, ‘Visual Pigments of Single Primate Cones’, Science 1964, 143, 1181–1183; P. K. Brown, G. Wald, ‘Visual Pigments in Single Rods and Cones of the Human Retina’, Science 1964, 144, 45–52. 13. George Wald (1906–1997) received the Nobel Prize for Medicine for his work on color vision. 14. C. P. Snow, The Two Cultures: and a Second Look. Cambridge University Press, Cambridge, 1964.

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2. Physics of Light and Color 2.1. The Nature of Light As already mentioned in Chapt. 1, the pioneering discovery in color physics was made by Newton when, as a student in 1666, he found that visible light can be split into spectral colors by a prism and then recombined once more by a second, inverted prism. This chapter begins with a short introduction on the nature of light in general, followed by discussion of the physics of color phenomena. Several effects of light were known and, to some degree, experimentally investigated in the Renaissance and later in the seventeenth century. Two competing theories on the nature of light were proposed over the forty year period from 1665 to 1704, but it was to be more than two centuries until the development of an unambiguous theory of light in the early twentieth century would demonstrate that the two classical theories were not, as had been assumed for 200 years, mutually incompatible. The first of these theories was proposed in 1665 by Robert Hooke (1635–1703), professor of geometry at Gresham College in London, and developed further in 1690 by Dutch astronomer Christiaan Huygens (1629–1695). They postulated that light has the characteristics of a wave in an invisible medium permeating all space: solids, liquids, gases, and vacuum. This medium they called the ‘ether’. Huygens was able to show mathematically that the fundamental geometrical laws of optics could be explained by assuming that a prism or lens slowed the speed of the light wave. Hooke’s and Huygen’s wave theory was vigorously resisted by Newton at a meeting of the Royal Society in the late 1660s and in his book Opticks or a Treatise on the Reflections, Refractions, Inflections and Colours of Light (see Sect. 2.2),published in 1704.In Newton’s opinion,light was a very rapid flux of imponderable particles, or corpuscles. Despite his famous saying ‘hypotheses non fingo’ (‘I do not invent hypotheses’), he did not hesitate to ornament his theory with explanations resting on little or no experimental support.His discovery of differential and integral calculus,and his work on mechanics and dynamics, however, had established his reputation throughout Europe, and, accordingly, his particle theory of light was also to dominate throughout the eighteenth century. Wave theory was virtually forgotten. 11 Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

Physics of Light and Color

Fig. 2.1. Young’s experiment of interference with two slits (see text)

In 1802,however,Thomas Young1 conducted the first experiments on interference of light,the outcome of which he explained by invoking light’s wave nature. Young based his experiment on the well-known observation that the edge of a shadow is never completely sharp, even if the light source is so small as to be considered a point.His main piece of equipment was a non-reflecting, opaque plate pierced by two narrowly separated parallel slits and mounted in front of a screen (Fig. 2.1). When light from a narrow portion of the spectrum was shone onto the plate, the pattern he observed on the screen was not the expected two bright bands with blurred edges, but an interference pattern of lighted bars, decreasing in intensity up and down from a position midway between the slits, as indicated by E in the intensity diagram in Fig. 2.1 (right). This outcome can be understood on the basis of wave theory, but is in stark contravention of the particle theory. Let us look at two waves emanating from the two slits on plate O to the screen in front of the slits. If the screen is at an appropriate distance from the slits, then the light from the left and right slit will reach the middle of the screen in the same number of wavelengths. The bright bar in the middle of the screen consists, therefore, of the sum of two waves. In the dark segment between the bars, the two waves cancel one another out, because the crests of one wave coincide with the troughs of the other. The following bars on both sides are due to the sum of two waves which traveled over a distance differing by two wavelengths. 12

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In the early nineteenth century, support for Young’s experiment came in investigations into interference phenomena and diffraction effects (spreading of light waves at the edges of solid objects; see Sect. 2. 4) conducted by Augustin Fresnel (1788–1827) in France and Joseph von Fraunhofer (1787–1826) in Germany.After these and other experiments, the particle theory of light was to lose more and more of its acolytes. The nineteenth century’s most important step in understanding the nature of light was the prediction by James Clerk Maxwell (mentioned in Chapt. 1 in the context of the tristimulus theory) that light is a combination of magnetic and electrical phenomena. When Maxwell began his work on the nature of light in the early 1860s, electrostatics and magnetostatics were already fairly well investigated experimentally. Field theories, however, were still in their infancy. Maxwell’s work culminated in 1873 in the two equations of the electromagnetic field which bear his name. While his highly sophisticated theory is above all a major nineteenth century achievement in understanding physics, it is also very interesting because of the way in which Maxwell developed his theory: he started his work with a model. Experiments and analogies with ‘models’ have been important means for solving difficult problems in science since antiquity. A classical example is the explanation of Archimedes (285–212 B. C.) of the origin of the buoyancy of ships in water. When he took a bath, he noticed that the level of water in his bath tub became higher,and realized that the apparent increase in volume and weight of water corresponded to his own weight. Like Maxwell, with his analogy between fluid flow and the electromagnetic field, Hans Kuhn used the analogy of the vibrations of a gas in a onedimensional box (or a string on a musical instrument) to explain the discrete energy levels of electrons and hence another modern example of a complex physical process, the light absorption (i.e., color) of colorants. It is discussed in Chapt. 3. Models also find widespread use in social sciences, such as in Berlin and Kay’s basic color terms (Sect. 6.3), in the linguistics of color words. Though immensely powerful, model analogies are also subject to severe limitations. Bertold Brecht astutely summed this up in 1949 in his remarks on the opening performance in Germany of his play Mother Courage and her Children. ‘Modelle zu benutzen ist so eine eigene Kunst; soundso viel ist davon zu erlernen. Weder die Absicht, die Vorlage genau zu treffen, noch 13

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die Absicht, sie schnell zu verlassen, ist das Richtige’. (‘The use of models is an art in itself; a given, often considerable amount, but by no means all can be learned from it. It is inappropriate neither that the model fit exactly, nor that it be abandoned easily’).These limits apply every bit as strictly to using models in science! Maxwell’s quintessentially nineteenth century model for electromagnetic phenomena draws its analogy from fluid dynamics. Hypothetical elementary particles of ether pervade a system of rotating vortices, which he represented by regular hexagons between which he inserted ball-bearings to act as idler wheels to decrease friction (Fig. 2.2). He assumed that the strength of the magnetic field was proportional to the speed of rotation of the vortices,and that the ball-bearings corresponded to electrical particles, which would carry a current in the presence of an electric field if they were free to move. Realizing that waves would propagate through this model system of vortices and ball-bearings, he calculated their speed on the basis of electrostatic and magnetostatic constants and found that it corresponded to the speed of light in a vacuum (c = 299800 km/s). He, therefore, concluded that light and electromagnetism must ultimately be the same in nature, and so must both be waves of electromagnetic radiation. In 1873, he implicitly abandoned the necessities of

Fig 2.2. Maxwell’s model of electromagnetic phenomena, as published in the Philosophical Transactions of the Royal Society (1861)

14

Physics of Light and Color

using ether or any mechanical model, basing his theory on two pairs of symmetrical equations, known now as Maxwell’s equations. It had been known since 1800 and 1802, respectively, that the visible spectrum continued into invisible forms of radiation at either end. William Hershel found the infrared spectrum by placing a blackened thermometer towards the red end of a sunlight spectrum and observed an increase in temperature. Ultraviolet radiation beyond the violet end was discovered by Johann Wilhelm Ritter when he placed silver chloride crystals there and observed that they darkened faster on exposure to the invisible radiation than when subjected to the adjacent visible light. A decade after Maxwell, Heinrich Hertz (1857–1894) conducted experiments on electromagnetic wave progagation in air and discovered that short-wavelength radio waves were of the same nature as light. His investigations enlarged the electromagnetic wave spectrum enormously. As shown in Fig. 2.3, the visible spectrum covers only a minute portion of all electromagnetic waves known today. The total range of their wavelengths covers no fewer than twenty orders of magnitude from the shortest to the longest wavelength,while the visible light range,approximately in the middle on a logarithmic scale, represents only half a power of ten out of the total twenty. Strangely, the twentieth century, like the nineteenth, also began with two discoveries which were to be crucial to our understanding of light.In 1900, Max Planck (1858–1947) recognized that the (very weak) spectrum of blackbody radiation can be explained only by postulating that the radiation does not consist of a continuous spectrum of wavelengths, but of one in which energy states are discrete and narrowly defined.With this interpretation he ushered in the concept of quantization of energy. The early twentieth century’s second great achievement in the theory of light is to be found in two of the three famous papers published by Albert Einstein in 1905. Einstein applied Planck’s concept of quanta to light and posited its wave-particle duality, a hypothesis initially received with great skepticism by the scientific community. He based his conjecture on the socalled photoelectric effect, observed when electrons are detected escaping from the surface of a metal plate when it is irradiated with light in an evacuated chamber. Einstein predicted the energy of the electrons ejected by light particles (later to be called photons) of different energies. His energy predictions for these electrons were later verified experimentally, and his theory slowly became established after Niels Bohr’s development in 1913 15

Physics of Light and Color

Wavelengths [m]

Fig. 2.3. Electromagnetic spectrum

16

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of his atomic model, in which the nucleus is made up of protons and (after their discovery in 1932) neutrons, with the electrons circling around it in various orbits in wave form. The circumferences of individual orbits correspond to an integral number of wavelengths. Bohr’s Copenhagen interpretation and Heisenberg’s uncertainty principle are mutually complementary and crucial to proper understanding of waveparticle duality. In a nut-shell, Bohr’s concept states that phenomena in the physics of elementary particles can often be described only by using not one, but two different and apparently mutually irreconcilable approaches. Heisenberg deduced the uncertainty principle from quantum mechanics, concluding that any two physical properties of an elementary particle, such as its location and momentum, or its energy and the time of energy determination, can never be determined beyond a certain (albeit very high) degree of precision. On the basis of these two propositions, the photon, as the fundamental unit of light, behaves as a particle of zero mass, but with energy hv, where h = Planck’s constant and v the frequency of its waves in cycles per second2. Paul Scherrer (1890–1969), in his physics lectures at the Federal Institute of Technology (ETH) in Zurich, used to explain wave-mass dualism with the analogy of a person holding two passports, but who only ever shows one in any situation, never both together. Here we should end the discussion on the nature of light because of the large number of other subjects to be incorporated in approaches to color. Light’s nature, however, is by no means definitively established as far as physicists are concerned. Maxwell and Hertz showed that electromagnetic fields are fundamentally the same as light, together with several other related wave phenomena, while Planck and Einstein used quantum effects and relativity to unite light and elementary-particle physics. Since that time, physicists have achieved better and better understanding of these interacting phenomena. This endeavor, which will be everlasting, currently sails under the name of quantum electrodynamics, or QED. Richard Feynman used the term as the title of a 1985 158-page booklet on the physics of small particles3, a book which even interested non-physicists find accessible. But, even with QED, no final answers are to be found in science. The grand old man of the philosophy of science, Karl R. Popper (1902–1994), emphatically stressed this point as early as his first book, Logik der Forschung4 (The Logic of Scientific Discovery), writing that only refutations of theories are ever definitive; corroborations are merely so much supporting evidence. This was the reason for his choice of the title 17

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Unended Quest for his autobiography5. A book on light for laymen, covering physics, history, and the humanities, was written some years ago by physicist A. Zajonc6.

2.2. Color by Refraction: Newton ’s Experiments Newton wrote that he conducted his light-refraction experiments in 1666; however, recent studies indicate that some of his early experiments date from 1664 or 16657. He published his investigations in 16728 (see also Fig. 1.2) and, in extended form, in his book on optics in 17049. The colors obtained from refraction had been observed, but not investigated further,by Leonardo da Vinci (1452–1519) some 150 years earlier.One of his notebooks shows a drawing of waves of sunlight shining into a (presumably dark) room and onto the edge of a table on which there is a glass filled with water. The parallel rays of light entering the glass are split by refraction into slightly outspread rays reaching the floor at three locations next to one another, representing three regions of the visible spectrum. The posthumous son of a Lincolnshire farmer, Newton studied at Trinity College, Cambridge. Shortly after he gained his Bachelor of Arts degree there in 1665, the university was obliged to close for over a year because of the Great Plague. During that spare time, Newton began work on three of his major scientific accomplishments: the theory of gravitation, the development of calculus10,and his theory of the color composition of light: three colossal achievements in theoretical physics, mathematics, and experimental physics, respectively. This period from 1666 to 1667 demonstrates how fruitful a time without formal working commitment may be for a real genius! Newton conducted his experiments with sunlight and prisms in the context of work to improve telescopes. Prismatic colors had already been observed before him, but Newton was the first to explain sunlight as the combination of all the colors present in the visible spectrum. Fig. 2.4 is taken from Newton’s laboratory notebook. The window on the right-hand side is darkened except for a small hole through which sunlight enters in,first falling on a lens to align the rays parallel to one another before the first prism. The path of the rays is bent at their entrance into the prism, as well as at their exit from it. Newton called this bending ‘refraction’.The rays are also split into the sequence of spectral colors which 18

Physics of Light and Color

Fig.2.4. Newton’s sketch of his experiments with prisms as given in his laboratory notebook in 1666 (reproduced by permission of the Warden and Fellows, New College, Oxford)

can be seen on the screen in the left of the room. Bending is greatest for the violet end of the beam, and red is at the lower end of the screen. From this part of his experiments, Newton concluded that ‘light […] is a heterogeneous mixture of differently refrangible rays’, as he wrote in his publication of 1672. His drawing also shows his second experiment: if there is a hole in any part of the screen – the sketch shows one in the red region of the spectrum – the red fraction of the light, while still bent, is not dispersed any further by a second prism placed behind the screen. In another experiment, Newton demonstrated that a narrow beam of sunlight, which had been dispersed into spectral colors by a prism, could be merged back into an achromatic beam if a second prism was put into the path of the bent beam,but rotated through an angle of 180°, as shown schematically in Fig. 2.5. This experiment he called the ‘experimentum crucis’. This experiment was the basis for his conclusion that achromatic light is a combination of all spectral colors; a claim which was to be the main cause of Goethe’s repudiation of him a century later (see Sect. 7.6). Newton described the spectrum which he observed in his experiments as consisting of seven colors and arranged them in a circle. As we will see in later chapters, the categorization of chromatic and achromatic colors is ultimately not a problem of the physics of light, but of colorimetry, color vision, culture, and art. We will, therefore, discuss Newton’s color circle in the context of the colorimetry of color vision (cf. Chapt. 4, Fig. 4.6). 19

Physics of Light and Color

Fig. 2.5. Newton’s arrangement of two lenses and two prisms for showing that a dispersion spectrum can be merged back again into colorless light

Newton’s color experiments should, however, also be viewed in the context of phenomena associated with light rays at the surface of a medium. Reflection of light in mirrors had already been investigated by Hero in Alexandria in the first century A.D. He found that reflected light leaves the surface of a mirror at the same angle as the incident light (Fig. 2.6). The law of refraction, i.e., the bending of a light beam observed when it falls on the flat surface of a transparent substance like water or glass, was investigated by the Dutch mathematician Willebrord Snellius (also known as van Snell van Royen; 1581–1625).He found in 1618 that the angle of refrac-

Fig. 2.6. Reflection, total reflection, and refraction at plain between two media. Law of reflection: a1 = a1′ Law of refraction: n1 sin a1 = n2 sin a2 n1 and n2 = refractive indices of medium 1 (above) and medium 2 (below) Total reflection: a3 = a3′

20

Physics of Light and Color Tab. 2.1. Refraction Indices n for Some Materials at a Wavelength of 550 nm Vacuum (by definition) Air Water Fused quartz

1.0000 1.0003 1.3300 1.4600

Crown glass Flint glass Diamond Lead sulfide

1.5200 1.1000 2.4200 3.9100

tion (the angle between the refracted ray in the second medium and the perpendicular to the surface) could be larger or smaller than the corresponding angle for the incident ray (angles a2 and a1 , respectively in Fig. 2.6; a1′ is the angle of the reflected ray, a1 = a1′). Fig. 2.6 also includes the case of total reflection, essential for comprehending the physics underlying the rainbow (see Sect. 2.3). Any ray emanating from the higher-optical-density medium (medium 2, n2 > n1) will not pass into medium 1 if sin a3 is less than n1/n2. Instead, it will be reflected back into medium 2 at the same angle (a3 = a3′). The refractive index of a medium x (nx) is related to the speed of light in that medium (vx) relative to the speed of light in vacuum (c), which, as mentioned earlier in this chapter, Maxwell calculated to be 299,800 km/s. nx = c/vx Refractive index does not only vary dramatically from one medium to another (see Table 2.1). It also varies with wavelength; for the visible spectrum, for example, nNaCl (table salt) increases from 1.48 for long-wavelength (red) light (l = 700 nm) to 1.51 for short-wavelength (violet) light (400 nm). For ultraviolet light, the effect of wavelength is much larger; for example nNaCl = 1.70 (200 nm). The characteristic luster of diamond is partly due to its high refractive index.

2.3. The Rainbow Of all natural color phenomena, the rainbow surely represents the most significant impression that color has made on the human mind, probably in every culture on earth. It was to be a source of myth and wonder, and also of early scientific curiosity11. We have all seen rainbows. In most cases, they occur unexpectedly, we are unprepared for them, and they may disappear fairly soon. We cannot see 21

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where this spectral arc to the heavens meets the ground, nor how high it is in the sky, and usually we do not take in the sequence and number of its colors. Sometimes a second, higher and fainter, rainbow can be seen: does it have the same colors and in the same sequence? And yet, rainbow-like phenomena may occasionally be seen much closer to us, near a waterfall, or in the driving spray of seawater on a sea voyage. Although we all know that rainbow formation needs water and sunlight, we still intuitively regard them as, in a certain sense, ethereal things. We cannot catch or touch a rainbow. It is, therefore, not astonishing that the rainbow has a very important symbolic value in the Bible. After the Flood, God decides not to repeat such a disaster for the earth, but to establish a covenant with Noah (Genesis, 9, 12–16): ‘And God said,This is the token of the covenant which I make between me and you and every living creature that is with you, for perpetual generations: I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth. And it shall come to pass, when I bring a cloud over the earth, that the bow shall be seen in the cloud: And I will remember my covenant, which is between me and you and every living creature of all flesh; and the waters shall no more become a flood to destroy all flesh’. In the New Testament, the rainbow is mentioned in the Revelation of St. John the Divine (4, 3), when he saw God: ‘[…] and he that sat was to look upon like a jasper and a sardine stone: and there was a rainbow round about the throne, in sight like unto an emerald’. Nor is it astonishing that the rainbow is also of primordial importance in Greek mythology. The goddess of the rainbow was Iris, daughter of Thaumas, the god of wonder.Aegean islands and coasts would not uncommonly be witness to rainbows with at least one arc rising out of the sea, home of Oceanus, and this arc of colors, joining heavens, earth, and sea, would have seemed the natural bridge between the worlds. Mythologically, the rainbow also serves as a means of conveying messages. Iris often has the role of a speedy, wind-footed messenger, bringing good and bad news, and commandments to men from gods like Zeus. The rainbow has this function in many other cultures,and also serves as a pathway to heaven for souls of the dead in diverse belief systems such as the native American, Polynesian and Japanese. The Norse Eddas also feature it in this role. 22

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Plato (427–347 B. C.) makes an interesting statement in his Theatetus, where he considers Iris and wonder in relation to philosophy: ‘This sense of wonder is the mark of the philosopher. Philosophy indeed has no other origin, and he was a good genealogist who made Iris the daughter of Thaumas’. The name Iris is a symbol of a mythic rainbow. In our modern culture, the term has disappeared, used now only for a flower and the small colored ring around the pupil in our eyes. The latter, growing or shrinking with the intensity of outside light reaching the eye, acts at the borderline between our outer and inner worlds, maintaining the healthy equilibrium of the light coming from one world to the other. Plato’s remark takes us to Iris as messenger to the world of science. A major question in Antiquity was the number of different colors in a rainbow.Aristotle was of the opinion that there are three, beginning with red at the outside edge of the bow and followed by green and purple. He was also aware that the rainbow is a segment of a circle. In ancient Greece it was generally assumed that the process of vision involved a ray originating in the eye and traveling to the perceived object, and so Aristotle assumed that this ray was projected from the eye to the cloud, somehow giving rise to the rainbow.In his view,the cloud consisted of mist,which acted as a very large number of small mirrors, reflecting the sight ray to the sun. Summarized in his book Metereologica, in which he mostly described and analyzed phenomena taking place within the sphere of the moon’s orbit, this hypothesis includes the first beginnings of a geometric analysis. The relative positions of sun, rainbow, and observer are also indicated in a poem by the Roman poet Virgil (70–19 B. C.). He observed a much larger number of colors, however: ‘Ergo Iris croceis per coelum roscida pinnis mille trahens varios adverso sole colores devolat…’(Aeneid 4,700) (‘As when the rainbow, opposite the sun a thousand intermingled colors throws…’). It is not known who first noticed that two rainbows can frequently be seen at the same time, the second one higher in the sky (see later in this section) and with a reversed sequence of colors. The first recorded account of the secondary rainbow can be found in Aristotle’s Meteorologica. Careful observers can see that the area between the two rainbows looks darker. The philosopher Alexander of Aphrodisias, head of the Lyceum in Athens between 198 and 211 A. D., concluded from theoretical considerations that it should be lighter, not darker. Therefore, the observed relative darkness was originally called the Aphrodisian paradox. An explanation was 23

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found, but only in the seventeenth century, by Descartes12, and the effect is now known as Alexander’s dark band. After the Middle Ages,interest in science increased in Europe in the twelfth century, when the works of Greek and Arabic scientists became accessible in Latin translation at the first universities. For his contribution to understanding the phenomenon of the rainbow, Robert Grosseteste, also known as Robert of Lincoln (ca. 1167–1253), should be mentioned first of all. He studied at the Universities of Oxford and Paris, taking a particular interest in languages and what we would today call philosophy of science. He developed a dual approach to experimental science, proposing principles which could be either corroborated or refuted; the method which, seven centuries later, would be the main preoccupation of Karl R. Popper, the most influential philosopher of science of our century! Grosseteste’s major work on the rainbow and on mirrors is the small booklet De iride seu de iride et speculo. In it, he shows that Aristotle’s explanation – that the rainbow is a product of reflection by a system of small mirrors in the cloud – cannot be correct, but that it must be a refraction effect: ‘Necesse est ergo, quod iris fiat per fractionum radiorum solis in rotatione nubis convexae’ (‘It is therefore necessary that the rainbow be made by the refraction of the sun’s rays in the moisture of a convex cloud’). About a hundred years after Grosseteste,Dietrich von Freiberg (also known as Thierry de Freiberg and Theodoric of Freiberg), a German member of the Order of Preachers,devised an explanation of the rainbow which anticipated by some 340 years the geometrical theory later published by Descartes12. A professor of theology in Germany who spent the later years of his life in France, von Freiberg (b. ?, d. ca. 1310) was the author of many works on metaphysics and on optics. Among them is the lengthy book De iride et radialibus impressionibus. Von Freiberg’s work on the physical origin of the rainbow is remarkable first of all for the approach he used. From Aristotle to Robert Grosseteste, all previous investigators had considered rainbow formation to be caused by the cloud as a whole. Von Freiberg, however, realized that it was necessary first to look at the interaction of light with individual raindrops. The very small size and the difficulty of isolating raindrops made their study practically impossible, however, and so von Freiberg scaled one up by substituting a spherical glass filled with water, which he exposed to sunlight. In this way,he saw that a ray is first refracted and then reflected at the inner surface at the rear of the vial, before it leaves the vial, undergoing a sec24

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ond refraction. He also postulated that an analogous process takes place in the formation of a secondary rainbow, but involving two reflections inside the sphere. This is shown schematically for both rainbows in Fig. 2.7. Descartes (1596–1650) accomplished a major advance on von Freiberg’s optical rainbow analysis. He was able to calculate the angles between incoming sunlight entering the raindrops and the rays traveling from both rainbows to the observer; these are 42° and 51° for the primary and secondary rainbows, respectively. Descartes included the basis of this calculation in the appendix La dioptrique to his Discours de la Méthode12. As discussed in the preceding section (see Fig. 2.7), the law of reflection was already known, but the law of refraction, although discovered by Snellius in 1618, was not published before his death. His manuscript, however, was known to the Dutch scien-

Fig. 2.7. Refraction (a) and reflection (b) in the primary and in the secondary rainbow (upper and lower figure,respectively).Angles 42° and 51° (see text)

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tist Christiaan Huygens (Traité de la lumière, 1690),and it may be that Descartes became acquainted with it during a visit to Huygens in Holland. He did, however, analyze refraction independently in a slightly different manner, including it in another appendix to Discours de la Méthode. His application of the law of refraction to the analysis of the rainbow was included in a third appendix Les météores. It is interesting to note that,despite many reported observations, Descartes was unable to offer an explanation for the colors of the rainbow. He stuck with the then traditional hypothesis that light was changed qualitatively when it passed through a raindrop. In his study of the rainbow, Descartes’ most fundamental and final goal was to demonstrate that it has the same origin whether it is seen in the sky or in the air close by, wherever drops of water meet sunshine. He, therefore, promoted the use of the term ‘arc-en-ciel’, which, with its equivalents, has been taken over in the Romance languages in place of the name of the Goddess Iris. Doesn’t that linguistic change indicate that he wanted to completely replace the divine origin of the rainbow by physics and geometry? But he hardly touched upon its colors,which,since antiquity,have been considered the most wonderful (in a literal sense) part of the appearance of a rainbow. Fig. 2.8 gives Descartes’ schematic representation of the primary and secondary rainbow. The sun, the observer, and the center of the two rainbow circles are in the same perpendicular plane, as indicated by the straight line between points S, O, and C. Newton, soon after he had conducted his classical experiments of light refraction and dispersion into spectral colors in 1666 (see Sect. 2.2), recognized that his results also applied to the colors of the rainbow. He recognized in his experiments with prisms that each color had its own characteristic degree of refrangibility (called index of refraction today), and concluded that this is also the case for refraction in water, as well as in glass.He included his interpretation of the colors of the rainbow in his first publication on color in 1672 (see Fig. 1.2). Calculating the angles of refraction analogously to Descartes (see above), but also taking into account the color dependence of the degree of refrangibility, as well as the one or two reflections required for primary and secondary rainbows, Newton obtained values of 42°2′ and 50°57′ for red, and 40°17′ and 54°7′ for violet. These angles also demonstrate why the colors appear in reversed sequence in the two rainbows.‘For those drops [of rain], which refract the Rays, dis26

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posed to appear purple, in greatest quantity to the Spectator’s eye, refract the Rays of other sorts so much less, as to make them pass beside it; and such are the drops on the inside of the Primary Bow, and on the outside of the Secondary or Exteriour one’12.

Fig. 2.8. Descartes’ scheme of the primary and secondary rainbow in relation to the observer and the sun (A and F)

Tertiary rainbows have been observed and described occasionally; in 1759 and 1878, for example. Their formation is easily explained on the basis of geometrical optics; they are caused by three reflections within the raindrops.Rainbows arising from four or more reflections are possible in principle, but there does not seem to be any unambiguous positive evidence of their observation. 27

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There are, however, other rainbows, which, for centuries, have occasionally been observed adjacent to the inside of the primary arc, and also, more recently, at the outside of the secondary arc. These are called supernumerary rainbows. Thomas Young found that they are clearly visible when the raindrops are sufficiently small (ca.0.33 mm in diameter),and postulated in 1804 that their cause can be understood on the basis of his interference theory,discussed earlier in this chapter.Various authors have verified this since.

2.4. Peacock’s Colors: A Phenomenon of Interference Historically, culturally, and scientifically, the physical color phenomenon of most intense fascination to mankind must be the rainbow. However, a physical color-display phenomenon which has probably commanded almost as much of mankind’s attention is to be found in the tail feathers of the peacock (Pavo critatus L.). Why only almost as much? The flightless peacock does not have the same symbolic relationship to the sky, to heaven and to the gods; it belongs to the earth as we do, material as we are, not untouchable like the rainbow. The peacock is indigenous to India, and its first known representations are from the Harappa culture in the Indus Valley (third and second millennia B. C.). It is mentioned in the Old Testament in a report on the ships of King Solomon (First Book of Kings, 10, 22). In Greece, peacocks do not appear to have been known at the time of Homer, although five hundred years later they were, but only as a rarity. In the Roman Empire, they were the emblem of empresses. The thrones of the great Moguls in India and of the Shah of Iran even until 1979 were named after peacocks, and their gold and silver surfaces decorated with representations of them. Because peacock feathers blend all colors together, the peacock became a symbol for completeness and, in Christian art, for immortality and the incorruptible soul.The alchemists were also very interested in their many colors because color changes of metal surfaces were often to be observed in alchemical experiments involving metals. Pictures of the peacock can, therefore, frequently be found in alchemical literature, mainly in the glass vessels used by the alchemists. A very nice example is a picture attributed to Salomon Trismosin in the work Splendor solis (Fig. 2.9), dated from 1582. Most of us will probably agree with the view of Charles R. Darwin (1809–1882), expressed in his book The Descent of Man, and Selection in Relation to Sex14: ‘the oval disc or ocellus [of the peacock] [...] is certainly one of the most beautiful objects in the world’. 28

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Bearing in mind what we have previously said about the rainbow and its mystical associations for us and our ancestors, it is notable that Darwin’s statement confines itself to ‘objects in the world’. It may be hair-splitting to debate the precise semantics of the word ‘object’, but even so, it is per-

Fig.2.9. The peacock in the hermetic vessel of the alchemist.Picture attributed to the alchemist Salomon Trismosin (dated 1582)13.

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tinent to ask how real an object is a rainbow in the context of the feelings it engenders in the onlooker (the same question is clear for a peacock). On the other hand though, using the word ‘object’ to describe an animal, a living organism of this world, does still seem rather pejorative to me. The beauty of the colors of peacock feathers stems from the fact that they are iridescent – the word derives from Iris, the rainbow goddess. To be iridescent, colors must have two characteristic properties for our color perception: firstly they must, physically, be of exceptionally high purity, reflecting a single wavelength of light (monochromatic light) almost exclusively in sunlight, and, secondly, the color must change when the iridescent surface is viewed from different angles. The color change may cover up to half of the visible spectrum with viewing angles of 90° down to 10°, progressing in the direction of colors of lower wavelength. Below about 50°, the pure color gets blacker and blacker, as reflection of the pure color decreases. The shimmering colors of peacock feathers arise from a combination of three optical phenomena: reflection,diffraction,and interference.Diffraction of light at small slits was observed as early as the seventeenth century, particularly by Huygens, who observed that it was reflected at the edge of a slit and partly diffracted when it passed through it. In the early nineteenth century, Joseph von Fraunhofer found that the effect could be multiplied if many parallel slits, set apart from one another by a constant, very small distance, were used in a plane. Light-diffraction effects also occur if very small round particles are present in an arbitrary arrangement in a gas or a liquid (John Tyndall, 1820–1893). This is why the sky, seen from within the atmosphere, looks blue, while from space it looks black. Similar centers of diffraction may also be present in a regular pattern as points in a three-dimensional, translucent lattice. However, unlike Fraunhofer’s two-dimensional gratings, such three-dimensional lattices with spacings of the order of one wavelength between mass points, were – and to my knowledge, still are – impossible to construct technically. That is the situation as it relates to wavelengths corresponding to visible light. In 1912, however, Max von Laue (1879–1960), followed shortly afterwards in 1913 by William H. Bragg (1862–1942) and his son William L. Bragg (1890–1971), found that the distances between atoms in crystal lattices are similar to the wavelengths of X-rays.Exploiting this finding,they were able to study diffraction, reflection, and interference effects of X-rays in crystals experimentally and in a quantitative manner. Their work was to result 30

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in chemistry’s most important technique for the elucidation of the threedimensional structure of large molecules. Fig. 2.10 shows the simplest case of such a lattice, for a cross-section of the first three planes of mass points in a cubic arrangement and with three electromagnetic rays, all of the same wavelength, which must be of the order of magnitude of the distance d between the mass points. Diffraction of the ray at the mass points is neglected for reasons of simplicity.It is clear for geometrical reasons that the distance traveled by ray 2 is longer than that by ray 1. If the extra distance for ray 2 is exactly half a wavelength, the reflected ray 2′ will completely cancel out the intensity of ray 1′. If the additional distance is one wavelength,or a multiple of that,however,the reflected ray will have its maximum intensity. This result is, therefore, a further application of Young’s law of interference (see earlier in this chapter). The correlation is expressed in Bragg’s law for interference intensity maxima: n 2d sin a = hv (h = 1, 2, 3…) If the frequency of the radiation used (v) and the refractive index (n) of the translucent medium used are known, the equation permits the calculation of the distance (d) between mass points (the atoms in the molecules making up the crystal) and vice versa. The phase displacement of ray 2′ relative to 1′ corresponds to the distance PB + BQ = 2PB = 2d sin a. Amplification by interference occurs if that distance is equal to one wavelength of the radiation used.

Fig. 2.10. Interference in a plane of a cubic lattice perpendicular to the surface of the reflected radiation, after Durrer15 (reproduced by permission of the Naturforschende Gesellschaft in Basel)

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As mentioned above, our technology is unable to produce three-dimensional gratings with regularly repeating spacings corresponding to visiblelight wavelengths. Nature can, however: the peacock’s colors are the result of lattices of this type in his feathers. This was shown by Durrer in a beautiful electron-microscopy investigation published in 196215.

Fig.2.11. Schematic representation of the three-dimensional grating of melanin rods (black) in the outer area of a peacock feather, after Durrer15. Top : Surface of the feather. Left : Section along the melanin rods. Right : Section perpendicular to the rods. Dotted area : keratin; striped area : tonofibrils; white : air bubbles (reproduced by permission of the Naturforschende Gesellschaft in Basel)

The iridescent portions of a peacock’s feathers are characterized by lattices of melanin rods in their surface zones.Melanins are black or dark brown organic polymers, chemically based on derivatives of benzene, and are opaque to visible light. A three-dimensional scheme of an iridescent portion of feather is given in Fig. 2.11. The straight melanin rods have a diameter of 98 to 125 nm and a length of about 1 µm16, each followed after only a short interval by another, oriented in almost exactly the same direction as its predecessor. They are embedded in keratin, a light-translucent protein,with between three and eleven planes of melanin rods extending down from the surface. The number and spacing of the planes determines the color of that particular portion of feather (see below). Fig. 2.12 is a schematic drawing of the tail feather of a peacock. The tip is called the outer zone (Ou) and is green to red17. While individual barbs in the outer zone are arranged relatively loosely, those in the four adjacent boundary stripes 4 to 1 are much more precisely coordinated: 4 is greenish golden,3 dark green,2 violet,and 1 is yellow.The central part accommodates the eye (or ocellus), with its three ocellated spots III to I. The reddish-brown ocellated spot III is egg-shaped, II is oval and turquoise, and I is kidneyshaped and dark blue, fringed with black. Below the eye spots the distances between individual barbs grow larger once more. This middle portion (M) iridesces between green and red. D represents the downs and Sp the quill. Fig. 2.13 shows electron micrographs of six zones of a peacock’s feather, with numbering as in Fig. 2.12. The interpretation of the electron micrographs is given in Table 2.2. The wavelengths of the reflected light, calculated using Bragg’s law (see above), agree well with those estimated from the visual color. The high hue purities of ocelli I – III are evident from the relatively high degree of consistency of the distances between melanin rods, i.e., the smaller standard deviations of these distances compared with those in the outer zones (Ou 1 and 2). In conclusion then, the variety of colors present in a peacock’s feathers is due to the variety of inter-melanin rod spacing in different parts of the

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Fig. 2.12. Schematic representation of an eye feather of a peacock, after Durrer15 (reproduced by permission of the Naturforschende Gesellschaft in Basel)

feathers.Other animals have also adopted the same physical means of color generation to their own,entirely different,ends.Cuttlefish, squid, and octopi, for example, use melanin-based colors for purposes of mimicry. Their ability to change color very rapidly (within a few seconds) comes from 33

Physics of Light and Color

Ou (green-red)

2 (violet)

1 (golden yellow)

III (reddish brown)

II (turquoise)

Fig. 2.13. Comparison of the electron micrographs of six zones of a peacock’s feather, after Durrer15.For numbering, see Fig. 2.12. Magnification: 40,000× (reproduced by permission of the Naturforschende Gesellschaft in Basel)

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I (dark blue)

Physics of Light and Color Table 2.2. Comparison of Visual Colors of Six Areas in a Peacock’s Feather with Electron-Microscopy Results and Wavelengths expected on the Basis of Bragg’s Law, after Durrer15 (reproduced by permission of the Naturforschende Gesellschaft in Basel) Areaa) Color (in diffuse illumination)

Ou 2 1 III II I a

green-red violet golden-yellow reddish brown turquoise dark blue

Wavelengths, expected on color [nm]

500 – 630 380 – 440 570 – 600 590 – 620 490 – 510 450 – 470

Electron-microscopy results Numbers of layers

Horizontal separation of melatonin rods [µm]16

Separation (d) Calculated of layers wavelength of reflected light [µm]16 [nm]

3–6 4–7 4–6 5–7 9 – 10 9 – 11

0.16 ± 0.012 0.19 ± 0.022 0.15 ± 0.024 0.15 ± 0.011 0.17 ± 0.013 0.15 ± 0.01

0.21 ± 0.015 0.19 ± 0.022 0.208 ± 0.016 0.21 ± 0.007 0.17 ± 0.005 0.16 ± 0.006

630 ± 45 570–690 624 ± 40 630 ± 21 510 ± 15 480 ± 18

) See Fig. 2.13.

expanding or contracting their melanin-containing skin,changing the distances between melanin grid rods and, hence, its structural color. In this manner, they are able to maintain a superficial resemblance to natural objects in their environment. Three-dimensional diffraction gratings also occur in natural inorganic objects, such as opal gemstones. The so-called play of color in black opals has been shown by electron microscopy to involve a regular three-dimensional array of equal-sized spheres of amorphous silica containing a small quantity of water (see Nassau’s review18). Many other color effects in animals – like those seen in butterfly wings, beetle carapaces, wasps’ bodies, oyster shells, and snakes’ scales – are produced by multiple, regularly ordered two-dimensional gratings. In many cases, a rapid play of color can be observed as these animals move, and could, obviously, serve to confuse a predator. The gratings are either the edges of stripes on a flat surface, as in some beetles and wasps, or more elaborate constructions like the vanes on the surfaces of butterfly wingscales19 (Fig. 2.14). These star-like gratings have the advantage that a large proportion of the light energy is directed into the first interference maximum, resulting in high color purity and intensity. They are called echelette gratings.

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Fig.2.14. A) Structure of vanes on the surface of scales on the wings of the butterfly M. rhetenor. B) Multiple interference produced by the ridge on these vanes (Fig. reproduced from Ref. 19 )

2.5. How Many Causes of Color Do We Know? We have discussed three instances of color arising out of diverse physical phenomena: refraction in a glass prism, combined refraction and reflection in a rainbow’s water droplets,and concerted refraction,reflection,and interference in peacock feathers. Many other causes of color formation, both in nature and in the laboratory, are known, produced by either physical or chemical processes. Nassau has collected and categorized fifteen in a review paper18 and a book20, and these are listed in Table 2.3. Each cause is given with some specific examples. His table has five main groupings, based on the fundamental mechanisms involved: I) vibrations and simple excitations by (external) heat transfer or energy transfer within molecules, II) excitation of unpaired electrons in transition metals, their ions and complexes, III) electron transition between molecular orbitals in organic and inorganic chemical compounds, IV) electron transition in solids; mainly metals, semiconductors, and related matter, and V) optical effects in matter which is partly or completely translucent to light. Color effects belonging to groups I, IV, and V are usually considered physical phenomena, whereas those belonging to group III are chemical processes. Group II effects are borderline between physics and chemistry. 36

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The causes of color discussed in the preceding three sections all belong to group V. Reasons of space preclude a more detailed discussion of other physical causes of color in this book. Interested readers are recommended to consult Nassau’s review18 or book20. The chemically-based group III and impurity-induced ligand-field (group II) effects are, however, briefly discussed in the following chapter on the chemistry of color. Rapid color changes in animals like chameleons and frogs are due in most, but not all, cases not to the physical and chemical causes listed in Table 2.3, but to expansion or contraction of the chromatophoric (color-bearing) cell structure. A frog is green because its chromatophores absorb all visible light except that of medium wavelengths (500–600 nm). If the frog is startled,it suddenly changes its color for camouflage to yellow by contracting all those chromatophoric cells which absorb long-wavelength visible light. Table 2.3. Nassau’s Classification of the Fifteen Causes of Color, with Examples 20 (reprinted by permission of John Wiley & Sons Ltd., New York) I. 1. 2. 3.

Vibrations and Simple Excitations Incandescence: flames, lamps, carbon arc, limelight Gas excitations: vapor lamps, lightning, auroras, some lasers Vibrations and rotations: water, ice, iodine, blue gas flame

II. Transitions Involving Ligand-Field Effects 4. Transition-metal compounds: turquoise, malachite, chrome green, copper patina, Thenard’s blue, some fluorescence, lasers, and phosphors 5. Transition-metal impurities: ruby, emerald, aquamarine, red iron ore, some fluorescence and lasers III. Transitions between Molecular Orbitals 6. Organic compounds: most dyes, most biological colorations, some fluorescence and lasers 7. Charge transfer: blue sapphire, magnetite, lapis lazuli, ultramarine, Prussian blue IV. 8. 9. 10. 11.

Transitions Involving Energy Bands Metals: copper, silver, gold, iron, brass, ruby glass Pure semiconductors: silicon, galena, cinnabar, vermillion, cadmium orange, diamond Doped semiconductors: blue and yellow diamond, light-emitting diodes, some lasers, and phosphors Color centers: amethyst, smoky quartz, desert amethyst glass, some fluorescence, and lasers

V. Geometrical and Physical Optics 12. Dispersive refraction: rainbows, halos, sun dogs, green flash of sun, ‘fire’ in gemstones, prism spectrum 13. Scattering: blue sky, red sunset, blue moon, moonstone, blue eyes, skin, butterflies, bird feathers and some other biological colors, Raman scattering 14. Interference: oil slick on water, soap bubbles, coating on camera lenses, some biological colors 15. Diffraction: aureole, glory, diffraction gratings, opal, some biological colors, most liquid crystals

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The white fur of polar bears is obviously also a camouflage color. White, however, has the disadvantage of not absorbing visible light energy; so the fur does not transform the polar summer’s sunlight into warmth for the animal. Therefore, the skin of the polar bear is black and the diffuse light passing through the fur helps maintain the creature’s heat-balance. The title of this section asked how many causes of color are known. The answer is that there are not only several, but several very different causes of color in this world, and that is one of the many reasons for the fascination it has for us. References and Notes 1. Thomas Young has already been mentioned in Chapt. 1 as the discoverer of the tristimulus theory of color. He was a true universal genius: besides his professional work as a practicing medical doctor and his mentioned achievements in optics, he was the first to use the word energy in its modern sense and invented the first absolute measurement of elasticity, by defining modulus as the weight which would double the length of a rod of unit cross-section from which it was hung (Young’s modulus). In addition, he mastered fifteen European and Middle-Eastern languages and, in 1819, laid the foundations for the decipherment of Egyptian hieroglyphic script. 2. The frequency is reciprocal to wavelength. In this book, in most cases, wavelengths are used in discussion of the visible spectrum. 3. R. P. Feynman, QED, The Strange Theory of Light and Matter, Princeton University Press, Princeton, NY, 1985. 4. K. R. Popper, Logik der Forschung, Julius Springer, Vienna, 1935 (9th edn., 1989, Mohr, Tübingen). English edns., The Logic of Scientific Discovery, Hutchinson, London, 1959/1980. 5. K. R. Popper, Unended Quest. An Intellectual Autobiography, Open Court Publ. Co., La Salle, Ill, 1974. 6. A. Zajonc, Catching the Light, Oxford University Press, Oxford, 1993. 7. A. E. Shapiro, ‘The Gradual Acceptance of Newton’s Theory of Light and Color, 1672-1727’, Perspectives on Science 1960, 4, 59–140. Probably the most authoritative and critical review of this oft-discussed subject. Many references. 8. I. Newton, ‘New Theory about Light and Colors’, Philosophical Transactions IV, 1672, 80, 3075–87; often reprinted; facsimile: Werner Fritsch, Munich, 1967. 9. I. Newton, Opticks: or a Treatise of the Reflections, Refractions, Inflections & Colours of Light,S.Smith and B.Walford,London,1704/1730; reprint of the fourth edn. (1730) Dover Publ., New York, 1952. 10. Calculus was invented independently by Gottfried Wilhelm Leibnitz (1646–1716) and by James Gregory (1638–1675) in the 1670s. 11. C. B. Boyer, The Rainbow. From Myth to Mathematics, Macmillan Education, Basingstoke, Hampshire, 1987. Very comprehensive, critical, many historical literature references.

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Physics of Light and Color 12. R. Descartes, Discours de la Méthode, 1637; English translation: Discourse on Method, Liberal Arts Press, New York, 1956. 13. Harley MS 3469, British Museum, London. 14. C. Darwin, The Descent of Man, and Selection in Relation to Sex, John Murray, London (two volumes), 1871; facsimile edition: Culture et Civilization, Brussels, 1969. Vol. I contains chapters on color phenomena in insects, vol. II on those in fish, birds and mammals (peacock: vol. II, 132–134). 15. H. Durrer, ‘Schillerfarben beim Pfau’ (Pavo cristatus L.). Eine elektronenmikroskopische Untersuchung, Verhandlungen der Naturforschenden Gesellschaft Basel 1962, 73, 204–224. None of the many books, reviews, and scientific papers on causes of color which I have consulted mentions this study. A probable reason is the fact that Durrer’s paper was published in a journal without wide international circulation. 16. µm = micrometer = 10–6 m; nm = nanometer = 10–9 m. For comparison; the size of a virus is 20 to 300 nm, bacteria 1000 nm and over; a water molecule 0.2 nm. 17. Colors mentioned are those seen viewing the feather perpendicularly. 18. K. Nassau, ‘The Fifteen Causes of Color’, Color Research and Application 1987, 12 (1), 4–26. 19. H. F. Nijhout, ‘The Color Patterns of Butterflies and Moths’, Scientific American 1981, 245, 104–115. 20. K. Nassau, The Physics and Chemistry of Color, The Fifteen Causes of Color, John Wiley & Sons, New York, 1983.

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3. Chemistry of Color 3.1. History of Colorants The term ‘colorants’ embraces all colored compounds, irrespective of their origin and utility for coloration or other purposes. They may be divided into natural and synthetic compounds, but the distinction is not generally meaningful, as some existing colorants originally had a natural source but are produced synthetically today. It is, therefore, questionable whether the word ‘biochromes’ should be used for natural colorants1. Colorants are either dyes or pigments. The terms are often used indiscriminately; in particular,pigments are quite often considered to be a subgroup of dyes. Ideal pigments are characterized by being practically insoluble in the media in which they are applied2.Pigment particles have to be attached to substrates by additional compounds, like a polymer in a paint, a plastic, or a melt. Dyes, on the other hand, are applied to various substrates (textile materials, leather, paper, hair etc.) from a liquid in which they are completely, or at least partially, soluble. Unlike pigments, dyes must possess a specific affinity to the substrates for which they are used. The rather inaccurate distinction often drawn between pigments and dyes has several origins. Firstly, the words ‘dye’ and ‘dyeing’ are much better known to the general public than the more technical term ‘pigment’, or even ‘colorant’. Furthermore, some dyes, like indigo, are applied as watersoluble derivatives but become pigments after application. Prehistoric man was already able to dye furs, textiles, and other objects using natural substances, mainly vegetable in origin, but also some animal products. Cave drawings like those in Altamira in Spain also attest to the use of (inorganic) pigments in prehistoric times. Ancient Egyptian hieroglyphic scripts contain a thorough description of the extraction of natural dyes and their application in dyeing. Further developments, stretching over thousands of years, led to quite complicated dyeing processes and high-quality dyeing3. Among these, the following deserve special mention: – Indigo, obtained both from dyer’s woad,indigenous to Europe,and from Indigofera tinctoria, a native plant of Asia; 41 Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

Chemistry of Color

– Ancient Purple, extracted from a gland of the purple sea snail by a process developed by the Phoenicians; – Alizarin, on which Turkey Red is based,was obtained from madder Campeachi wood extract imported from Africa. In 1856, the young English chemist William Henry Perkin (1838–1907) was working under the direction of the German chemist August Wilhelm von Hofmann (1818–1892) at the Royal College of Chemistry in London.He suspected that the natural product quinine, at that time the only known treatment for malaria, might be produced by the oxidation of a compound – containing carbon, hydrogen, and nitrogen – which von Hofmann had isolated from coal tar. At that time, chemists’ knowledge of the structure of organic compounds was limited to determining the number of atoms of each element in a compound (its molecular formula). They knew nothing about the different ways in which the different atoms might bind to one another and the three-dimensional structure thus created (the compound’s structural formula). Quinine and Perkin’s coal tar derivative were similar to one another only in respect of their molecular formulae; not at all with regard to their structural formulae,and so Perkin could never have succeeded. Nonetheless, in 1856 he found that a related coal tar derivative produced an intense bluish purple solution with methyl alcohol, and that this compound dyed silk a rich color that neither washed out nor faded after a week’s exposure to sunlight. A patent was issued to the 19-year old Perkin, and with the financial help of his father, he started production of the compound in 1857. The product was given the name ‘Mauve’ or ‘Mauveine’. This brilliant hue on silk stimulated other chemists to carry out similar experiments. In particular, silk dyers became very interested in the potential of organic chemical oxidation reactions for dye production. The next step was made by Emanuel Verguin in Lyon, center of the French silk industry, when he obtained a red dye which was called ‘Fuchsine’. For reasons relating to French patent law, several French chemists emigrated to Basle in Switzerland, a center of the silk-ribbon industry, and began to produce dyes there in the early 1860s. This was to become the nucleus of the Swiss dyestuff industry. The best recent historical study on the development of the dye industry in the nineteenth century was written by Travis4. Another stimulus was to prove even more important to the young dyestuff industry’s long-term development than its ties to silk production.This 42

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was the accumulation of general knowledge in organic chemistry, particularly at German universities,which was already underway before Perkin’s discovery of 1856.The Royal College of Chemistry’s decision in 1845 to offer a professorship to von Hofmann demonstrates this. The development of dyestuffs in the 1860s owed most to the work of August Kekulé (1829–1896), professor at the universities of Gent (until 1865) and, later, Bonn. Kekulé is today chiefly remembered for his discoveries that carbon is tetravalent (1858), and that benzene is a six-membered ring of carbon atoms (1865). Even at that early date, he had realized that the conventional representation of benzene, with alternating double and single bonds (= and –, respectively,in the formula 1) did not fit with the compound’s reaction characteristics, which would require all six bonds between carbon atoms to be identical. The structure as given in the formula 1 is, therefore, only formally correct and does not accurately represent the compound’s reactivity. Kekulé based his conclusion on experiments such as this. It is possible to replace two neighboring hydrogen atoms in benzene with other atoms, such as chlorine (Cl). Were the classical formula correct, we would expect two different products, one with a C=C bond between the two chlorinebearing carbon atoms (Formula 2a) and one with a C–C bond (2b)5.Experimentally, however, only one product was formed, both in this instance and in all other analogous cases attempted. These observation led Kekulé to the conclusion that, in the real structure, all six C,C bonds must have the same character. A satisfactory theoretical understanding of Kekulé ’s conclusion was, however, possible only after Erich Hückel applied quantum mechanics to bonding theory in the 1930s. It is an eminently important fact that benzene does not have alternating single and double bonds. However, chemists have recently demonstrated that it is possible to force some of its derivatives to adopt that character, provided that some very unusual groups are attached to the benzene ring. ‘Tribicyclo[2.1.1]hexabenzene’ is one such compound, its three C=C bonds fixed in the positions shown in formula 3. It was synthesized in California by Jay S. Siegel and co-workers, and its structure corroborated by X-ray analysis by Hans-Beat Bürgi in Switzerland, both in 1995. Several German dyestuff producers began their activities in the 1860s; a consequence of Perkin’s pioneering work and the high quality of teaching and research in organic chemistry at German universities. The way was open for the planned preparation of synthetic dyes, as well as the artificial production of natural dyes. Success first came in 1868 with Graebe and

H H

H

H

H H 1

Cl Cl

2a Cl Cl

2b 43

Chemistry of Color

Liebermann’s elucidation of the constitution and,immediately afterwards, synthesis of alizarin, the basis of the metal-complex dye Turkey Red. The structural elucidation (Adolf von Baeyer, 1835–1917, Nobel Prize 1905, University of Munich, 1883) and synthesis (Karl Heumann, ETH-Zurich and BASF Ludwigshafen, 1890) of indigo required research work extending over several decades. The largest class of synthetic dyes, the azo dyes, however, can be traced right back to Peter Griess’s discovery of their diazocompound precursors at the University of Marburg in 1858. At the end of the nineteenth century, the dyestuff industry was to be the birthplace of other organic fine-chemical industries, the pharmaceutical industry prominent among them. By the early twentieth century,ca. 80% of all synthetic dyes were produced in Germany. Switzerland accounted for about a further 10%, and various other European countries (Great Britain, France etc.) for the rest. Natural dyes were no longer used by industrial dyers, except to a relatively small degree for Persian carpets and for local use in Africa. As a consequence of the two World Wars, the German proportion of total output decreased, while the Swiss share grew. Dyestuff production also started in the United States, eastern Europe, Japan, and India. Other fareastern countries have joined their number in recent years. Some dyes and pigments, both naturally occurring and industrially manufactured, serve purposes other than coloration of objects. In plants, for example, nature uses the so-called episematic6 colorants to color flowers in order to attract bees and other insects for pollination. The chlorophylls represent a class of natural functional dyes geared to a different purpose. Although responsible for the green color in leaves, they are not there for coloration, but for a photochemical process. The assimilation and photochemical transformation of carbon dioxide produces precursors of the sugar glucose, the crucial intermediate from which more than one million different compounds occurring in living organisms ultimately derive. The pigment mainly responsible for the red color of tomatoes is lycopene, a compound of the carotenoid class, and one of the most powerful antioxidants yet tested. These compounds are able to deactivate free radicals, i.e., very reactive chemical intermediate species which attack the insides of blood vessels in humans and animals. Lycopene and other colored antioxidants hence decrease the risk of cardiovascular disease and several forms of cancer. 44

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Are lycopene and other carotenoids, like the well-known vitamin β-carotene, chemopreventative agents against cancer? Three careful American investigations,published in 19967,found no chemopreventative properties for β -carotene administered in pill form or as a food additive, but only as a constituent of fresh food, such as fruit and salads, or vegetables cooked gently for a short time. In the same investigations, 34,000 older women were interviewed with respect to their eating habits. Analysis of the results found a smaller risk of death from heart disease for women whose diet was rich in vitamin E. Vitamins A and C were not found to influence risk of heart disease8. Neither was any benefit against lung cancer found for β-carotene and vitamin A in male smokers and workers exposed to asbestos. The sensitizing dyes used in silver-salt-based photography offer ideal examples of synthetic functional dyes in an industrial process. The silver salts used in photography only react to light of wavelengths at the lower end of the visible spectrum, like violet and blue. In a paper copy of a black and white negative made with a film with no sensitizer, a red object will be reproduced as black instead of grey. To produce a picture which reflects human (black and white) perception more accurately, it is necessary to extend the sensitivity range of the silver salt over the whole visible spectrum (it can even be extended into the infrared if necessary), and this is achieved through the addition of appropriate sensitizer dyes to the film emulsion.It is also possible to selectively sensitize silver salts for only parts of the visible spectrum; this effect is indispensable for color photography. In a textbook example of the rules of color mixing, the film emulsion is constructed with three silver-salt layers, each with a sensitizer for one of three regions of the visible spectrum. One application of functional colorants, discovered and developed in the last two decades, can be found in laser dyes. Using these, it is possible to construct lasers with almost monochromatic light of any desired wavelength. The information on computer diskettes and compact discs can be engraved using near-infrared laser light, which is absorbed on the disc by colorants with absorption bands in the corresponding region of the spectrum. A related application, this time therapeutic, of functional dyes is photodynamic therapy (PDT),used against certain cancers. It stems from the observation that some near-infrared colorants are preferentially taken up by cancer-damaged cells. Subsequent irradiation of the tissue with near45

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infrared light results in preferential absorption of light energy; the cells become too hot and are consequently destroyed, leaving the neighboring healthy cells intact. The first scientist to observe that cells of living organisms adsorb certain dyes more readily when damaged by bacteria or other causes was Paul Ehrlich (1854–1915). This observation was the stimulus for his search for therapeutic dyes, culminating in the discovery of Salvarsan, the first medicine effective against syphilis. He received the 1908 Nobel prize for medicine for his work.

3.2. Inorganic Pigments As already mentioned, the use of pigments for coloration in art is one of mankind’s oldest cultural activities. The first colorants used in cave paintings were inorganic pigments such as charcoal, iron oxides, and manganese dioxide. The artists of the Stone Age, however, used them without binder.The fact that these paintings are still visible today is thanks to seepage water, which usually contains minerals: these encase the colored pigments in transparent, colorless mineral layers. Most ancient Greek buildings were brightly colored, but their paints were not preserved by mineral layers. Hence, historical monuments like the Acropolis no longer have their original colorful appearance. Industrial production of inorganic pigments began in the nineteenth century. They are the pack horses of the colorant industry, and their production, at over 3.5 million tons annually, is some five times that of organic dyes and pigments. There are, however, only about a hundred chemically distinct inorganic pigments produced today, compared with close to 10,000 organic colorants. The inorganic pigments are unique for a coloristic reason: they are the only class of colorants to include almost white products, and nearly 70% (by weight) of all pigments are white.Virtually all white pigments are one chemical compound: titanium white, or titanium dioxide (TiO2).Although the metal titanium had been known since 1791,titanium-white production started only in the 1920s. Classical natural white pigments were gypsum (calcium sulfate) and chalk (calcium carbonate). White was the color of the first synthetic pigment, white lead, which the ancient Greeks made out of metallic lead and vinegar in an enclosed shed with tanner’s bark. In the enclosed building, a vapor of vinegar, oxygen, and water can form, turn46

Chemistry of Color

ing the lead into basic lead carbonate. White lead was the most important pigment used in painting. In the nineteenth century, the Chinese began production of a zinc-oxide-based white pigment. Why is demand for white pigments so large? Any chemically pure material appears white if it fulfills three criteria. First, it should not absorb any part of the visible spectrum, or else it will appear colored. Second, it should scatter incoming light at its surface as much as possible.The physical cause of scattering is the presence of areas of different refractive index. Liquid water, for example, is essentially uniform in its refractive index and is transparent to visible light,but snow is white,unlike a homogeneous block of ice. Third, a white material has to be physically and chemically very stable in order to keep its whiteness almost infinitely. These three criteria are fulfilled only in some inorganic materials, and so demand for these white pigments is correspondingly great. Light scattering is greatest if the difference between the refractive indices of the material and the surrounding medium is greatest; snow has a much higher refractive index than air, for example. On the other hand, lacquers and plastics used as media have a lower index than the white pigments embedded in them. The scattering effect is also heavily dependent on pigment particle size. In the production of man-made fibers, white pigments are added for an additional reason, as a delustrant. With the exception of carbon black, colored inorganic pigments are all metal oxides and salts. Fig. 3.1 gives a few examples from across the visible spectrum. Iron oxides cover a large range of hues, thanks to differences in oxidation states and the nature of binding to oxygen. Fe2O3, for example, is red, FeOOH (iron oxyhydroxide) yellow, and Fe3O4 (a combination of two- and three-valent iron) black. Important pigments are obtained by forming mixed crystals of chemically similar compounds. Cadmium sulfide, reddish yellow in its pure natural state, provides a good example. Addition of zinc sulfide gives a pure yellow, but cadmium selenide affords a dark bluish red. Mixed crystal pigments can be obtained if colored metal ions are introduced into colorless crystals. Most colored precious stones owe their tints to this phenomenon in a variety of guises. The colors obtained are sometimes unexpected, like the example of ruby, which consists of a small quan47

Chemistry of Color

Fig. 3.1. Some examples of synthetic inorganic pigments

tity of chromium trioxide (Cr2O3) in an aluminum trioxide (Al2O3) matrix. Al2O3 is colorless, while Cr2O3 is green if dissolved in various forms of Al2O3 in any appreciable quantity. It appears red only if present in minute amounts, as is the case in natural ruby. A convincing explanation for this was found by Orgel9 forty years ago: his ligand-field theory, discussed briefly in the last section of this chapter,predicts that the absorption wavelengths of a metal ion will change if the distance between it and its neighbors is perturbed. Orgel calculated that a relatively small decrease in the Cr–O distance should change the color of Cr2O3 from green to red. It is not unreasonable to assume that this is the case in ruby, because the aluminum ion (the Al3+ cation) is significantly smaller than the chromium ion (Cr3+). Hence, the relatively few chromium trioxide molecules can only be accommodated in the Al2O3 matrix by ‘squeezing’ them into it, shrinking the Cr–O distance. Larger percentages of Cr2O3, however, can no longer be constricted by the pressure,and so the mixed crystal reverts to green under these conditions. Even a ruby becomes green if heated, a phenomenon which is not difficult to understand in the light of this. The introduction of colored metal ions into colorless matrices is also used for the production of synthetic pigments. Addition of small amounts of 48

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cobalt, chromium, or nickel to host crystals of spinell, a colorless magnesium aluminum oxide (MgAl2O4), results in brilliant blue to green pigments (see Fig. 3.1). Many natural ores, as well as precious stones, owe their color to effects of this kind. Some of these mineral colors are fluorescent, ruby’s red fluorescence finding application in the ruby laser. Extraterrestrial matter may sometimes contain inorganic pigments not yet found on earth. An example is the green pigment cosmochlor, a sodiumchromium silicate discovered in the Coahuila iron meteorite in the 1970s. The chemistry, technology, and color of inorganic pigments is discussed in a book edited by Buxbaum10.

3.3. Organic Colorants Organic colorants tend to be classified by particular features of their chemical structure. Only a few specific examples of the most significant colorant classes will be mentioned here, just to give the non-chemist an idea of what types of structures are important and characteristic for colorants. Besides the many thousands of natural colorants, over a million different colored compounds have been synthesized, and about ten thousand of these are or have been produced on an industrial scale. Every dye or pigment which has ever been produced for large-scale coloration purposes (coloration of textiles, plastics, paints, and printing inks, for example) is registered and briefly described in a definitive publication known as the Colour Index11. My own book on color chemistry12 is intended for chemists, and also includes material on functional dyes (see Sects. 3.1 and 3.4) and major natural dyes.Only one comprehensive modern book concentrates on natural dyes13. Experience as a chemist has taught me that non-chemists detest all but the simplest chemical formulae. The following discussion, therefore, concentrates on as few classes as possible and on only a few specific examples in each class, even if other compounds in that class may be more important from one or another point of view. We begin with open-chain polyenes. The cause of visible-light absorption in these compounds is simply a sequence of carbon atoms connected to one another by alternating double and single bonds (methine groups; –CH=), or what chemists call a chain of conjugated double bonds. 49

Chemistry of Color

Compound 4 absorbs only ultraviolet light; not visible light. Its number of conjugated double bonds (three) is too small.Compound 5,however,is yellow, has seven double bonds and is a natural dye, called crocetin. One derivative of it is the colored ingredient of saffron, the spice used in risotto Milanese, among other dishes. Crocetin contains seven double bonds which are responsible for its color; the four methyl (CH3) and two carboxylic acid (COOH) groups have practically no influence on that, although they are very necessary for its flavor. A similar polyene, but containing eleven conjugated C=C bonds, is β-carotene, the source of the orange color of carrots, as the name suggests. Its orange hue is a sign that it absorbs light at longer wavelengths than crocetin; the larger the number of conjugated C=C bonds in a compound, the more its coloration is shifted in the sequence yellow → orange → red → violet → blue. This rule, however, applies only to polyene chains without certain groups attached to them. Replacement of the carboxylic-acid groups at either end of the crocetin (5) chain with certain dissimilar nitrogen-containing groups results in a dye absorbing in the infrared, i.e., at wavelengths longer (> 700 nm) than the visible spectrum. In this system, two conjugated C=C bonds are sufficient to give a crocetin-like yellow dye, rather than seven. Dyes like these, however, are not naturally occurring compounds, but synthetic. They are the most important silver-salt sensitizers, used in both color, and black and white photography, as mentioned earlier in this chapter. Benzene’s structure, a cyclic system with three conjugated C=C bonds,has already been discussed in Sect. 3.1. Several other aromatic hydrocarbons consisting of fused rings with conjugated C=C bonds exist: naphthalene (6) and anthracene (7), with two and three rings, respectively, are two examples.

50

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Although they themselves are not or only faintly colored, compounds 6 and 7,together with many other aromatic structures,can be found in derivatized form incorporated into many synthetic colorants.Their color intensities can be increased by substitution of methine groups (–CH=) by other groups, such as carbonyl (C=O) moieties, or by using bridging groups to link two aromatic constituents. An important class of dyes and pigments, all essentially made up of fused six-membered rings with conjugated C=C bonds and two to four C=O groups are the so-called vat-dyes. These are the dyes with the highest light and washing fastness on cotton and other cellulose-based textiles.Violanthrone (8) is a typical reddish-blue dye of that class.

Two classical natural dyes,indigo (9) and Ancient Purple (10), have already been mentioned in Sect. 3.1. Chemically, they are related to the synthetic vat-dyes like violanthrone (8), although they contain only two benzene rings. Their formulae 9 and 10 show that Ancient Purple is a derivative of indigo, in which two hydrogen atoms have been substituted with two bromine atoms. Despite this chemically small difference, indigo (9) is a plant product, yet its brominated cousin is formed only in a small family of sea snails (see Fig. 6.7).

51

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The industrial synthesis of indigo (9) in 1890 was a resounding triumph. Within a relatively short time it had superseded natural indigo, which had been produced mainly in Indian plantations. Development of indigo derivatives in the early twentieth century enabled some technical disadvantages in its dyeing properties to be eliminated, but even so, synthetic dyes like violanthrone (8) and others slowly but steadily usurped its role entirely: by the mid-twentieth century, there was only one large dyestuff manufacturer in the world still producing indigo. In the fifties and sixties, though, it underwent an unexpected revival, thanks to its suitability for dyeing denim. The word ‘denim’ comes from the French ‘de Nimes’ (a town in southern France) and seems to have been in common usage in the San Francisco harbor area in the mid nineteenth century. The words ‘jeans’ and ‘jeannets’ were already common in early nineteenth century England, used for the cotton cloth made there before Manchester corduroy became popular. For reasons of fashion, ring dyeing, the inhomogenous dyeing of the fiber cross-sections, is employed here, as it leads to low abrasion fastness. As briefly mentioned above, one much favored strategy for making colorants is inserting a bridge between two or more aromatic constituents, thus combining the different conjugated aromatic C=C bond systems into one. The most important bridging group consists of two nitrogen atoms joined by a double bond: the so-called azo bridge (–N=N–), key component of the azo dyes and pigments. A little over 50% of all commercial colorants used today belong to this class. Most contain only one azo group (monoazo dyes and pigments), but others incorporate two, three, and four (disazo, triazo and tetrazo compounds).A classic example is Naphthalene Orange G (11), a wool dye in production for over 125 years. 52

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Examining its structure, we can see first of all that the azo group forms a bridge between a benzene and a naphthalene ring system. The hydroxy (OH) group plays a very important role in the efficient absorption of visible light, while the sulfo (SO3H) group confers the right degree of water solubility. Another large and highly diversified class of colorants is made up by the metal-complex dyes and pigments. A large number of heavy-metal elements, the so-called transition metals, have a tendency to form strong bonds to oxygen, nitrogen, and certain other atoms in organic compounds. These bonds are particularly stable if the metal atom can form two of them to suitable atoms in the organic moiety, and, in the process, produce a five- or six-membered ring. The name given to this ring formation with organic compounds is chelation,and the increased stability it confers is very important for natural and synthetic colorants. Striking examples are offered by the class of porphyrin pigments, whose basic structural element is the highly symmetrical compound porphyrin (12): its key feature is the sixteen-membered central ring, incorporating four smaller five-membered nitrogen-containing rings. The two hydrogen atoms bonded to nitrogen atoms in porphyrin derivatives (–NH–) can easily be substituted by metal ions. A magnesium ion, for example, takes their place in chlorophyll a, one of the two green leaf pigments mentioned earlier in this chapter as functional colorants for photosynthesis of organic matter from carbon dioxide (CO2) in plants. This process is the so-called carbon assimilation (better described as carbon dioxide assimilation). The lightfastness of chlorophyll is low, and, during spring and summer, it is continuously resynthesized in leaves. This process ceases in the fall, and leaves turn yellow,brown, or red as the chlorophyll fades away.Trees’ beautiful autumn foliage, of which New England’s so-called Indian Summer offers probably the finest example, are caused by carotenoids, like xanthophyll and β-carotene, of better lightfastness than chlorophyll. While on the subject of plant colors, we should mention another group: the colorants of flowers. These mostly belong to the class of compounds known as anthocyanins, themselves a sub-class of the chemical class of flavonoids.The defining feature of flavonoids is their incorporation of two or more fused six-membered aromatic rings, one of which must contain 53

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a ring oxygen atom. Their bright orange, red, purple, or blue colors make them key elements in the plant-insect interaction leading to pollination. This vital symbiosis is discussed in the context of bees’ color vision in Sect. 5.6. Returning to porphyrins, another example which should on no account be overlooked is that of the iron complex hemin, the coloring agent of hemoglobin in vertebrate blood. Hemin also occurs in some invertebrates, in yeast, and in certain leguminous plants. The complex formulae of chlorophyll a and hemin have not been given as they contain additional chemical groups which distract from the chelate rings’ fundamental characteristics. These are much more readily appreciated in a related synthetic metal-complex pigment, copper phthalocyanine (13).This pigment was discovered purely accidentally in 1927 and then rediscovered in the 1930s, when its extremely high chemical stability and outstandingly brilliant turquoise hue were recognized. As a pigment and, solubilized in various ways, as a dye, it is now by far the most important turquoise colorant. Its impact has even been cultural, as discussed under color naming in Sect. 6.5. The formula of copper phthalocyanine (13) clearly shows the close relationship to that of porphyrin (12): both compounds have sixteen-membered rings with four five-membered rings incorporated into them. The major difference is in the number of nitrogen atoms around the center: four in the case of porphyrin, against phthalocyanine’s eight. Metal ions also form commercially interesting complexes with azo colorants.For this it is necessary that the azo compound possesses hydroxy (OH) groups on both sides of the azo bridge; not just on one side like Naphthalene Orange G (11).Complex formation is schematically represented below. M2+ denotes a metal ion with a double positive charge (a copper dication, for example). The general formula 14 of the azo colorant shows the two hydroxy (OH) groups attached to aromatic (= Aryl) residues, such as benzene or naphthalene. In the process of complexation, the hydrogen atoms in the two OH groups are substituted by the metal ion, which concomitantly forms an additional bond to one of the azo nitrogen atoms (see 15). As mentioned in the preceding section,the largest class (by weight) of inorganic pigments is that of white compounds. White organic colorants, though, do not exist in any proper sense, either as pigments or as dyes. 54

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However, one class of white organic compounds does play an important role in coloration. This is the class of optical brightening agents; compounds which absorb near-ultraviolet light and then release the energy in the visible light range, by fluorescence. They can be applied to textiles, to paper, and to other substrates, causing the surfaces of these substrates to actively emit visible light. Provided no other color is present, this causes the material to look whiter. (Although a white body, by definition, reflects all visible light, undyed textile fibers or paper very often appear slightly yellowish or brownish, as a result of impurities in these materials. This soiling may be removed either chemically, by bleaching with oxidants, or physically, using these fluorescing compounds to compensate for the light absorbed by the soiling impurities.) If colored (dyed) materials are treated with optical brighteners, they usually appear more brilliant. Another type of white compounds which absorb ultraviolet radiation are those used in textile materials for protection against sunburn. These compounds were developed in Australia, a country in which skin cancer caused by sunburn is a major problem. It was found that lightweight clothing (shirts, beachwear etc.) only protects efficiently against sunburn if dyed in dark shades, like black or navy blue. Dark clothes can also be uncomfortably warm, however, as they absorb infrared radiation. The problem was overcome using white or pale-colored textiles impregnated with specially developed white organic compounds absorbing in the ultraviolet. These have been in commercial use since the mid-1990s. Some dyes both absorb and fluoresce. Optical brighteners and fluorescent dyes reveal themselves by their characteristic glow. Optical brighteners appear usually bluish white, depending on the fluorescent agent, when viewed in ultraviolet light in a dimly lit room. 55

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Chemical processes can also be a source of light. Most obviously, organic compounds of all kinds burn in the presence of air. The high temperatures obtained in these processes cause the starting material or the intermediates formed by oxidation to glow.There are some chemical processes, however, in which light is generated even at low temperatures. This phenomenon is known as chemiluminescence,and is observed when an intermediate in a chemical reaction is formed in a sufficiently high energy state. Usually, excited states of this type emit their energy as heat, but some lose it by light emission. Most chemiluminescent reactions involve the decomposition of peroxides, compounds in which a bridge of two oxygen atoms connects two portions, X and Y, of a molecule (i.e., X–O–O–Y). Some animals, like fireflies and certain marine crustaceans such as Cypridina, have the ability to chemiluminesce. Fireflies use their light production capability for courtship display. Technology finds application for chemiluminescence in the Cyalume lightstick, used for air-sea rescue signaling, on aircraft emergency chutes and similar instances where electrical power is unavailable.

3.4. Correlations between Chemical Structure and Color Investigation into the correlation between chemical structure and color began in the early days of dyestuff chemistry. Graebe and Liebermann recognized in 1868 that all dyes contain a system of conjugated double bonds. The first comprehensive theory was proposed on an empirical basis by Otto Nikolaus Witt (1853–1915) in 1876 and later developed on by various chemists. The main physical foundation of the theory was proposed in 1937 by Sklar, who realized that light absorption by colorants has the effect of promoting electrons in the molecules into higher energy levels (called ‘excited states’ by chemists), and that energy of the absorbed light corresponds to the energy difference between the ground state and the first (or higher) excited state(s). In principle, it should be possible to calculate these energy states on a quantum-mechanical basis, as developed from Planck’s and Einstein’s work (see Sect. 2.1) from the 1920s onward. The Einstein-Bohr frequency condition states that the energy difference ∆E between the ground state and a particular excited state is directly proportional to the observed frequency v, and hence inversely proportional 56

Chemistry of Color

to the wavelength λ of the absorbed light:

∆E = hv = hc/λ (h = Planck’s constant, c = speed of light) An important outcome of early quantum mechanics in the late 1920s was a great improvement in scientists’understanding of the energy states, locations, and functions of electrons in atoms and simple molecules. In earlier models,it had been assumed that electrons orbited around atomic nuclei in fixed paths, like planets around the sun. Now, as a consequence of Heisenberg’s uncertainty principle (see Sect. 2.1), it was no longer acceptable to assume that an electron’s position and velocity could be defined so precisely, and so the simple particle came instead to be viewed as a smeared-out distribution of negative charge, its density varying from place to place. An electron is no longer said to be in an orbit, but to be distributed in an orbital,a given volume in which ‘most’(arbitrarily defined as over 95%) of the electron density is calculated to reside. Only the s orbitals of an atom (those of lowest energy) are spherical in shape; all the higher orbitals (p, d, f…) have more complex geometries. It is possible to explain bond formation between atoms on this basis. The purely ionic bond in simple salts like sodium chloride was already known: it is simple electrostatic attraction between the positively charged sodium cation (Na+) and the negative chloride anion (Cl–). The simplest case of a covalent bond is found in the hydrogen molecule H2: two hydrogen atoms bound together. H–H was a mystery, however, before the quantummechanical approach was developed. The molecular-orbital (MO) concept shows how a covalent chemical bond can result from overlapping of the outer orbitals on different atoms, so as to concentrate electron density between the atom cores. A so-called s bond is formed. At a higher energy level, a antibonding (unoccupied) MO is formed concomitantly; this, when occupied, corresponds to the excited state. The energy difference between the occupied and the unoccupied MOs corresponds to ∆E in the Einstein-Bohr equation. Much more complex than bonding of two hydrogen atoms are the bonds in compounds containing elements from the second and higher rows of the Periodic Table. In atoms of the second-row elements, like carbon, oxygen, and nitrogen, the outer electrons reside in atomic p orbitals. These can be combined with one another and with the s orbitals. The method of combination, called hybridization, determines the geometry of the molecule: in sp3 hybridization, an s orbital is combined with three p orbitals, 57

Chemistry of Color

giving rise to a tetrahedral molecule,like methane (CH4), in which the carbon atom is at the center of a tetrahedron bonded by s bonds to the four hydrogen atoms at the four apexes. In sp2 hybridization, only two p orbitals are involved, together with the s orbital. The third p orbital takes no part in hybridization, but overlaps with a p orbital of the neighboring atom to form a π (molecular) orbital. In a classical chemical formula, the electron pair in the π orbitals forms the ‘extra’ bond in a ‘double bond’, as shown in ethene (16; also known as ethylene). Ethene is not tetrahedral,but flat.All six atoms are in the same plane,while the two π electrons (those of the double bond) occupy orbital lobes above and below that plane (cf. 17). The black and white parts (which represent the sign of the function and describe the electron distribution) indicate whether the atomic p orbitals form a bonding or an antibonding π orbital. In 17, the antibonding situation is shown (dashed lines). The first MO calculations of the ground and excited states of all these bonds were accomplished in the 1930s and 1940s. They predicted that the energy difference ∆E for π bonds is much smaller than that for s bonds, and that it is smaller for conjugated double-bond systems than for systems with an equivalent number of isolated double bonds. This is consistent with observations, as discussed in the context of the structure of organic colorants and of Witt’s empirical theory (see above). Agreement between these calculations and experimental results was only qualitative, not quantitative, however. Better agreement was not possible at that time, as highly sophisticated computational methods were not yet available. In the late 1940s, Hans Kuhn introduced the free-electron model (FE)14. This model considers the wave functions of molecular π electrons to be analogous to waves in a ‘box’, whose size corresponds to the shape of the dye particle. At its simplest, the box may be regarded as one-dimensional, corresponding roughly in length to, for example, a polyene chain as depicted in formulae 4 and 5 above. This simplification, although highly artificial, does make theoretical calculation of the energies of an electron in this situation much less complex.Wave mechanics and the quantization of energy states require that only a whole number of electron wavelengths for different electron-energy states can fit into the box exactly. In other words, an electron can pass from one energy state to another only if an exactly defined quantum of energy is added to or removed from the box. These stationary states correspond to standing waves,the length of the box (L) in which the electron is free to move being equivalent to an integral 58

Chemistry of Color

multiple (n = 1, 2 , 3…) of the half wavelength l (L = nl/2). The energy levels en corresponding to these waves are occupied,each by 2 π electrons,from n=1 onwards.They correspond to molecular orbitals. The energy difference between the highest occupied molecular orbital (HOMO) and the unoccupied one above it (the lowest unoccupied molecular orbital, or LUMO) corresponds to ∆E in the Einstein-Bohr frequency condition, discussed above. The wavelength of light absorbed may, therefore, easily be calculated. Fig. 3.2,a, shows the energy levels en = 1–5, with their associated waves, for the cationic dye (CH3)2N–CH=CH–CH=N+(CH3)2. A member of the group of functional dyes used as silver-salt sensitizers in photography, it is related to the polyene dyes. Fig. 3.2,b, shows the same five energy levels as they are represented in Hückel molecular orbital theory (HMO) by combination (overlapping) of atomic orbitals. To obtain reliable predictions or corroborations of experimental data for structurally more complex π electron systems, it is necessary to replace the simple FE model with two- and three-dimensional FE methods. The FE model has the enduring advantage of being conceptually the clearest method for teaching purposes15. For more complicated interactions involving several electrons,more sophisticated methods are needed.These have become available over the last three decades with the development of faster, higher-capacity computers. John A. Pople received the 1998 Nobel Prize in Chemistry for the sophisticated approximation methods he developed for calculation of energy levels and other properties of organic molecules. Chemists’ knowledge of the color of inorganic compounds has developed in a different way from that of organic colorants. Since the dawn of modern chemistry in the late eighteenth century, it had been known that most inorganic compounds are colorless, with the marked exception of certain metal salts. The color intensities of various salts of the same metal, however, vary dramatically. Anhydrous copper sulfate, for example, is practically colorless, and, in aqueous solution, it is only moderately bluish. The solution becomes a very intense blue, though, if ammonia or amines are added.In the nineteenth century it was assumed that this intense blue color was merely the product of some kind of association, rather than of the formation of a chemical bond. It was possible to characterize such complexes, and even to determine their configuration (Alfred Werner, 1866–1919; Nobel Prize 1913). Cases such as cobalt chloride’s complexes with ammonia were still not understood: in a fresh solution of CoCl3·6 NH3, all three 59

Chemistry of Color a)

b)

+

Fig. 3.2. Five π molecular orbitals of the cationic dye (CH3)2N–CH=CH–CH=N (CH3)2 in the FE model (a) and in the HMO theory (b) (reproduced by permission of the Editor of Chemie in unserer Zeit, Wiley-VCH Verlag GmbH, Weinheim)

60

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chlorines can be precipitated out as chloride anions by adding silver salts16. In CoCl3·5 NH3, however, only two of the three chlorines can be removed from cobalt in the same way. Today we call these elements transition metals. Their defining characteristic is that their different electron shells do not fill up in the way which would be expected from their position in the Periodic Table. Unlike the socalled main-group elements, the transition elements can accommodate electrons in their outer shells before all their inner shells are completely filled. This is the root cause of observations like the inertness of the third chlorine in CoCl3·5 NH3. This conclusion was drawn in the early days of quantum chemistry, first in valence-bond theory (Linus Pauling, 1901–1993; Nobel Prizes for Chemistry 1954 and for Peace 1962),then in the crystal-field theory and finally in ligandfield theory. It can be shown that the contribution of ligand molecules and ions, like the ammonia in the cobalt complexes mentioned above, is not, as originally assumed, confined to one occupied molecular orbital overlapping with one empty inner orbital of the transition element. The ligand’s electrical field affects these empty orbitals much more profoundly, and causes them to differentiate further, into sub-groups,hence altering the separations between different energy levels. The degree of splitting induced depends on the type of ligand, increasing in magnitude from chloride ion to hydroxyl ion, water, ammonia, and cyanide ion. The oxidation level of the metal is also crucial, as increasing positive charge results in increased Coulomb attraction between cation and anion.These interactions influence several physical and chemical properties of these complexes, such as their magnetic properties, chemical stability and absorption spectra, or color. The series of copper complexes [Cu(H2O)6]2+ to [Cu(NH3)4(H2O)2]2+ provides a good example of the effect of changing the type of ligand. Substitution of one to four of the six water molecules in [Cu(H2O)6]2+ with ammonia molecules shifts the position of the absorption band from 770 nm (a very weak, hardly visible blue) to 588 nm with a six-times stronger absorption. Another example is the green to red color change arising from compression of the chromium-oxygen separation in ruby, mentioned earlier in this chapter. In conclusion, we may say that the increasing refinement of these theoretical approaches to explaining molecular orbitals,although highly complex in nature, is leading us to deeper and deeper understanding of the bonding in metal complexes. 61

Chemistry of Color

References and Notes 1. H.-D.Martin,‘The Function of Natural Colorants: The Biochromes’,Chimia 1995, 49, 48–68. 2. W. Herbst, K. Hunger, Industrial Organic Pigments, VCH, Weinheim and New York, 1992. 3. F. Brunello, The Art of Dyeing in the History of Mankind, Neri Pozza, Vicenza, 1973; Italian edn. 1968. 4. A. S. Travis, The Rainbow Makers. The Origin of the Synthetic Dyestuff Industries in Western Europe, Lehigh University Press, Bethlehem PA, and Associated University Presses, London, 1992. 5. In benzene and related structural formulae, hydrogen atoms bonded to carbon are normally not shown. 6. Episematic: from sema, sign. 7. C. H. Hennekens et al., G. S. Omenn et al., L. H. Kushi et al.‘Long-term Supplementation with b-Carotene (etc.)’, New England Journal of Medicine, 1996, 334, 1145–1162. 8. The results of this investigation with vitamins A, C, and E are mentioned here only because they were made in the context with those of b-carotene.We emphasize, however, that vitamines C and E are not colorants. 9. L. E. Orgel,‘Ion Compression and the Color of Ruby’, Nature 1957, 179, 1348. 10. G. Buxbaum (Ed.) Industrial Inorganic Pigments, 2nd edn., Wiley-VCH, Weinheim, 1998. 11. Colour Index, third edn. (4 vols.) with supplements (5 vols.) and a volume on pigments and solvent dyes. Soc. Dyers Colourists, Bradford, and Amer. Assoc. Text. Chem. Colorists, Research Triangle Park, NC, 1971–93. 12. H. Zollinger, Color Chemistry. Syntheses, Properties and Applications of Organic Dyes and Pigments. 2nd edn.,VCH Publishers, Weinheim and New York, 1991. 13. H. Schweppe, Handbuch der Naturfarbstoffe. Ecomed Verlag, Landsberg/Lech Germany, 1993. 14. H. Kuhn, ‘Elektronengasmodell zur quantitativen Deutung der Lichtabsorption von organischen Farbstoffen’, Helv. Chim. Acta 1948, 31, 1441–1455. 15. H. D. Försterling, H. Kuhn, Principles of Physical Chemistry, Wiley, New York, 1999. 16. AgCl is formed. It is practically insoluble in water.

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4. Colorimetry 4.1. Color Measurement ‘Colorimetry’ has various meanings in different activities in science. In astronomy, for example, before (direct) spectral photometry became available, it used to refer to the determination of stars’ temperatures by comparison with artificially colored light sources. In analytical chemistry today, it still means the determination of concentrations in colored solutions. ‘Colorimetry’, however, is also a term used for the quantitative description of colors as they appear to the human eye, combining color physics (spectra) and neurobiological processes in the eye and brain. Although colors are sensory impressions in the mind, it is necessary to be able to describe them quantitatively, not only for scientific investigation and the application of dyes and pigments, but also for understanding complex psychophysiological color phenomena, as discussed in Sect. 4.2 and, in greater depth, in Chapts. 5–7. In colorimetry it is not sufficient to measure the transmission spectra of dyes dissolved in solvents or in transparent films. Of greater importance are the reflection spectra of materials colored with dyes or pigments. Only in the twentieth century was it fully appreciated that there are three fundamental ways in which colors may be described quantitatively: 1) Purely physical representation of spectra, from plotting the transmission of dyestuff solutions, the emission of light sources, or the reflectance of a dye on a substrate as a function of wavelength (or frequency). This method does not take into account any color-vision-dependent factors. 2) Systems based on the responses stimulated in the human eye by visible light of various wavelengths and intensities. The CIE (Commission Internationale de l’Eclairage) system is the most widely adopted. It is based on the fact that light reflected from any colored surface can be visually matched by a mixture of red, green, and blue light in suitable proportions. The description of colors as color stimuli in the human eye, therefore, becomes a three-dimensional problem (see below). 3) Systems based on the measurement of sensations in color vision. These are dependent on three basically physiological parameters: brightness, 63 Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

Colorimetry

hue, and saturation (i.e., saturation with respect to monochromatic light or ‘pure’ hues). A three-dimensional ‘color space’ is the result. However, the parameters are chosen in such a way that pairs of equidistant points in that solid correspond to the same perceptual differences of brightness, hue, and saturation. We will first discuss emission, absorption, and reflection in terms of simple physical concepts. If we see a natural or artificial light source which does not have a very similar distribution of visible-range rays to that of the sun, it appears colored to us (do not forget Newton’s remark, quoted in Sect. 1.2, that ‘…rays […] are not coloured’, however). Light consisting of rays in the range 400–450 nm looks violet to us, 450–500 nm looks blue, 500–540 nm green, 540–590 nm yellow, 590–650 nm orange, and 650–700 nm red (see also Fig. 2.3). This is the situation for seeing light emitted from a light source. It is different when we see a colored object. A dyer or a painter knows that he or she adds dyes or a pigment to an object. From a physicist’s point of view, however, the effect of coloration is based on taking away color, removing one or more of the colors of white light. A red car withholds green light, absorbing light of wavelength 490–550 nm, so that the reflected light looks red to us. Similarly, absorption bands at 400–430 nm, 430–490 nm, 550–600 nm and 600–700 nm produce the appearance of yellow, orange, violet, and blue, respectively (Fig. 4.1). Solids which look green are characterized by two absorption maxima, at 400–450 nm and at 580–700 nm. Qualitative inspection is not able to determine whether a green color originates from one dye with two absorption maxima, or from a mixture of a blue and a yellow dye. If sunlight or equivalent artificial light encounters a solid which reflects all visible light in a diffuse manner and with complete reflectance, it appears white to the human eye. On the other hand, if the solid absorbs all light, we see it as black. If it absorbs a constant proportion of light over the whole range between 400 and 700 nm it appears gray. White, gray, and black are called achromatic colors and are characterized by a constant level of absorption in the 400 to 700 nm range (Fig. 4.1), in contrast to the chromatic colors discussed above. The application of two or more dyes with different absorption spectra to achieve a particular hue always leads to less brilliant shades than those obtained by applying a single dye of the required color. Such a combina64

Colorimetry

100%

Absorption

black

gray

white

0% 400

500

600

700

Wavelength Fig. 4.1. Schematic representation of the light absorption of colored solids

tion is called a subtractive mixture (Fig. 4.2). In extreme cases, black is obtained as the mixture, the sum of all dyes applied ‘subtracts’ all parts of the visible spectrum. Analogously, the color of a solution of colored compounds in a liquid can be explained in terms of absorbed and transmitted (rather than reflected) light. The subtractive process outlined above may be contrasted with the superposition of light from different sources. When light from a number of sources is combined in such a way that the sum of their contributions matches the relative intensities of the visible spectrum of sunlight, the impression obtained is of colorless (‘white’) light. Such a combination is called an additive mixture of colors (Fig. 4.3). Additive mixture is used in color television, and can be demonstrated in an impressive way.If the (switched on) television screen is examined under a powerful magnifying glass, it can be seen that the colorless plane consists of a regular sequence of red,green,and blue rectangles. Additive mixing of these gives the impression of whiteness when viewed from the proper distance. By varying the light intensity of each rectangle, it is possible 65

Colorimetry

to obtain practically all hues. The light sources are so-called phosphors, coated onto the inner surface of the glass screen, which become luminescent when bombarded with a beam of electrons. These phosphors are not derivatives of the element phosphorus, but inorganic compounds doped with metal ions. Red-light-emitting rectangles, for example, contain yttrium oxysulfide (Y2O2S) doped with europium ions (Eu3+), while the greenand blue-light-emitting rectangles are both zinc-sulfide-based, doped with copper (Cu+) and silver (Ag+) ions, respectively. For color perception in the human eye, it is not only the position of the wavelength of maximum absorption which is important,but also the shape of the band. The smaller its width and the steeper its slopes, the purer and more brilliant appears the resulting hue. Fig. 4.2. Subtractive mixture of three paint colors1

Fig. 4.3. Additive mixtures of three light beams1

Contrary to popular belief, qualitative inspection of physical spectra does not always correctly predict the color of a colored solution or solid. The sensitivity of the human eye varies over the visible spectrum, and the shape of the absorption band turns out to be every bit as important as the location of its maximum. The stimuli-based and sensation-based colorimetric systems take this sensitivity of the human eye into account. As mentioned, it varies over the whole visible spectral region, exhibiting a pronounced maximum at 555 nm and decreasing to zero at 400 and 700 nm. Therefore, a dye with an absorption maximum at, say, 500 nm, but with an extended slope on the long-wavelength side, does not appear reddish-orange, as the maximum would suggest, but a dull bordeaux red, as the slope extends into the eye’s highest sensitivity region (555 nm). In isolation, absorption at 555 nm would appear as bluish red, but it is dulled because of the simultaneous sensation in the reddish-orange region. It would be tempting to surmise that visual sense is being overruled by a fastidious palate here, preferring a good claret to an orange juice. It would be wrong to conclude this, however. Moreover, bodies with different reflection spectra may appear the same color under particular lighting conditions, but then change in color if the spectral composition of the illuminating light is altered.This effect is called metamerism (and the colors metameric). Metamerism plays an important part in applications of dyes, in cases such as the so-called evening colors: metameric dyeings which are visually identical in daylight, but which exhibit distinct color differences when lit using a light source of different spectral composition, like filament lamps. The opposite phenomenon, of

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objects retaining their color under illumination with a wide variety of different light sources, is also known and is called color constancy. Several astonishing experimental demonstrations of color constancy effects have been devised by Edwin Land and are discussed in Sect. 5.5. The International Commission on Illumination established the CIE system for describing colors in 1931. It is based on three primary colors (red, green, and blue). Spectral response curves derived from the averaged results of color-matching experiments conducted on a number of subjects with normal color vision were used to describe the matched colors. Caution was necessary because the color sensitivity of the eye varies from one individual to another (people who are partially or completely ‘color blind’ are, of course, not included in this discussion). The CIE standard observer response curves X (red),Y (green), and Z (blue), give the amounts of the red, green, and blue primaries required, respectively. In addition,Y is arranged to correspond exactly to the average luminous curve for an average eye, and so is a direct measure of luminosity. Graphic presentation of tristimulus values would require a three-dimensional coordinate system, and so new quantities, the chromaticity coordinates x, y, and z, are derived, using a simple transformation. Since x + y + z = 1, x and y alone are sufficient to describe a color, if luminosity is disregarded. The CIE chromaticity diagram, with x as abscissa and y as ordinate, is shown in Fig. 4.4. The colors of the spectrum lie on an almost parabolic curve, the ends of which are connected by a straight line representing purples; they are colors which do not occur in the optical spectrum. Introduction of the luminosity coordinate perpendicular to the diagram leads to the CIE color solid. The achromatic colors, white, gray, and black, lie on a line perpendicular to the base, near the centre of the solid. Using the two-dimensional CIE diagram, it is possible to determine two other characteristics of a color: the dominant wavelength ( lD) and purity (pa = a/b) (Fig. 4.4). The dominant wavelength is given by the point at which a straight line drawn from the neutral point (W) through the color point (F) intersects the spectral color curve. The nearer the color point lies to the spectral color curve, the greater the purity. Extrapolation of the straight line in the opposite direction gives the dominant wavelength of the complementary color (C). In luminosity, dominant wavelength, and purity we have three physically measurable quantities which correspond 67

Colorimetry

Fig. 4.4. CIE Chromaticity diagram. W = Neutral point (white), F = color point of the dominant wavelength λD, C = complementary color1.

68

Colorimetry Table 4.1. Luminosity (Y), Dominant Wavelength (λD in nm), and Purity (pa in %) of Some Colors Color

Y [%]

λD

pa [%]

Blue Slate gray Navy blue Golden yellow Cream Olive green

32 12 3 45 55 14

476 476 475 576 575 572

27 10 20 70 25 45

roughly to the subjective properties commonly used to describe color: brightness, hue, and saturation. The examples of Table 4.1 show, for two groups of colors, how visual color impression is influenced by luminosity and purity, despite the approximately equal dominant wavelengths. In contrast to previous attempts to specify colors, the CIE system takes into account the differing sensitivity of the eye to different parts of the spectrum. That the system is, however, based on the measurement of stimuli, and not perception, is shown by the fact that equal visual differences between pairs of colors in regions of different hue do not correspond to equal distances in the CIE diagram. This can be recognized by simple visual inspection of the colored version of the CIE diagram: the hues in its green and yellow regions change much less than those between yellow, orange, and red. Although the CIE system can provide unambiguous descriptions of colors, it does not suffice to define tolerances among specimens differing in hue.This problem can be solved only with a system based on measurement of sensations. The most rigorous investigations carried out concerning the problem of uniform color spacing were based on the color system developed from 1905–1910 by the American painter Albert H. Munsell, and later modified by various workers. Munsell classified a large number of samples of colored chips by hue, saturation,and brightness,spacing them in what he judged to be almost equidistant steps. He obtained an onion-shaped solid of irregular outline. The Munsell solid is represented by a series of charts, each corresponding to a particular hue: for example, yellow, yellow-red, red. The charts are arranged radially within a cylinder, with equiangular spacing (visually equal steps), the inner steps all lying on the cylinder axis, which represents 69

Colorimetry

achromatic colors ranging from black through gray to white.Colored chips are arranged upon each chart such that all the chips in a particular horizontal row are of equal lightness (Munsell term ‘value’), but vary in degree of saturation (Munsell term ‘chroma’) in equal visual steps with the least saturated colors on the inner edge. Corresponding sample rows on different charts also have the same lightness (value). The chips are arranged so that those equidistant from the axis (i.e., in corresponding vertical columns) have the same chroma, but vary in lightness from very dark (black at the bottom of the axis in the neutral column) to very light (white at the top of the axis). The number of chips in the columns, of course, decreases with increasing saturation, and the number of rows varies with lightness. The position of the row containing most chips (i.e., including the chip of highest chroma) varies with color, being at a high value for a ‘light’ color such as yellow and at a low value for a ‘dark’ color such as blue. Although this first perception-based colorimetric system was developed by an artist and before the First World War, it much later became very important for various coloration industries (textiles, paper, leather, plastics, and color printing among them) to have a practical color measurement system suitable for computer control of procedures such as dyeing and color matching. From the 1940s onward,various quantitative relationships were derived between the Munsell parameters value, hue and chroma and the CIE tristimuli. Finally, in 1976, the CIE recommended the socalled LAB system as standard. L, A, and B refer to the three axes of the system: a lightness axis (L) and two axes representing both hue and chroma, one red-green (A) and the other blue-yellow (B). This system has the advantage that color differences between two samples or two recipes can be determined with the aid of relatively simple computer programs. More recently, the Optical Society of America has developed the OSA Uniform Color Scale samples,which,it is claimed,are perceived as more equally spaced than the set of Munsell samples2. Since about ten years ago,these computer-based color measurements have completely replaced the classical colorist, who followed each dyeing lot visually, manually fine-tuning the shade by careful addition of small amounts of appropriate dyes until the match with the master sample was perfect.There are several modern treatises on color measurement,emphasizing the physical and technological aspects3–5. Tempora mutantur, nos et mutamur in illis! (The times change, and we with them). 70

Colorimetry

4.2. Color: Harmony or Contrasts? The title of this section may look rather odd in a chapter on colorimetry. In the preceding section, present-day systems of describing color were introduced with no reference to historical background. Modern colorimetry clearly demonstrates that color, as perceived by the human eye and brain, is not a simple physical phenomenon based on one, two, or three parameters. The border with non-physical aspects is fluid. It is, therefore, appropriate, as a bridge to color vision and color in culture, to briefly evaluate colorimetry’s historical development directly after discussion of the modern CIE system and its relatives. Since the early seventeenth century, some thirty or more graphical color systems have been proposed, both by scientists and artists6. It is remarkable that most of them are highly symmetrical; they are circles, isosceles triangles, hexagons, spheres, pyramids, cones, and double cones. The oldest is the color sphere of the Swedish theologian Aron Sigfrid Forsius (1611; Fig. 4.5). Newton, in the context of his work on the visible spectrum obtained by refraction of sunlight by a glass prism, proposed a color circle in 1704 (Fig. 4.6). It is characterized by several important features: 1) Newton distinguishes seven colors: yellow, orange, red, violet, indigo, blue, and green. They are given different weightings, and this segmentation, therefore, destroys the symmetry within the circle. Newton was

Fig. 4.5. Color sphere of Aron Sigfrid Forsius (1611)

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Colorimetry

inspired to this seven-step arrangement by analogy with the musical scale, a correlation originally proposed by René Descartes (1650)7. In the manuscript for his Cambridge lectures on color (1669), Newton mentioned that the close relationship between the two ends of the color spectrum corresponded to the two ends of the octave in music. Today we know that this apparent correlation has no physical basis: in any (natural) musical octave, the frequency and wavelength change by a factor of exactly two, whereas no such simple numerical relationship exists either for frequency or for wavelength in the case of visible light. Despite this remark in his 1669 lecture notes,and at least one paragraph in a manuscript intended for Opticks, neither of Newton’s two publications on color (the Transactions paper of 1672 and the 1704 book Opticks; see Refs. 8 and 9 in Chapt. 2) make any mention either of color harmony or correlations with the musical scale. It seems he had finally abandoned these ideas. 2) Newton mentioned indigo as a color8. The distinction between indigo and blue is not immediately evident. Dyeings using natural indigo have a mid-blue hue; they are not, however, brilliant, but relatively dull. Importation of indigo from India began in the late seventeenth century, and it seems that Newton was fascinated by it. The Indian product from Indigofera tinctoria gave a blue color on cotton which was a little

Fig. 4.6. Newton’s color circle (1704)

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Colorimetry

more brilliant and, therefore, more attractive than that from European woad (Isatis tinctoria). There is, however, little reward in speculating too much about certain of Newton’s color terms: some of them he used inconsistently, and it is likely that he did not ultimately consider these questions important. This is also evident from a hand-written note stating that the color terms used in the 1672 paper were coined by an assistant. 3) When Newton described his color circle (Fig. 4.6) in Opticks, he wrote: ‘….they are all the Colours of uncompounded light9 gradually passing into one another, as they do when made by Prisms; the circumference DEFGABCD, representing the whole Series of Colours from one end of the Sun’s colour’d image to the other’. In other words, he drew the two extremes of the color spectrum together to form a circle, leaving no room for the hues between red and violet, i.e., purple10, which do not appear in the prismatic spectrum.He did discuss purple in Opticks, however:‘Lastly, if red and violet be mingled, there will be generated according to their various proportions various Purples, such as are not like in appearance to the Colours of any homogenial Light…’. Therefore, if all visual colors are to represented,it is necessary to add space between violet and red for purple hues.The CIE chromaticity diagram (Fig. 4.4) provides for this with its so-called purple line,running between the corners labeled 400 and 700 nm and containing the hues obtained from mixing light of wavelengths 400 and 700 nm in various proportions.The almost parabolic curve corresponds to the colors produced by a prism. Important problems common to colorimetry, color vision, and art are the so-called primary colors and complementary colors. There is unfortunately no unambiguous definition of a primary color. Robert Boyle (1627–1691) wrote in 1664: ‘…is of advantage to the contemplative naturalist, to know how many and which Colours are Primitive and Simple, because it both eases his Labour by confining his most solicitous Enquiry to a small number of Colours upon which the rest depend, and assists him to judge of the nature of particular compounded Colours…’. The German engraver Jakob Christof Le Blon, who discovered the fourcolor (black, yellow, magenta, cyan) printing method in about 1710, defined primary colors as those hues which cannot be obtained by mixing. Later, the question of whether green is also primary was repeatedly discussed, as a pure green is always perceived as without any resemblance either to yellow or to blue. Some authors, therefore, use ‘pure’ for the four, and ‘primary’ for Le Blon’s three chromatic colors. For additive mixtures, however, the term ‘primary colors’ is also used for red, green, and blue. 73

Colorimetry

a)

It is likely that Renaissance and Baroque artists already had at least an intuitive knowledge of complementary colors11. Newton, it seems, was less interested in these color pairings. They had, however, aroused interest since the eighteenth century in the study of so-called negative after-images. If an observer stares fixedly for some time at a colored disc on a black plane, and then replaces the disc by a white sheet of paper, an after-image persists of the disc in the corresponding complementary color.After looking at a red disc, for example, the after-image is a bluish green. The effect is nicely described by Goethe in Zur Farbenlehre (see Ref. 60 in Chapt. 7): ‘Als ich gegen Abend in ein Wirtshaus eintrat und ein wohlgewachsenes Mädchen mit blendend weissem Gesicht, schwarzen Haaren und einem scharlachroten Mieder zu mir ins Zimmer trat, blickte ich sie, die in einiger Entfernung vor mir stand, in der Halbdämmerung scharf an. Indem sie sich nun darauf hinwegbewegte, sah ich auf der mir entgegenstehenden weissen Wand ein schwarzes Gesicht, mit einem hellen Schein umgeben, und die übrige Bekleidung der völlig deutlichen Figur erschien von einem schönen Meergrün’12. The after-image Goethedescribed is called successive contrasteffect.He made detailed study both of this and of simultaneous contrast, observed primarily in shadowed areas when sunlight is especially brilliant (see Sect. 7.6). Simultaneous contrast was important for Impressionist painters and is discussed in the context of Monet’s painting of his step-daughter (Fig. 7.10).

b)

Fig. 4.7. Color circles a) of Harris (1766) and b) of Biernson (1962)

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An appraisal of primary colors was included by Moses Harris in a color circle published in his The Natural System of Colours (1766) and by several authors subsequently. Fig. 4.7,a, demonstrates the principles of the Harris circle, which is based on subtractive mixing of pigments. The complementary color pairs in Fig. 4.7,a, are those which, when combined, yield achromatic grays13. However, results are different if additive mixtures of pairs of approximately monochromatic lights are tested (Fig. 4.7,b, from Biernson8). Here, yellow and blue are in opposite positions on the circle, whereas in the Harris circle it was yellow and violet. Red and green are no longer in direct opposition. Simple comparison of these two circles, however, is of dubious worth. This is easy to appreciate if it is borne in mind that a subtractive mixture will produce an increasingly darker gray on increasing the concentrations of the two colors at constant ratio, whereas increasing the intensities of monochromatic light sources in additive mixing should always give white. Furthermore, it is practically impossible to find colorants with absorption and reflection spectra with sharp cut-off points in their spectral curves (i.e., rectangular absorption and reflection spectra).

Colorimetry

Moreover, it must be emphasized that, for the two circles of Fig. 4.7, it is not known exactly which particular shades the authors had in mind. Even given this information though, it might be questionable whether somebody interested in complementary colors would necessarily concur with the choices made by the authors of these two representations. Red, yellow, green, and blue all have so-called unique hues14: those entirely free of any tinge of neighboring colors. Unique green, for example, is neither yellowish nor bluish. Colors intermediate between two primary colors do not have unique hues: we always recognize that orange, for example, is a mixture of yellow and red. These (four, by this definition) primary colors share a further characteristic: we never have the feeling that one of these colors is mixed with its opposite color. Experience tells us that we never saw a greenish red or a yellowish blue15. The unique hues seem reasonable candidates for drawing comparisons between the two color circles of Fig. 4.7. The dominant wavelengths of the unique hues are known. Fig. 4.8 shows them in the CIE diagram. It also includes straight lines between red and green, and between yellow and blue16. The positions of yellow and blue (dominant wavelengths lD =583 and 447 nm, respectively) are on an almost perfect straight line through the achromatic center. However, this is not the case for red and green (lD = 698 C and 515 nm, respectively). Unique red and unique green, therefore, are not complementary colors according to the definition given above, since they do not add to give white in any ratio of unique red and unique green. Neither are they compensatory colors, which would add to give neutral gray. Fig. 4.8, however, indicates that the complementary color of unique green is purple; a conclusion which can indeed be found in corresponding literature written by artists. Some scientists, though, also emphasize the position of purple as the complementary color of green. Konrad Lorenz (1903–1989, Nobel Prize for Medicine 1973 for his work in behavioral science), for example, wrote in 1963 in a discussion of the evolution of visual contrast phenomena that they are an ingenious discovery of nature that certain pairs of spectral colors are perceived as white17. The physical basis of these colors, however, is not in any way circular, but is a (one-dimensional) variation in wavelength, and so there is no fundamental reason to assume the existence of a complementary partner to the middle part of the spectrum (green). The non-spectral color purple, a combination of the red and violet ends, conveniently fills the gap and enables the spectral ribbon to be closed into a circle. Therefore, Fig. 4.7,b, is the better representation. It could be further improved by a slight displacement of the posi75

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tion of purple, so as to give a straight line from it, through the center, to green. Important contributions to our understanding of complementary colors were made by French chemist Michel Eugène Chevreul (1786–1889), who in the 1820s was given the task of improving the brilliance of dyes used in the Manufacture Royale des Gobelins.His conclusion was that complementary combinations give optimum harmony of contrast. Possibly, Chevreul was the scientist whose work was best known among artists. His book De la loi du contrast simultané des coleurs (1854) was published in several editions and translated into English and German. Next best known is probably Wilhelm Ostwald (1853–1932), winner of the 1909 Nobel Prize for his work in physical chemistry.He was also very interested in color theory and in 1920 developed a three-dimensional color space, consisting of a double cone with the achromatic colors in the axis and the chromatic colors along the circumference. Ostwald himself considered his contribution to color science his most important work18. Another three-dimensional color space,this time spherical,had already been proposed by Philipp Otto Runge in 1809, however. In its original form, the Harris circle also includes a third

Fig. 4.8. Positions of unique red, green, yellow, and blue in the CIE diagram (red – on the purple line – is given by the ‘theoretical’ complementary wavelength, indicated by C )

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dimension, with outer circles representing increasing hue purity, in addition to the simplified representation of Fig. 4.7,a. In the twentieth century, the scientific approach to colorimetry has been dominated by the development of color representations based on quantitative data from measurements of eye sensations. The CIE and LAB diagrams were the result. Independently of the activities of the CIE, artists have remained active in color theory, albeit on a smaller scale since the mid-nineteenth century. Their theories, not so far removed from the subject of this section, are discussed in detail in Sects. 7.2 and 7.3. In summary, we may note that the search for regularities in color combinations, with the aim of rationalizing their esthetic value and position, dates back at least to Aristotle. Today, over two millennia later, we find an abundance of schematic representations of color in circles, spheres and double cones. These serve as exemplary allegories for the tendency of Western culture to try to force colors into some kind of circular system, perhaps because a circle is related to principles of color harmony.In nature, though, there is nothing circular about the sequence of spectral colors; it is a simple, linear function of wavelength. Scientific investigation has demonstrated that, for formal systematization of all visible colors, at least three dimensions are necessary.These three dimensions, however, are still insufficient for any truly comprehensive appreciation of color. I shall return to this major mystery of color in Sect. 5.5, in the context of psychophysical investigations into vision. References and Notes 1. The colors used for these figures are not colorimetrically optimized as primary colors and their combinations. 2. B. Fortner, T. E. Meyer, Number by Colors. A Guide to Using Color to Understand Technical Data, TELOS/Springer, New York, 1997. 3. A. Berger-Schunn, Practical Color Measurement, Wiley, New York, 1994. 4. H. G. Völz, Industrial Color Testing. Fundamentals and Techniques, VCH, Weinheim, 1995. 5. R. McDonald (Ed.), Colour Physics for Industry, 2nd Ed., Society of Dyers and Colorists, Bradford, 1997. 6. Schematic representations of colors were made as early as in the Middle Ages. These had no colorimetric or scientific intent, however (see Figs. 54 and 55 and the corresponding comments in Gage’s book (see Ref. 4 in Chapt. 1)). 7. Compare also the graphical representation of harmonic proportions given by Gioseffo Zarlino (1573) and of colors by François d’Aguilon (1613), both reproduced in Gage’s book (see Ref. 4 in Chapt. 1).

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Colorimetry 8. G.Biernson,‘Why did Newton see Indigo in the Spectrum?’, Amer. J. of Phys. 1972, 40, 526–533. 9. By uncompounded light, Newton means monochromatic light. 10. For the hue of the natural dye Ancient Purple, see Fig. 6.9. 11. Leonardo da Vinci, for example, used the expression ‘retto contrario’ (directly opposite) for contrasting pairs, but he also used it for combinations which are clearly not complementary in the modern sense. 12. ‘As I entered an inn towards evening, a well-favored girl with a brilliantly fair complexion, black hair, and scarlet bodice came into the room, I looked attentively at her as she stood before me at some distance, in half shadow. As she afterwards moved away, I saw on the white wall, which was now before me, a black face surrounded with a white light, while the dress of the perfectly distinct figure appeared a beautiful sea-green’. 13. Such pairs are, therefore, often called compensatory or contrasting colors, whereas mixtures of complementary colors should produce white. 14. The unique or unitary hues were originally called psychologically pure hues (or colors). 15. We know, of course, that adding some yellow pigment or paint to a large amount of blue pigment produces a greenish and not a yellowish blue, but that adding some green pigment to blue results in a recognizably greenish mixture. In contrast to these examples of subtractive mixtures, the (ideal) additive mixture of blue and yellow light is not green, but white. 16. It is worth drawing attention to the position of unique red: it is not a monochromatic color, but is located on the purple line; i.e., it is a mixture of light of wavelength 400 nm with a little of 700 nm. 17. K. Lorenz, Gestaltwahrnehmung als Quelle wissenschaftlicher Erkenntnis, Wissenschaftliche Buchgesellschaft, Darmstadt, 1963. 18. For an analogous comment by Goethe, see Sect. 7.6, p. 212, 213.

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5. How Do We See Colors? 5.1. Perception and Cognition of Color Newton’s fundamental point (1672) that the rays are not colored tells us clearly that color is not a purely physical phenomenon. Vision is prerequisite for color. Although several physical processes are involved in seeing colors, vision is truly the result of a complex sequence of neural reactions. It consists of three phases: sensation, perception, and cognition, although it is not possible to disentangle these from one another precisely. Anatomically, the eye is an outgrowth of the brain. It is, of course, necessary for vision, and so it is correct to say that we see with our brains.When light enters the eye, it is focused onto the retina, where it is absorbed by photoreceptors and converted into neural signals. Distinction between various types of sensation (movement,color etc.) is made even at this early stage. This information is transmitted via the optic nerve to the brain, where the sensory imput then undergoes complex processing and evaluation. A major role in this act is played by the sensory experiences already stored in the brain, including those previously processed (memory in the broadest sense), and these experiences have also established precedents for which aspects of a stimulus can be ignored. Emotions, individual preferences, and other psychological factors all also play a role in this complex cognition system. Any discussion of color vision from sensation to cognition must therefore encompass also psychology, esthetics, and even disciplines such as linguistics (as we express our perception of colors by giving them names). With the rise of rationalism in the seventeenth and eighteenth centuries, anatomists investigated human vision in the light of knowledge gained from optics. Fig. 5.1 is taken from D. Diderot’s (1713–1784) Encyclopédie ou dictionnaire raisonné des sciences, des arts et des métiers (1751–1780). It shows how an arrow seen by the two eyes is portrayed on the retina of each eye upside-down. Diderot assumed that a subsequent process must somehow return this arrow to the same orientation as the original arrow. Vision, however, is fundamentally irreconcilable with this purely rationalistic explanation1.It does not merely constitute the formation of an image somewhere in the brain, but is a highly complex transformation of light stimuli into perception and cognition. 79 Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

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Fig. 5.1. The process of vision, as explained in Diderot’s Encyclopédie ou dictionnaire raisonné des sciences, des arts et des métiers (Paris, 1751)

Diderot’s proposal for the form vision mechanism looks naive in the twentieth century, but a more general problem, already under discussion in Diderot’s time, underlies it. Today we call it the mind-body relationship. In 1983, Teller and Pugh2 discussed potential approaches to (at least partial) solutions to related problems in color vision, employing terminology from two logically distinct spheres of discourse: hard-science-based neurophysiology and psychophysical measurement in uneasy juxtaposition with subjective perception-dependent qualities such as redness and brilliance. Essentially, the authors handle such problems by attempting to identify what they call linking propositions. Teller and Pugh’s method is worth applying to color vision in cases where links are relatively short and both ends of them can be treated quantitatively.Such factors of color vision as emotion, esthetics, subjects’ cultural backgrounds and so on seem doubtful candidates for such an approach. Let me give one example of such a case. Bornstein3 intensively studied the psychophysics of color perception in infants. He found that infants (aged four months) exposed to monochromatic stimuli tend to prefer unique hues (yellow, green, blue, red) to complex ones. This result correlates with psycholinguistic findings, where these simple hues also dominate (see Chapt. 6). How, though, can we find any linking proposition between Bornstein’s psychophysical results, which are based on normative perceptual processes, and the cognition levels of infants? Are the two ends of the link too far apart from one another? The conclusions of the Swiss psychologist Jean Piaget (1896–1980) on the genetic epistemology of child development gives at least a partial answer to this question. He proposed that the development of cognition in infants and children involves incorporation of what is learned into constructs. These constructs undergo adaptation, as new knowledge is assimilated 80

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and accommodated during development,and are not,as assumed by Kant, present a priori.

5.2. Anatomy of the Human Eye The two eyes are the organs in which rays emitted by a light source or reflected by an object are focused and their intensities adjusted to suit the eyes’ light sensitivity. The rays give rise to an image on the retina, a photosensitive surface at the back of the eye. Their energy triggers a photochemical reaction, which converts this image into nerve signals. Fig. 5.2 shows a horizontal section through an eye. Its outer casing consists of two separate components: a transparent membrane, the cornea, at the front, and the white sclera, of which a portion (the ‘white of the eye’) is visible around the colored iris, making up the remainder. In the center of the iris is a circular opening, the pupil, the diameter of which can be varied between 2 and 8 mm by contraction and dilation of muscles in the iris. Luminance, the light intensity reaching the retina, can be adjusted by a factor of approximately ten by these involuntary muscle actions. Focussing is a function shared by the cornea, the lens, and the aqueous and vitreous humors, although the major proportion occurs at the outer

Fig. 5.2. A horizontal cross section of the right human eye

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corneal surface; the difference in refractive index between it (1.37) and air is much higher than those between the interfaces. Its curvature is higher than that of the lens. The lens, however, is solely responsible for accommodation, i.e., for the ‘fine-tuning’ of focussing on objects at different distances from the eye. Unlike a classical photographic lens, or the lenses in binoculars, that in the eye is elastic. Relaxation of the ciliary muscles causes curvature of the front surface of the lens to decrease, allowing the eye to focus on a more distant object. When the rays from an observed object arrive at the retina, the image is indeed upside-down, as postulated by Diderot (Fig. 5.1). However, in the last three centuries, nobody has found a second optical phenomenon turning the ‘picture’ back the right way up. The process between lens and retina is a purely optical, and hence physical, phenomenon. Subsequent processing, however, is neurophysiological in nature. It is not a simple, quasi-mechanical ‘reinverting of the picture’, but an extremely complex process, relying not only on chemical and physical mechanisms, but also on other phenomena, which are, quite probably, intrinsically incapable of being understood by man. I shall briefly discuss such problems in Chapt. 8. The retina is a very complex organ, despite being only 0.2 mm thick. It is located between the vitreous humor, from which it is separated by the internal lining membrane, and the front of the choroid and sclera (see Fig. 5.2 and also Fig. 5.4, below). As its name implies, the retina is a network. Histologically, it is part of the central nervous system. Before describing the retina, we should devote some time to the structure and interactions of nerve cells in general. Nerve cells are called neurons and are characterized by several fiber-like extensions around the cell. There are two types of nerve cells: axons and dendrites. Axons send electrochemical signals via synapses at their ends to the dendrites of other neurons (Fig. 5.3). Axons may be very long; some stretch from the brain to the spinal cord, or from spinal cord to the toes. At the synapse, a relatively small organic compound, known as a neurotransmitter, is released, carrying the signal onwards. Neurotransmitters identified in retinae are amino acids: they also perform the same function in other parts of the nervous system.

Fig. 5.3. Schematic representation of a synapse

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Information processing between neurons is analogue in nature; not digital as in most computers.

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Fig. 5.4. Cross section of the vertebrate retina, illustrating schematically the basic cellular and synaptic organization (for abbreviations, see text) (from Encyclopedia of Neuroscience, Ed. G. Adelman, Birkhäuser, Boston, 1987, Vol. I, p. 1062; reproduced by permission of Birkhäuser, Boston)

Fig. 5.4 illustrates schematically the basic organization of the vertebrate retina. It contains three cellular layers (shaded in the figure) in which the six major classes of retinal neurons are located.Two synapse layers (white) separate the cellular layers. The neuron classes are divided into a total of about sixty sub-classes. The first class consists of the two types of photoreceptor cells in the outer neu83

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ron level (ONL), identified by microscopy as early as 1866. Called rods (R) and cones (C) because of their different shapes, they are responsible for transmuting light energy into a chemical reaction which initiates all subsequent neurochemical processes.Light is no longer required after this point. The bases of the photoreceptors (called pedicules) contain several pits for signal transfer to dendrites, as shown in Fig. 5.4. From the evolutionary point of view, it is astonishing that light has to pass through all of the layers of the retina before reaching the photoreceptors. All these layers therefore have to be transparent (with the exception of the pigment epithelium (PE)), but are also highly structured (see the diagram in Fig. 5.4). It is far from self-evident that these layer structures do not cause ‘optical noise’ in the light signal on its way to the photoreceptors (especially for many old people). The great German physiologist and physicist Hermann von Helmholtz commented as early as around 1850 on the astonishingly high accuracy of perception in human vision, despite its low optical precision.‘The eye’, he remarked, ‘in spite of its admirable performance, is an optical instrument so full of defects as to put any artisan who brought me such an instrument out of business’4. At about the same time,another genius of biology,Charles Darwin, called the eye a ‘perfection of structure and co-adaptation which justly excites our admiration’. No doubt Helmholtz and Darwin would have expressed similar opinions today,when we know much more about the eye and are even more astonished about the performance of the retina! In the outer plexiform level (OPL), three types of synapses (represented as triangles and as open and closed circles) are responsible for signal transmission from rods and cones to the bipolar (BD and BH), horizontal (H), and amacrine (AS and AT) cells. BD, BH, and H cells report to interplexiform (IP) cells, which are located in the inner nuclear layer (INL). The second synaptic layer is the inner plexiform layer (IPL),which houses the synapses between the B, H, A, and IP cells of the INL, and the ganglion cells (G) of their corresponding layer (GCL; ganglion cell layer). From there, nerve fibers (ganglion cell axons) conduct each ganglion cell’s information output to the optical nerve.The retina contains over one hundred million photoreceptor cells, but only one million ganglion cells sending information to the brain. Receptors appear to drive both the bipolar and the horizontal cells. Horizontal cells provide a pathway for lateral interactions between distant receptors and bipolar cells. In the inner plexiform layer, bipolar cell syn84

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apses transmit information to ganglion cells. When a cell receives a signal, a change occurs in chemical potential (an oxidation-reduction equilibrium) or acidity. This reaction is conveyed from the horizontal and the bipolar cells to the ganglion cells in a graduated fashion, similar to the action of an analogue computer. From the ganglion cells to the brain, however, transmission is in the form of pulses, changes in potential rising to a maximum and falling very rapidly (within about a millisecond) to zero. The maximum is constant, obeying the ‘all or nothing’ principle, and the magnitude of the information is communicated by the number and frequency of the pulses. The activity of the receiving neuron is itself changed after the arrival of a signal. It may start or cease activity (an ON or an OFF response), or it may do both (ON-OFF). This is shown in Fig. 5.4 for three ganglion cells. In the interest of clarity, we shall ignore the fact that two main types of ganglion cells (small and large ones, also known as Pα and Pβ , respectively) have been identified. Small ganglion cells distinguish between the three cone types and linearly combine information received from them, in a sophisticated manner involving both positive and negative signalling. It can be seen then that the output into the optic nerve is already color-specific. The large cells, however, process the sum total of all information received from the three types, and thus accommodate achromatic color intensities. Recent investigations in anatomy, though, have revealed that the brain does not maintain completely separate and independent pathways for rod and cone signals. Rod-cone interaction is clearly a subject for further research. These are merely some short comments on the functional organization of the retina. They cannot give an explanation of the neurobiological process in vision, but are presented only with the goal of demonstrating that the combined efforts of many scientists have produced some knowledge of these processes. We must all be aware that this knowledge is still far away from a proper understanding of the basic results of clearly defined and reproducible psychophysical experiments discussed later in this chapter. To end this section, we will briefly discuss color vision deficiencies, commonly called color-blindness inasmuch as they result from deficiencies of cones in the retina. Cerebral color-blindness will be dealt with in the section on the visual cortex. Retinal deficiencies are caused by abnormal or non-existent spectral sensitivity in one or more of the three cone types. The most common type of deficiency is the red-green differentiation defect. It is sex-dependent (ca. 8% of male Caucasians, 4–5% of Asians and 1–4% of Native Americans; very few females) and inherited. Grandsons 85

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inherit this deficiency from their grandfathers via daughters who are color-normal. There are several potential causes of the red-green defect: either the sensitivity curve of the middle-wave cones (Fig. 5.5) is located too close to that of the long-wave cones, or, vice versa, the curve of the long-wave cones is shifted to too short a wavelength. Persons with these defects are called protanomalous and deuteroanomalous, respectively. The other two possibilities are that one or the other of these cone types is completely lacking. Individuals suffering these conditions are said to be protanopes and deuteranopes. Anomalies in which the short-wave cones show deficiencies are very rare. The same applies for total color-blindness (i.e., vision by rods only, tritanopia). It is interesting that, among the 8% of Caucasian males with a red-green deficiency, a significant percentage do not recognize this defect as children. This suggests that the defect is not critically grave, as least not for our way of life. Tests for color deficiencies are conducted with pseudoisochromatic devices. Particularly well-known are the plates introduced by Shinobu Ishihara in 1917 5, sporting a large number of colored circles, some of which form a numeral character of a specified chromaticity while others in the background are of different chromaticity but equivalent brightness.Other tests have been developed in which the test subject has to arrange various colored chips in order of spectral color (Farnsworth-Munsell test). The anomaloscope is an instrument used mainly by ophthalmologists for investigating red-green deficiencies. The subject has to mix additively a red and a green light in such proportions as to produce a yellow test light. Substituting lights of different color allows defects other than red-green to be studied. The first scientific investigation into color deficiencies was carried out in 1798 by John Dalton (1766–1844), one of the most famous early chemists, who established that the elements differ from one another not because of their shape,but because of their (atomic) weight.He had difficulties in distinguishing between scarlet and green, or pink and blue, and described these observations in a lecture to the Manchester Literary and Philosophical Society6. Therefore, red-green deficiency is today often called daltonism. We know today that he was a deuteranope (not a protanope), a conclusion reached via DNA analysis by Hunt, Mollon, and co-workers7. The genetic background for this analysis and for red-green deficiencies in general was established by Nathans in the 1980s in his work on photoreceptor genetics (see below). 86

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5.3 Photochemistry in the Retina We will not go any further into the neural mechanisms involved in general information processing of achromatic light by neurons, but will concentrate on the role of the retina in color vision. It has been known for several centuries that people can easily differentiate colors in daylight, but hardly, or not at all, in dim lighting or at night. At night all cats are gray, as they say. More recently, it has been found that the sensitivity of the rods is so high that the retina registers the absorption of a single photon, the smallest possible quantity of light. There are about 120 million rods and seven million cones in the retina. An isolated retina’s reddish purple hue is, therefore, mainly due to the rods. The highest concentration of cones occurs in the small central depression in the retina, the fovea centralis, which contains about 25,000 cones, but no rods. It, therefore, has the highest visual acuity for color vision. On the other hand, the number of cones in the far periphery of the retina is vanishingly small, and our ability to recognize colors in the far periphery of our visual field is practically nil. We hardly recognize this fact, however, as we move our eyes without realizing it when an object at the edge of our field of vision attracts our attention. Thomas Young postulated in 1802 that color vision is based on three ‘resonators’ which might exist in the eye. He was right, but convincing experimental evidence for his claim was only found 162 years later. Astonishingly, it was found by two independent teams (those of MacNichol and Wald; see Ref. 12 in Chapt. 1) at the same time (1964). By putting single cones on a microscope stage and measuring absorption of light passing through them as a function of wavelength, these scientists found it was possible to measure their visible spectra. Different cells from humans and from monkeys did indeed have absorption maxima variously in the blue, green, or red part of the spectrum. These results have been corroborated with improved techniques several times since 1964. Results from different animals are discussed in Sect. 5.6. For man, the three cone types have absorption maxima at about 419, 531, and 558 nm. The maximum for rods is at ca. 495 nm. The absorption spectra of cones and rods are not the same as their sensitivity spectra. These can be determined by measuring the intensity of monochromatic light necessary to produce a neural response over the 87

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whole visible spectrum. Using this technique, Baylor et al.8 found peak sensitivities for macaque monkeys near 450,530,and 560 nm. Spectral sensitivity of human color vision can be obtained with the aid of psychophysical measurements from matching monochromatic lights with mixtures of blue, red, and green light, a technique developed more than one hundred years ago. Results, shown in Fig. 5.5, demonstrate astonishingly close correspondence between human peak sensitivity wavelengths and the neurophysiological results from the macaques. The sensitivities of shortwave cones are lower than those of middle- and long-wave cones. Rods are more sensitive than middle- and long-wave cones by two logarithmic units. Recent investigations have concluded that it is not only the sensitivity maxima which are relevant for color perception (e.g., that only the short-

Fig. 5.5. Spectral sensitivities of short-, middle-, and long-wave cone cells of macaque monkeys, relative to their maximum sensitivities (logarithmic scale), after Baylor et al.8 (from Cold Spring Harbor Symposia on Quantitative Biology 1990, LV, 638; reproduced by permission of Cold Spring Harbor Laboratory Press, Plainview, NY)

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wavelength cones react to monochromatic light of wavelength 430 nm, i.e., blue). Rather, neural processing relies on simultaneous comparison of the stimuli at two photoreceptor types, or on the difference in sensitivities of two cone types for a specific wavelength. As already mentioned in Sect. 1.2, the designation of cone types as blue-, green-, and red-sensitive should no longer be used. What is the photochemical reaction which triggers the neural signals? The light-sensitive molecules are proteins,i.e., amino-acid chains.These chains are embedded in a lipid membrane, resembling a concertina. Fig. 5.6 represents such a chain, called rhodopsin, as a string of 348 pearls, each pearl representing one amino acid. Seven parts of the chain adopt a helical conformation. In nature, these seven helices are present in an oval palisade arrangement9, but, in Fig. 5.6, they are shown schematically in a row of rectangles. Outside the membrane are nonhelical loops. The seventh helix of the membrane includes lysine,i.e.,amino acid no.296, which binds the chromophore, the colored portion of the photopigment. This chromophore, retinal, is a derivative of vitamin A, and, as formula 1 shows, it is a cationic (positively charged) group. The chromophore is surrounded by the palisade of helices9. As shown by Mollon10, the amino acids, indicated by the straight lines in Fig. 5.6, have an influence on the wavelength of visible-light absorption, although there are no direct chemical bonds between them and the chromophore. No chemical bonds are cleaved or formed in the photochemical reaction of the rhodopsin in the cone pigment. The reaction is purely stereochemical: a rotation of 180° takes place between carbon atoms C(11) and C(12), straightening out the ‘bent’ part of rhodopsin shown in 1. One single photon suffices for this, changing it into form 2. This change in the retinal part of the pigment, in turn, pushes away parts of the protein chain: this is possible because these chains are flexible. The adoption of this new threedimensional structure by this part of the protein, in the immediate vicinity of the retinal moiety, activates a large number of molecules of transducin, another protein (not shown in Fig. 5.6), in the membrane. This and all subsequent reactions take place in the dark; light is no longer required. Two of these reactions should be mentioned. Firstly, the retinal part easily reverts from form 2 back to form 1, and so rhodopsin is able to undergo the photoreaction numerous times. The other important reaction is initiated by the transducin protein. Through the intermediacy of a small messenger compound, membrane pores are closed, blocking entry of sodi89

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Fig. 5.6. The photoreceptor pigment rhodopsin (above) and its chromophoric retinal side chain before (1) and after (2) absorption of a photon. R = Binding site at the protein chain (in part after Mollon10) (reprinted with permission from Nature 1992, 356, 378; Macmillan Magazines Limited). Straight lines: see the corresponding text on p. 89 and 92.

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um cations (Na+) into the cell. This process results in a change of about 40 mV in the interior voltage of the cell. The change in membrane potential regulates the release of neurotransmitters from cone and rod synapses to the bipolar, horizontal and interplexiform cells (BD, BH, H, and IP, respectively, in Fig. 5.4). Let me mention briefly four other important and intriguing aspects of the visual photoreceptors. First, there is the fact that the structure of the pigments is similar both in the rod cells and in all three types of cones.Second, even more intriguingly, the same chromophore is also present in photoreceptors used in other organisms, such as bacteria. Halobacterium halobium, a salt-loving organism, uses a retinal prosthetic group as a light-driven proton pump, regulating acidity in the cell fluid. Third, the heptahelical construction of photoreceptors is found elsewhere in sensory receptors in living organisms: the corresponding sensory cells for smell in the nose, for example, make use of it in a form lacking the retinal chromophore. Finally, it is known that a single rod contains about one hundred million rhodopsin molecules, and that these are renewed every ten days throughout a person’s life! Referring back to the discussion of colorants based on conjugated doublebond chains (see Sect. 3.4), we note that formulae 1 and 2 in Fig. 5.6 show cationic dyes with six double bonds. Compounds incorporating the same chromophoric group would be expected to show the same absorption bands. A dyestuff chemist would assume that their absorption wavelengths would vary little with the nature of the R group in compounds 1 and 2, provided that R contains no additional conjugated double bonds. Experiments, however, contradict this expectation. For the simplest case (R = H), the absorption maximum is at 440 nm, while for bovine rhodopsin, obtained from the eyes of cattle, it is at 498 nm. For bacteriorhodopsin it is at 568 nm, while human rod cells show it at 491 nm, and S-, M-, and L-wavelength cones at 419, 531, and 558 nm, respectively10,11. This variation in wavelengths remained inexplicable until a proper understanding had been achieved of what is called the secondary and tertiary structure of proteins, especially as pertains to visual opsin. Chains, consisting of a hundred or more amino acids, in a particular protein not only have a clearly defined unique sequence of up to twenty different amino acids (the primary structure), but also exhibit distinct spatial relationships between atoms and atom groups with no direct chemical bond connection between 91

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them. The fact, especially relevant for proteins, that physical and chemical properties can be different for compounds with different secondary and tertiary structures has only been appreciated for the past forty or fifty years. As regards the variation in visible absorption spectra of the various types of eye pigments, it gradually became fairly clear (although the precise details are still not entirely explained) that the wide spread of wavelengths is probably the result of some form of interaction between the retinal chromophore and amino acids in the adjacent membrane, different amino acids resulting in different absorption characteristics, as it was mentioned above in the context of Mollon’s investigation (straight lines in Fig. 5.6). Genetics provided a breakthrough in our knowledge of the differentiation of rhodopsin and the three cone pigments when in 1986 J. Nathans12, then a graduate student at Stanford University, identified the genes for the four pigments. The DNA base codes of these genes translate into the aminoacid sequences of the corresponding proteins, and showed that the degree of amino-acid-sequence similarity between rhodopsin and each of the three cone pigments was 40–45%. The S- and the M-wave pigments are 96% identical.All four proteins adopt conformations in the same palisade of seven membrane-spanning helices (see Fig. 5.6). Nathans was also able to explain the observation that M- and L-cone absorption and sensitivity maxima vary in the range from 530 to 560 nm (see Fig. 5.5) for various individuals,showing that these variations are due to slightly different gene loci coding the photopigments. Nathans et al. found that the genes for the M- and L-wave cone pigments are located next to one another on the X (sex) chromosomes, whereas those for the S-wave (blue) cone pigment and for rhodopsin are on chromosomes 3 and 7, respectively, and are less similar to one another. The close spectral proximity of the M- and L-wave cone pigments suggests relatively recent evolutionary divergence, a hypothesis for which good evidence exists, as we will see in Sect. 5.6. It was found, even in subjects without color deficiencies, that some amino acids in photoreceptors may differ for genetic reasons. Psychophysical tests do indeed give slightly different results in color vision. Jordan and Mollon13 went one step further on the basis of Nathans’ results: women have not one, but two X-chromosomes, and so also have two sets of genes for the M- and L-wave pigments. Thus, it may be that a woman inherits from her parents two different versions of either one of these 92

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genes. Such evolutionary variation may lead to the possibility of finding tetrachromic vision in some women. Jordan and Mollon’s pyschophysical studies of women with and without known inheritance of color vision defects (as shown by their sons) are still in progress, but no cases of tetrachromasy have been found to date. The possibility of tetrachromic vision is not excluded by this, but evolution is slow.

5.4. What the Eye ‘Tells’ the Brain To start this section, here are two comments made by David H. Hubel, pioneer of vision neurobiology and Nobel Prize laureate (together with Torsten N. Wiesel and Roger W. Sperry; see later). They date from 1988 and 199514, respectively. ‘The output of the eye, after two or three synapses, contains information that is far more sophisticated than the punctuate representation of the world enclosed in the rods and cones’. ‘I have not made any attempt to incorporate recent research [recent = period from 1988 to 1995] on the visual cortex. To extend the coverage to include areas beyond the striate cortex would have required another book’. In the last four decades, some very important results, representing pieces of a large puzzle, have been found by neurobiologists and brain anatomists. However, they are still not sufficient to explain color perception by the brain. Various techniques have been used in finding pieces of the puzzle. Chief amongst these are: i)

use of microelectrodes, permitting measurement of action potentials of single neurons in various parts of the brains of living animals, ii) psychophysical tests with healthy human subjects, brain-disordered patients and post-surgery split-brain patients and, finally, iii) positron emission tomography (PET)15

Anatomical and physiological investigations on animals supplement clinical experience and post mortem pathological results from human patients and human psychophysical results. For the pathways and mechanisms involved in color vision, experience shows close similarity between man and macaque monkeys. 93

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Let us begin with a short survey of the anatomy of the brain and its connection to the eyes, as shown in Fig. 5.7. The optical nerve fibers from the two eyes meet at the optic chiasma.‘Chiasma’ comes from its similarity in form to the Greek letter chi (χ). Here, roughly half of the fibers from one eye cross over to the other eye’s side, the other half remaining on the original side. Information relating to eye and pupillary movement splits off here. It has been shown that if an incision in the optical tract is made (in Fig. 5.7, right side), some of the information received from the right eye will nevertheless reach the right side of the cortex, but via the left side of the cortex and corpus callosum.

Fig. 5.7. Schematic representation of the connections in a horizontal cross section of the brain from the two eyes to the cortex (partly after Hubel 14)

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The new nerve fibers for vision proper arrive at the left and right lateral geniculate bodies. Here the first major processing of signals takes place in a sandwich-like structure of six layers of neurons. In any one such layer, the terminating fibers originate exclusively from one eye, while in the next layer only information from the other eye is processed. This alternation continues throughout the structure. From the lateral geniculate bodies, information from the eyes then proceeds to the cerebral cortex, mostly to the primary visual cortex. The cortex (Latin: bark) exists as a folded sheet of tissue covering the two brain hemispheres as a layer with a thickness of about 2 mm. Anatomists over the last hundred years or more have established that the cortex is not homogenous, but can be divided into different cortical areas. Area V1 is the primary visual cortex. Located in both hemispheres at the occipital lobe (the inner back part of the cerebral cortex), it is characterized histologically by striated layers in cross section, and so is also known as the striate cortex. In modern brain surgery, for certain types of epilepsy, for example, it is possible to open and retract sections of the skull under local anesthesia. To remove only diseased brain, without impairment of speech, motor, and sensory functions, the surgeon first has to identify topographically the areas of the cortex responsible for these functions.This is done by mechanical stimulation at several points and observation of the patient’s corresponding reactions. The alert brain does not signal ‘pain’ when electrically microstimulated, and so these investigations are carried out on alert patients. The surgeon will mark the critical points and then perform the surgery proper at the appropriate location. The whole cortex is folded and crinkled, its surface area amounting to about 1,000 cm2 in humans. Microscopic cross-sections, though, also exhibit small-scale folding and formation of mushroom-like structures. Various histological staining techniques have been developed since the late nineteenth century, and the terms ‘gray matter’ and ‘white matter’ were coined in this early period. Gray matter is the cortex, with neurons and their axons, dendrites, and synapses. White matter, which takes its name from the fact that it does not stain easily, contains embedded nerve fibers. Let us briefly follow the pathways taken by visual information after its arrival at the primary visual cortex. Around V1 is V2, and information proceeds from there to at least three occipital areas, called MT (medial temporal), V3, and V4 (see Fig. 5.9). However, it also travels back from these 95

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areas to preceding parts of the visual cortex, to other areas on the temporal, parietal, and frontal lobes and, last but not least, to deep brain structures like the hypothalamus. All these multidimensional outputs and inputs are evidence of the dramatic increase in processing (physiological) complexity from eye to brain, even though the brain is based on exactly the same type of information-system principles – neurons,axons,dendrites, and synapses – discussed in Sect. 5.2 on the anatomy of the eye. It has been known for over a hundred years that the two hemispheres of the brain are anatomically very similar, but that their tasks in processing neural information are in part different. For most people, verbal activity (speaking, reading, and writing) and calculation are concentrated in the left hemisphere. However, Roger W. Sperry (1913–1994), in psychophysical investigations with split-brain subjects, found that the dichotomy of the two hemispheres is more profound and more complex in form. He showed that the right hemisphere has a higher verbal capacity than previously assumed. It also has a higher analytical competence for complex spatial correlation and for music, but has difficulties in verbal expression. Sperry showed that the brain’s higher functions are possible only because of the organization of neurons into larger cerebral networks. In this respect, the two hemispheres differ from one another. At present, however, little is known about the cooperation of large neural systems within and between the two hemispheres. For cooperation between the hemispheres, the corpus callosum (Latin: tough body) is important for obvious anatomical reasons (see Fig. 5.7). It used to be assumed that the purpose of its very large number of nerve fibers (containing some 200 million axons) is to interconnect precisely corresponding cortical areas in the hemispheres. Crucial experiments on cats with a surgically divided chiasma and/or corpus callosum by Sperry’s group have at least provided evidence that the corpus callosum is more than just a link between two cerebral systems. Earlier in this section, the use of microelectrodes was mentioned as a major aid in our understanding of vision processing. Before discussing their application in color research in the brain,two historically important investigations into nerve cells in the retina should be outlined. Microelectrodes were first used by Stephen W. Kuffler in the early 1950s, when he recorded the responses of cat retina ganglion cells to spots of white light. In the absence of light, all cells fired slowly and irregularly. If the retina was lit with a small spot of light, the low-frequency firing of a 96

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ganglion cell would change in one of two ways. Either it would switch to a higher firing rate and revert to the dark firing mode immediately after the light was turned off (after 2.5 seconds), or else a reversed behavior would be observed, slow firing ceasing during the 2.5-second light flash. Kuffler called these cells on-center and off-center cells, respectively. Even more exciting was the result, for both types of ganglion cells, when the light spot was moved a small distance further away from the eye, resulting in a larger spot, influencing a larger number of retina photoreceptor cells. Under these lighting conditions, those ganglion cells which previously had fired rapidly stopped firing under a spot which only lit up the surrounding area. Vice versa, those cells which had ceased firing on exposure to the small spot fired rapidly if the surrounding zone was lit. The exact size of the spot and associated surrounding area necessary to provoke these response patterns varies by a factor of 100. It strongly depends also on the distance from the fovea centralis. Within this area, called the receptive field, all of the photoreceptors report to one ganglion cell in the retina. On average, the receptive field contains about one hundred photoreceptors. Shortly after Kuffler’s work with achromatic light, the first investigations using chromatic light began. In 1956, Svaetichin reported on several types of neurons (later identified as horizontal cells) in the retinas of fish which were excited by retinal stimuli at one group of wavelengths and inhibited by stimuli of the complementary wavelength, both for red/green and for yellow/blue light. Cells were also found whose excitation reflected overall eye sensitivity, i.e., for white light. Fig. 5.8 gives examples of Svaetichin’s measurements for yellow/blue- and red/green-sensitive cells. The next year, de Valois and co-workers conducted the first measurements of the effect of chromatic light on brain cells. Their results, from cells in the lateral geniculate bodies of macaque monkeys, were astonishingly similar. Again,three groups of cells were found: a) excitators,responding with increased firing rates to stimuli of all wavelengths, b) inhibitors, responding to all stimuli with a decrease in firing, and c) opposing cells, responding to red/green or yellow/blue stimuli with respective increases and decreases in firing rates. Some were excited by red and inhibited by green (+R-G cells), others showed the opposite behavior (–R+G cells). An analogous arrangement was found for yellow and blue. The pioneering investigations of Kuffler, Svaetichin, and de Valois were combined by Hubel and Wiesel in 196614. From earlier observations, these 97

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Fig. 5.8. Examples of Svaetichin’s electropotential measurements with chromatic light sources at retinae of fish eyes: blue/yellow, yellow/blue, and green-red sensitive cells

two neurobiologists already knew that, if one eye is not used in the first month after birth, irreversible changes in the cerebral networks may be observed. From their results, it can be concluded that neural networks in the brain, arising from fundamental genetic processes, must have a vital role to play. Without corresponding exposure to the environment, however, these cannot establish themselves; they are plastic and need environmental modification. 98

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Some hundred billion neurons are already formed in the infant’s brain before birth. In the most active period of brain development in a foetus, some 25,000 neurons are formed every minute. After birth, the long-distance network from the cortex (gray matter) into the white matter behind it develops, and, within one or two years, the child has significantly more synapses than an adult.At the same time and up to the age of about seven, however, almost half of the neurons formed before birth die,as their axons find no ‘useful’ connections. This plasticity of the brain, one of the most important features of the central nervous system, is the vital basis for learning. It is rather unlikely that this huge neural network is the result only of genetic pre-programming: the human genome, after all, is only about 1.5 times bigger than that of a mouse.Vision provides a good demonstration of the hypothesis that interaction between genome and environment is essential for the formation of the complex infant brain. Pictures from the two eyes (or, more accurately, the sensations from corresponding rods and cones in each eye) must be combined and integrated in the cortex. In the first few years of an infant’s life, the necessary specific coordination establishes itself out of the plenitude of possible ones. In this way, a relatively small set of genetic instructions can result in the formation of very complex and efficient brain structures. We shall return to the theme of brain development in another context: the question of whether a child learns language only after birth or whether it is innate, some universal grammar already being ‘pre-installed’ in the human brain. Adapting Kuffler’s center/surrounding technique for their investigations of color vision, Hubel and Wiesel studied the responses of small ganglion cells in the lateral geniculate body of cats to chromatic light spots shone into the eye. Using red light, they found some ganglion cells reacting as on-centers and others as off-centers, both responses, obviously, coming from L-wave cones. Analogous results, once more with on- and off-reactions, were obtained with other ganglion cells sensitive to green, blue, or yellow light. Surprising results, however, were obtained when the corresponding surrounding areas were exposed to chromatic light: in one case with a L-wave on-center cone, the reaction of the surrounding was that of an inhibitory response of a M-wave cone. Analogous results were found for all possible red-green combinations: the r+g– combination as mentioned,together with r–g+,g+r–,and g–r+.A second grouping of cells included the combination of S-wave cones supplying the centre, and M- and 99

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L-wave cones supplying the surrounding. All these cell types make up about 75% of the ganglion cell population in the lateral geniculate body. Two other ganglion cell types are located in the lower four layers. One of these has the receptive field of a center only, with red-green or blue-yellow opponency. The other, found only in the two lowest layers, are larger and light-dark-sensitive only,with no color preferences.They receive their information from the large ganglion cells in the retina (Pβ) mentioned in Sect. 5.2. The wavelength-sensitive area (with the small-size ganglion cells) in the lateral geniculate body is called the parvocellular system and the lightdark-sensitive area the magnocellular system. In 1968, Nigel Daw found an even more complex system of ganglion cells in the fish retina. These double-opponent cells are characterized by an opponent cell center surrounded by a ring of opponency. Margaret David Livingstone and Hubel discovered a similar double opponent cell system in the monkey, not in the retina, but in the cortex. They exhibit red-green or yellow-blue opponency in their centers and antagonistic yellow-blue or red-green (respectively) opponency in the surrounding. This short summary of the neural responses in the visual system of monkeys clearly demonstrates that their neurons are to a large extent color coded. Our knowledge about their interplay is still insufficient, however. For a long time, relatively few neurons with wavelength-selective properties were known in the primary visual cortex. Since the early 1980s, however, Livingstone and Hubel have been able to identify color-sensitive cells in the so-called blobs: cells which had already been identified histologically. These cells were called double-opponent because they exhibited redgreen or blue-yellow opponency at the center, coupled with antagonistic behavior of the surrounding to any center response, whether on or off. In general, interblob cells, found around the blobs in the cortex, are lightresponsive, but not color-responsive. P. Gouras has demonstrated the existence of additional cells (although not blob cells) responding to color borders.This separation fits in with our approach in considering form and color as separate aspects of vision. Information on luminance is sent from the magnocellular system to the primary visual cortex (V1) and then to visual area V2.There the signals are analyzed in a more refined manner to give information on motion and depth. The parvocellular system of the blobs is also connected with area V2, where its color information is combined with that relating to space and depth. Semir 100

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Zeki and others, however, have also found segregation of different visual functions in areas other than those mentioned: in the middle temporal lobe and visual area V4, for example. Zeki has provided strong evidence for the involvement of area V4 in form analysis and color perception. Our knowledge of the function and interaction of various areas of the visual cortex has been significantly enhanced by the recent development of new analytical methods for physiological processes. Positron emission tomography (PET) is the most important of these15. It can be used to measure changes in cerebral blood flow and enables the triggering of such changes by perception stimuli to be determined with great precision. As it is a non-invasive method, changes can be measured without causing harm to living subjects. The observed differentiation of the various areas of the visual cortex in Fig. 5.9 is in part due to PET investigations performed by Zeki and Lamb16. Their results on area V5 and V5A came from study of motion perception, a part of the vision process not really within the scope of this book (see, however, the comments on kinetic art in Sect. 7.3). It seems plausible to assume that analogous instances may also be found for color vision. The visual areas and their locations in the brain as shown in Fig. 5.9 are those of macaque monkeys. Zeki has identified a homologue of area V5 in the human brain and found that it is specialized for motion and only very weakly responsive to stationary form stimuli. Area V1 informs area V5 of the results of its operation,reciprocating input with return inputs,and also informing layer 4B of the magnocellular part of the lateral geniculate body. In addition, other routes from the retina to area V5 have been found to bypass area V1, assumed years ago to be the central entrance gate to the cortex for all visual stimuli17. Summarizing these results, akinetopsia, visual-motion-blindness syndrome is, therefore, the result of a lesion in area V5. This has been corroborated in normal subjects, where the perception of visual motion can be transiently and reversibly compromised by direct magnetic stimulation of area V5. The discovery of a satellite area of V5, specifically responding to rotary motion, demonstrates the complexity of processing visual inputs. Let us return to color vision. Zeki’s conclusion that color, movement, and form vision are carried out by separate pathways18 is supported by clinical evidence. One example is a type of color-vision deficiency called achromatopsia,a type of color-blindness due not to defects in photoreceptors in the eye, but to damage in the brain. Patients suffering from this rare 101

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Fig. 5.9. Horizontal cross section (B) through the visual cortex of the brain of the macaque monkey (A). Area V1 is shaded (after Zeki 18) (reproduced by permission of Blackwell Science Ltd., Oxford)

illness see forms and motion in the visual world, but they report it as being composed only of various achromatic shades. Some 20 cases of achromatopsia have been reported in the medical literature since the 1880s. Pathologists and brain researchers soon realized that this illness was very different from the various types of inherited color-blindness arising from retinal abnormalities. Achromatopsia patients’ brains were, therefore, investigated post mortem for pathological deficiencies, with the aim of gathering information on the question of whether color vision sense can be located in specific areas of the brain. L. Verrey, one of the early investigators, postulated as early as 1888, on the basis of the autopsy of his patient, that the color center is located in the lower part of the occipital lobe of the cortex, probably in the rear of the lingual and fusiform gyri19. His conclusion was hotly debated, several illustrious individuals being hostile to any localization of a color center and accumulating apparent evidence against it. Only since the 1970s 102

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has Verrey’s claim been generally accepted. The development, reviewed by Zeki20, demonstrates how difficult and complex brain research is. Other color-vision defects of the brain are color agnosia and color anomia, in which colors are seen, but either are not recognized (agnosia) or cannot be named (anomia). People suffering from dyschromatopsia confuse colors. Interestingly enough, chromatopsia is associated with prosopagnosia, the inability to name (or difficulty in naming) faces. It is also interesting, however, that prosopagnosia patients are still able to recognize the expression on a face, whether the person is happy or sad, for example. Psychological studies have also shown that color perception is slower and less sharp than that of movement and stereoscopic depth, and also that movement perception is color-blind. These observations are consistent with separate pathways for color, movement, and form. I close this section with a personal experience which has always impressed me when I watch the Engadine Ski Marathon. An almost solid mass of over ten thousand skiers starts on the Lake of Sils in Maloja. Watching from the other end of the lake, one sees a black mass during the first three kilometers, after which it dissociates into black points for the next two kilometers. Only in the last few hundred meters can chromatic colors be distinguished. Not more than about 10% of the competitors are dressed predominantly in black, while another 10–15% have navy-blue outfits. Since the late 1980s, the really fashionable, and hence dominant, colors have been violet and purple, perhaps 30%, and all other chromatic colors, with the surprising exception of brown, represented in about equal proportion. Gray is very rare, white occurs in relatively few cases. These observations are consistent with the fact that rod cells are so much more numerous than cones. My above-mentioned observations are psychophysical in a methodological classification. More sophisticated and more predictive psychophysical experiments in human color vision are discussed in the following section. The Marathon observation is, however, also useful for understanding the so-called Pointillist technique in painting, and so is mentioned once more in the chapter on color in art (Sect. 7.2).

5.5. Psychophysical Investigations into Color Vision Study of the performance of the visual system in normal subjects, and its dysfunction in patients with well-characterized lesions, relies on modern 103

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non-invasive procedures. Brain-mapping techniques such as positron emission tomography (PET)15, mentioned in Sect. 5.4, have proven particularly valuable. The aim of investigations such as these is to discover correlations with neurobiological observations, as discussed in Sect. 5.4, and also to understand the basis of the influence of color on language and on other human cultural activities. Trichromacy, the fact that all colors can be produced by quantitative mixing of three primary colors, was already known to Thomas Young. He was the first to realize (in 1802) that its basis lies not in physics, but in the physiology of the human retina. Maxwell, who performed the first quantitative analysis of color vision21, and von Helmholtz shortly afterwards, clarified this further, as discussed earlier. The trichromatic theory developed by Young, von Helmholtz, and Maxwell (Y-H-M mechanism) is, therefore, clearly based on psychophysical methodology. It remains fundamental for modern technical applications and was corroborated by Wald’s and MacNichol’s neurologically inspired investigations into the spectra of the three types of cones in the retina in 1964 (see Sect. 1.2). There are, however, two psychophysical phenomena apparently inconsistent with the trichromatic theory. Firstly, it has been known for a long time that there are four chromatic hues perceived by observers as pure in the sense that none of them (red, green, yellow, blue) shows any similarity to any of the other three. In particular, nobody sees any yellow or blue in pure green,although we all know that,physically,green paints can be made out of yellow and blue ones. The dominant wavelengths of these so-called unique hues have been determined (see discussion of Fig. 4.8 in Sect. 4.2). It should be repeated in this context that unique red is actually a mixture of the two ends of the color spectrum, situated on the purple line. The second problem relates to the achromatic colors white, gray, and black. Psychologically, it is a fact that we acknowledge no relationship between achromatic colors and chromatic colors, even though it is easily demonstrated experimentally that white results from the additive mixing of three tristimulus colors and from Newtonian recombination of the spectral colors using a second prism, while black arises from subtractive mixing of tristimulus colors.

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The German physiologist Ewald Hering (1834–1918), considering the four unique chromatic hues and their apparent fundamental difference from the achromatic colors, recognized very early that colors like bluish yellow or reddish green do not exist. He interpreted this with his ‘opponent color’ theory (1874), postulating that vision involves three antagonistic pairs of opponent physiological processes. The three pairs correspond to black/white, red/green, and blue/yellow sensation; each member of a pair ‘opposing’ the other. Hering’s postulate of positive and negative responses was revolutionary at the time, because no physiologist then could conceive of how nerve cells might generate negative signals. It was only in the 1930s that it became clear that nerve signals are generated by changes in differences in nerve cell potentials, as discussed in Sect. 5.2 (cf. Fig. 5.3). Direct experimental support for Hering’s theory was provided by electrophysiological investigations performed by Svaetichin and by de Valois in the late 1950s (see Fig. 5.8 in Sect. 5.4).Additional momentum was supplied by Hubel and Wiesel’s research demonstrating that the center and the surrounding of ganglion cell receptive fields were sensitive to antagonistic (green-red and yellow-blue) colors, and that neurons in certain areas of the cortex were sensitive to luminance, but not to color (see Sect. 5.4). In recent decades, psychophysical measurement of color phenomena has largely been the preserve of experimental psychologists. Research from about the 1950s onwards mostly led to conclusions supporting Hering’s opponent-colors theory, although results of more recent studies are claimed to contradict it. The former group’s principle figures are husband and wife Leo Hurvich and Dorothea Jameson22. They began their work in the 1950s with additive color-mixing experiments based on matching of stimuli in proper proportions: a hue-cancellation technique. The apparatus they developed for these experiments consisted of three monochromators. One provided a series of twenty test stimuli, spaced at 10-nm wavelength intervals. The second supplied one of the four unique hues: yellow, green, blue, or red. The third projected the surrounding for the two test lights in the form of a 37° adapting field of chromatically neutral white. The first and the third light source were of equal luminance, and the observer was asked to add sufficient of the appropriate opponent unique hue to the light from the spectral test stimulus as to extinguish the unique hue of the first monochromator. If, for example, it was desired to measure the amount of yellow chromatic response to a spectral test stimulus perceived as yellow, whether pure (unique) yellow, reddish yellow, or greenish yellow, the observer 105

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added increasing amounts of unique blue stimulus to it, until perceiving a hue or a neutral sensation in which the yellow of the test stimulus was exactly cancelled, but without any bluish tint becoming discernible. For example, a unique yellow original spectral test sample would be cancelled to give white, whereas reddish yellow would give pinkish. Abramov and Gordon23, among others, have systematically explored different procedures for scaling hue. Their results (Fig. 5.10) were obtained for monochromatic light spaced across the spectrum. There is very little overlap of red with green and of yellow with blue. The small degree of overlap which was found is the result of inter-subject and inter-trial variability, and so is meaningless for general conclusions. The red function has two components: a short-wavelength branch with its maximum at ≤ 440 nm and a long-wavelength one with maximum ≥ 660 nm. This accords well with the observation that unique red is not a monochromatic hue, but a mixture of the two ends of the color spectrum, situated on the purple line in the CIE chromaticity diagram (see Fig. 4.8). Abramov and Gordon also demonstrated the saturation of monochromatic light. Saturation is defined as the percentage of the entire sensation

Fig. 5.10. Hue scaling of monochromatic lights, measured with a dark surrounding for six subjects (mean values), after Abramov and Gordon23

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(chromatic and achromatic) that is chromatic24. For the group of subjects the results of which are presented in Fig. 5.10, saturation is highest for blue (440 nm), almost as high for red (660nm), slightly lower for green (490 nm), and much lower for yellow (580 nm). These results agree with our perceptual experience: we cannot imagine a very dark yellow (very dark yellow is what we call ‘brown’). Yellow is always very light,or in other words, contains much achromatic light, whereas blue may be very dark, containing very little achromatic light. Abramov and Gordon’s definition of saturation, therefore, is a parameter qualitatively related in an inverse sense to luminosity (Y) in the CIE system of colorimetry (see Sect. 4.1). The opponent color process as postulated by Hering is based on a qualitative phenomenology. More recent work on chromatic antagonism is psychophysically based, employing quantitative measurement of threshold detection, discrimination and adaptation. Modern measurement of the spectral sensitivities of the three cone cell types (see Fig. 5.5) demonstrates that their wavelength dependencies are not symmetrical, but in each case exhibit a steep sensitivity decrease on the long-wavelength side, but only a minimal decrease on the short-wavelength side. The peaks, therefore, look very different from dye-absorption peaks (see the schematic representation in Fig. 4.1, for example). The asymmetrical form of sensitivity curves has important consequences for our understanding of color recognition in vision. These have been clearly established only in the last two decades: in color vision of chromatic light – even monochromatic light – at practically all wavelengths from 400 to 700 nm, not only one, but two of the three cone types react. If we consider only monochromatic light for a moment, recognition of its color is consistent with the comparison of the activities of two cone types, or in other words, with subtracting the signals from the cone type with the lowest sensitivity for the particular wavelength from those of the other cone type. The system of cells postulated for the Hering process, on the other hand, assumes segregation of sensations of redness and greenness, or yellowness and blueness. Light of one of the unique hues reaching the eye would be expected to stimulate only one cone type; not two as would be the conclusion from the sensitivity curves of Fig. 5.5. How can these hypotheses be tested by psychophysical experiments? Several approaches can be found in the literature. Here an adaptation experiment designed by Webster and Mollon25 and a color-mixing experiment devised by Nijhawan26 are briefly summarized. 107

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A circular color display (white in the center,all the spectral colors and purple on the periphery) was used to adapt one eye of a test subject to variation along a number of particular axes in that space according to a procedure developed by Webster and Mollon. The subject was then required to adjust a reference color projected onto his or her unadapted retina in order to match the color falling on the adapted retina. Colors lying along the adapted axis were closer to white after adaptation, while colors orthogonal to the axis were slightly altered in appearance. Results were very similar for all orientations of the adapting axis. No enhanced selectivity for two orthogonal axes was found, as one might expect for a Hering opponent colors process. Studies of the perception of yellow hues permitted Nijhawan26 to differentiate the Hering model from the Young-Helmholtz-Maxwell (Y-H-M) mechanism.Hering proposed that yellow results from the activation of one distinct retinal-neural pathway. In the Y-H-M process, however, yellow results from the combined activation of the two pathways for green and red. Nijhawan asked subjects to watch a moving green bar with a thin line produced by the red flash of a strobe light, in two slightly different situations.When the viewers saw the green bar only for an instant,the red strobe flash on the green bar appeared yellow. When the partition was removed and viewers were able to perceive the moving green bar until the red strobe flashed, however, no one saw yellow inside the green bar. Instead, they saw a red line lagging behind the green bar. Their visual systems were unable to compensate for the neural delays in registering the sudden flash, and so the sensation could not be re-mapped to the place in the visual cortex where composition of the red and green into yellow would have taken place. Visual signals require about 50 milliseconds to travel from the eye to the brain. Therefore, moving objects do not appear in exactly the same place in the visual cortex map as in the retinal map without correction of the discrepancy. It follows that human and primate retinas are not capable of sensing motion on their own. Composition (in regular vision) and decomposition (in these experiments) occur only in the brain. A frog’s eye, however, can detect motion and send signals directly to its muscles without passing through the brain, on seeing a fly, for instance. This is why frogs can react so quickly and catch flies but we cannot, since we need our brains to perceive color and motion. As I showed some time ago27, the two pairs of unique hues are not equivalent.In the CIE chromaticity diagram (Fig. 4.8),only the straight line joining the yellow-blue pair passes almost through the central point for 108

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white; the straight line for the red-green pair does not. This finding is in agreement with conclusions drawn by artists. Although psychophysical investigations such as those of Webster and Mollon25 demonstrate that the simple opponent-colors concept for color vision cannot be correct,the existence of the four unique chromatic colors is welldocumented and cannot be ignored. This is also the opinion of Mollon, who wrote ‘The special phenomenal status of the four pure hues is perhaps the chief unsolved mystery of color science’28. Several other color-vision phenomena, some of them known for a century and more, have been extensively investigated using psychophysical methods but remain only poorly understood. Particularly interesting and exciting is the phenomenon of color constancy. Among men (far less so among women), only experienced subjects with a particular interest in color, like artists,dyers,and colorists,are aware that the color of an object may change markedly if the light source under which it is viewed is altered, from sunlight to tungsten light, for example. This is why women, when they buy a dress, take it from the shop out into the street to see if the color ‘changes’. If it does, the fabric is said not to have a good ‘evening color’. The technical term for such color changes is metamerism (metameric dyeing; see also Sect. 4.1). Although the specific observation that an object’s color is dependent on the spectral composition of the light source is easy enough to understand on the basis of physics, our cursory experience is to the contrary. Our visual system is obviously able to handle the problem in some way, with the result that, in most situations, people do not realize that color constancy is physically non-existent. Edwin H. Land (1909-1991), the inventor of artificial light polarizers in 1932 (Polaroid sunglasses) and instant photography (1947),investigated the problem of color constancy with the aid of psychophysical experiments, using pictures consisting of several rectangular fields of varying sizes and colors29. Their geometrical form is reminiscent of paintings by the Dutch artist Piet Mondrian (see Sect. 7.3), and so Land’s tables are called Mondrians. In lecture demonstrations, Land showed that a green rectangle,for instance,does not reflect the same color under all lighting conditions. By changing these conditions, a red rectangle from the same table may look the same as the green one did under the initial illumination. Even so, observers will still say that the red rectangle is red under both conditions.In other demonstrations, 109

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Land prepared three slides of the same scene, but each through different color filters,and projected them from three projectors onto the same screen. Even if he varied wavelengths and intensities of the projector lights widely, the scenes still remained almost the same for the observers. These experiments corroborated Helmholtz’s conjecture, already made in the late nineteenth century, that, in vision, the color of objects is determined by an unconscious judgement, in which the influence of the illuminant is discounted. Land also showed that this method of perception is present in animals’ color vision: it can be of vital importance to recognize the ripeness of fruits under varying lighting conditions, for example. Land worked diligently for at least two decades to develop a predictive theory for his experimental results. His theory is constructed around an independent neural system called Retinex. The term deliberately leaves open the question of whether the system is located in the retina, in the cortex, or in both.The Retinex theory has been developed in various versions over the years, but is at present not in a sufficiently complete state to be discussed here. A very prestigious named lecture given by Land in Germany in 1984 was reported in the science section of a well-known newspaper as having falsified Newton’s theory after 300 years. It is highly unlikely that Land made such a statement, as it is quite clear that Newton’s refraction of sunlight into spectral colors is a purely physical experiment, whereas Land’s experiments are based on visual perceptions of physical experiments. The human (and probably animal) brain’s ability to keep color recognition constant under varying illumination conditions is a psychophysical phenomenon of vision whose neuroscientific basis is still unknown. Before concluding this section, I present three optical illusions demonstrating color constancy problems. Fig. 5.11 demonstrates one of Land’s brightness experiments. The rectangle on the right is much brighter than the one on the left (they reflect 80% and 40% of incoming light, respectively), and each appears more or less uniform over its entire extent. If a pencil or similar long, narrow object is placed over the gap separting the rectangles, their brightness immediately seems to change, so that they become virtually indistinguishable from one another. Somewhat less noticeably, both rectangles now look a little darker on the right hand side, compared with the left30. This experiment, and the experiments with Mondrians discussed above, lead us to the conclusion that the color we see of an object is determined 110

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Fig. 5.11. One of Land’s brightness experiments

not only by the wavelength composition of the light reflected from it, but also by that reflected from surrounding objects. Claude Monet made similar observations, as is evident in, for example, his paintings of back-lit haystacks in a yellow field, where the shadows of the haystacks are bluish. Which of the two doors in Fig. 5.12 is nearer, the red or the blue one? Which is larger? Both are the same distance away, and are the same size. This illusion was already known to Leonardo da Vinci, who wrote, ‘Quello che voi che sia cinque volte più lontano, fallo cinque volte più azzuro’ (‘Paint the object you want to show at a five times greater distance five times bluer’). This effect is related to the distinction between warm and cold colors, of which we discuss various aspects in Chapt. 6. Are the green parts of the upper half of Fig. 5.13 the same shade as those in the lower half ? They are indeed, as can be seen by putting a pencil over the horizontal line or, better, by looking at them from an angle. Color constancy and optical illusions are excellent demonstrations that vision is not only an elementary physical process, but a highly complex functional system of neural units, organized and reorganized in eye and brain to combine information into a complete whole. An excellent summary of the brain’s processing of form, color, and spatial information, and 111

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Fig. 5.12. Distance to the doors

Fig. 5.13. Shades of the green parts

of optical illusions, including examples demonstrating the segregation of these functions in the brain, has been published by Livingstone31.

5.6. Color Vision in Animals A short summary of animal color vision should help us to place recent findings about our own color vision in a broader perspective. 112

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Let us first look at the general development of vision in the animal kingdom. The simplest organs for vision are isolated light-sensitive cells located on the surface of some animals, like earthworms. These only allow perception of brightness.In some animals, like snails,these cells are surrounded by pigment walls, forming cup-like ocelli, which enable the direction of the incident light source to be determined.If the opening of this cup structure is made very small, the light-sensitive cells can discern a rough image of the surroundings.Eyes of this type are found in nautili and other cephalopod molluscs in the southern Pacific and the Indian Ocean. In jellyfish and other hydrazoa, this opening is replaced by a lens. Fully developed eyes with lenses, i.e., a dioptric system including at least a cornea,vitreous humor,and a retina,are present in some annelida,a proportion of the cephalopoda and in all vertebrate animals. A completely different visual system, the compound eye, is typical for the arthropods, like insects and crawfish. It consists of a large number of individual eyes arranged in a honeycomb-type pattern (ommatidia). The optical resolving power of these screen-like compound eyes is of course significantly lower than that of dioptric eyes with retinas. On the other hand, the compound eyes of fast-flying insects are capable of analyzing up to 300 light flashes per second, compared with only about 25 for the human eye. Fig. 5.14 gives a schematic comparison of the most important characteristics of dioptric and compound eyes. In contrast to the compound eye (b),

Fig. 5.14. Scheme of picture formation in dioptric (lense) and compound eyes (reproduced by permission of Prof. K. Kirschfeld (Naturwissenschaftliche Rundschau 1 9 8 4, 37, 352) and Wissenschaftliche Verlagsgesellschaft mbH, Stuttgart)

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the dioptric eye (a) gives an image which is upside-down and side-inverted, as Diderot described in 1751 (see Fig. 5.1). These three different animal vision organs serve as excellent examples of the fallacy of the popularly held view of Darwinian evolution; that all species, extinct or living, are arranged in some sort of one-dimensional hierarchy with amobae at the bottom and humans at the pinnacle. Evolution is not a linear process, but like a bush with many branches. In this bush’s development, the lenses in cephalopod molluscs developed very early in that one particular branch. Later, the compound eye arose in another branch, developing into a biological instrument which today, on some measures of performance, surpasses dioptric eyes. Evolution of the compound eye began before that of the dioptric eye, and these two types later evolved in a parallel fashion, but entirely independently of each other. Regarding the evolution of vision generally, it is also interesting to note that the sense of vision may sometimes in the course of evolution be lost. This tends to happen if, for example, the preferred habitat of certain species changes to caves or if an organism develops into a parasite living inside a host animal. Very little is known about the early evolution of color vision. We will first discuss color vision in animals with compound eyes, and afterwards that of animals with dioptric eyes. This sequence seems appropriate because, next to our monkey cousins, the animal whose vision is best investigated is the honeybee. This is thanks mainly to Karl von Frisch (1886–1982)32, recipient of the 1973 Nobel Prize for Medicine for his work on honeybees, a biologist whom I am proud to have known personally, and the one who held the living creature in the highest esteem. It has long been known that bees belonging to different swarms in an apiary can find the correct entrance to their hive if these entrances are marked with different colors. As early as 1793, Berlin schoolteacher Konrad Sprengel deduced that flowers are colored so that bees could see where they could find their main food, nectar, and that, in their search for this nectar, the bees made the plant’s reproduction possible, by pollination. Karl von Frisch investigated bees’ color-vision sense, and vision more generally,from 1914 onwards.He found that bees,surprisingly enough,see different colors than we do. They cannot see the long-wavelength end of our visual spectrum, but they do see ultraviolet light. Like us, worker honeybees have three kinds of photoreceptors, but with maxima of absorption 114

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at 532, 432, and 336 nm, as recent measurements have shown. By analogy with the nomenclature used in this chapter for human photoreceptors, these may be called M-, S-, and UV-wave receptors. Like us, bees are trichromats, operating over a similar spectral range, but shifted to shorter wavelengths (300–620 nm, compared with our 400–700 nm). The same object, therefore, looks very different to bees than it does to us. This is particularly important for seeing flowers. What we see as a yellow poppy, with petals and sepals of very similar brightness, for example, the bee sees with very light petals and dark sepals, which tell the bee where to find the nectar. Chittka and Menzel33 have recently shown that the bee’s photoreceptor sensibility range is optimally positioned on the wavelength scale for differentiating flower colors. All the scientists investigating bees’ vision have been fascinated by their ability to learn colors as a stimulus associated with a reward. This ability enables bees to differentiate between known and unknown flowers, and between profitable food sources and inefficient ones. Another noteworthy phenomenon is that they can analyze polarized light. As discussed in Sect. 2.1, light has wave properties. It is well-known that particles can move periodically in alternating opposite directions; the equilibrium of waves of mass particles in water or sound waves are two examples. If the motion takes place in one direction only, like the up and down pattern at a lake’s surface, then the waves are said to be polarized. The same principle also applies to electromagnetic waves like light, and bees can recognize the polarization of sunlight as long as they can see at least a small spot of blue sky. From this, they are able to locate the sun and, indirectly, find the way to their hives. The ability of bees to navigate by analysis of light polarization was discovered by von Frisch in 1949. It was later found in many other arthropods (insects, spiders, crawfish). It is a very interesting question whether, in the course of evolution, nectar-seeking animals with color vision or plants with colored flowers evolved first. It is known that the extensive proliferation of angiosperm plants with colored flowers began in the middle Cretaceous or late Triassic age, about one hundred million years ago. How then did insects living, for instance, two hundred million years ago see the world? The question can be answered by looking at arthropod animals whose evolutionary lineages diverged from those of bees before colored flowers existed. If the photoreceptor systems of these animals are not 115

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different from those of bees, then insect color vision is likely to predate the evolution of flowering plants. An astonishingly large number of arthropod species (29) has been investigated in this respect, using intracellular measurement of the number of different photoreceptors and their wavelength sensitivities in their compound eyes. Most of them are trichromats, only one is a dichromat, while two have four different photoreceptors. All possess an ultraviolet-sensitive receptor. Besides insects, some, but not all, crustacea and chelicerata (e.g., jumping spiders and horseshoe crabs) also have ultraviolet receptors in their compound eyes. A bewilderingly large number, up to eleven, of photoreceptors was found in stomatopoda34, including an ultraviolet receptor. From these results, Chittka and Menzel33 inferred that insects were well-preadapted for flower-color discrimination more than five hundred million years ago. The same authors also measured color vision in bees and other hymenopteran insect species (wasps etc.) in color-discrimination-based behavioral tests. They evaluated the results using a model in which color is coded on the perceptual level by processes based on two opponent color pairs. It might be worth comparing this evaluation with those of human vision discussed in Sect. 5.5. Another interesting correlation between color vision in bees and in humans was found recently by Rüdiger Wehner. In bees, color plays an important role in searching for food, but their vision of moving objects is achromatic. As mentioned earlier in this chapter, perception of motion in the human brain is dominated by the magnocellular system, rather than the parvocellular system mainly responsible for color perception. Considering the differences between compound and dioptric eyes, these functional similarities between insect and human vision are striking. One important difference is the fact that no specialized low-intensity-light detectors, like the rods in vertebrates, have been found in animals with compound eyes. As we have discussed thoroughly, color vision in humans and in other vertebrates relies on cone receptors. Here we come across another case which demonstrates that evolution is not a linear progression. Full color vision, as we have seen, is based on three types of cones having different wavelength sensitivities. Reduction or abolition of the effectiveness of one or more of an individual’s cone types results in various forms of partial (or, 116

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very rarely, complete) color blindness. If not all vertebrates possess color vision based on these cone types, then it might be assumed that mammals, the most recently evolved vertebrates, should have three types, but that ‘primitive’ fish, and maybe also birds, might not. This assumption is completely false: species with three cone types are found in all of the orders of vertebrates mentioned, and in the others. As percentages of the total number of species in an order, however, mammals probably come last.Table 5.1 lists known cone absorption maxima for some arbitrarily selected animals from four different orders. Although it is difficult to obtain comparative quantitative data for colorvision proficiency in various animals, it is likely that birds and fish are superior to mammals in color vision. Birds’ range of vision includes some ultraviolet,and it is likely that owls’eyesight extends into the near-infrared spectrum. Using color-mixing experiments, Neumeyer35 was able to show in 1992 that goldfish are able to distinguish between pairs of fields covering the wavelength range from 360 to 641 nm. These results allow the conclusion that goldfish have four different cone types, and hence tetrachromatic vision. In another investigation, she demonstrated that the visual behavior of the turtle is also consistent with tetrachromatic vision. Mammals, humans included, do not possess ultraviolet receptors: their lenses are insufficiently transparent to ultraviolet light. Nathans’ isolation of the genes encoding the human eye’s color-sensitive photoreceptors (see Sect. 5.3) yielded new clues about the evolution of color vision. The pronounced homology between the rhodopsin gene and the cone genes strongly suggests that all four genes evolved from a common ancestor. Similarities in the gene sequences indicate that the S-wave cones diverged from rods at an early stage, some eight hundred million years ago. About two or three hundred million years ago, a cone gene arose on Table 5.1. Absorption Maxima of Visual Pigments of Some Vertebrates Absorption maxima of cones [nm] Class

Species

UV

Short wavelength

Middle wavelength

Long wavelength

Fish Reptiles Birds Mammals

Goldfish Turtle Pigeon Macaque Man

ca. 360 ca. 360 ? – –

455 462 461 415 419

530 522 514 535 531

625 623 567 567 558

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the X-chromosome, coding for a precursor cone in the M- or L-wave region of the spectrum. This X-gene diverged again relatively recently, between nine and thirty-five million years ago: the M- and L-wave cones were the result. On the evolutionary time scale, this last divergence took place not too long ago, and its effects are still evident as Mollon10 showed recently. Old-world monkeys and other primates do have three cone types. Their full trichromatic color vision co-evolved with relatively large,colored fruits, like mangos, bananas, papayas, and citrus fruits. Recent ecological studies have demonstrated that monkeys prefer orange or yellow fruits, either dehiscent with arillate (possessing a fleshy appendage) seeds or succulent and fleshy, weighing between five and fifty grams. Fruits consumed by birds, on the other hand, are red or purple, and smaller, whereas those favored by ruminants and rodents (e.g., squirrels) are green or brown, with dry, fibrous flesh. This explains how trichromatic vision benefits old-world monkeys: they do not eat the fruits at the parent tree, but fill their cheekpouches and move on to another place to eat the contents. Small seeds are not crunched, but swallowed and excreted intact. With fruits with large seeds, the soft flesh is separated from the seed, which is spat out after the monkey has reached its feeding place.Thus, the monkey acts as a disperser for the tree, rather than a robber. The situation is different for new-world monkeys, like the squirrel monkey and the marmoset. These diverged from our own ancestors about thirty million years ago, and, with the exception of one species, the males are always dichromatic. The female, however, having two X-chromosomes, may inherit two different versions of the gene from her parents, and so may have an additional, third cone type in the sensitivity range of M- and L-cones.New-world monkeys,therefore,constitute a special case,intermediate between dichromatic and trichromatic animals. No ecological and behavioral studies related to color vision in new-world monkeys have, to my knowledge, been completed to date. The large majority of mammals are, however, dichromatic. Some, though, have other special features relevant for vision. One such relates to the pigment epithelium (PE in Fig. 5.4). In most species, this is black melanin, the same polymer as in peacock feathers (mentioned in Sect. 2.4). Melanin absorbs light after its passage through the (transparent) retina, keeping it from being reflected and scattered inside the eye (the black paint inside a camera has the same function). Some nocturnal animals, however, like cats, foxes, cattle, opossums, alligators, and some fish, have a reflective 118

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epithelium. Reflection increases the likelihood that photons are absorbed by the rods. A modern technological analogy to the process is found in lasers, where parallel mirrors amplify the stimulated emission by multiple passage through the laser cavity. This reflection in cats’ and foxes’ eyes can be seen when they are caught in automobile headlights at night when hunting. Mammals’ color vision was reviewed by Jacobs36 in 1993. References and Notes 1. This is a homuncular explanation; see also Chapt. 8. 2. D. Y. Teller, E. N. Pugh,‘Linking Propositions in Color-Vision’, in ‘Colour Vision. Physiology and Psychophysics’, J. D. Mollon, L. T. Sharpe. (Eds.), Academic Press, London, 1983, p. 577–589. 3. M. H. Bornstein,‘Quality of Color-Vision in Infancy’, J. of Exper. Child Psychology 1975, 19, 401–419. This and other publications by Bornstein (e.g., review in Brain and Language, 1985, 26, 72–93) demonstrate that an astonishingly rich body of experimental data on color perception of infants is available. 4. Translated from L. Koenigsberger, Hermann von Helmholtz, Vieweg, Braunschweig, 1902, Vol. I, p. 240. 5. Much earlier (ca. 1877), J. Stilling developed similar plates in Germany. 6. A certain Giros von Gentilly (1781) in Germany and Thomas Young (1807) suggested that one of the three cone types is not working in patients suffering colorvision defects.‘Giros von Gentilly’ may be the pseudonym of George Palmer (see Ref. 10). 7. D. M. Hunt, K. S. Dulai, J. K. Bowmaker, J. D. Mollon, ‘The Chemistry of John Dalton’s Color Blindness’, Science 1995, 267, 984–988. 8. D. A. Baylor, B. J. Nunn, J. L. Schnapf, ‘Special sensitivity of cones of the monkey Macaca fascicularis’, J. Physiol. 1987, 390, 145–160. 9. J. M. Baldwin, ‘The Probable Arrangement of the Helices in G-protein-coupled Receptors’, EMBO Journal 1993, 12, 1693–1703; J. M. Baldwin, G. F. X. Schertler, V. M. Unger, ‘An Alpha-carbon Template for the Transmembrane Helices in the Rhodopsin Familiy of G-Protein-Coupled Receptors’. J. Mol. Biol. 1997, 272, 144–164. 10. J. D. Mollon,‘The Uses and Origins of Primate Colour Vision’, J. Exper. Biol. 1989, 146,21–38; J. D.Mollon, ‘…aus dreyerley Arten von Membranen oder Molekülen’: George Palmer’s Legacy’, in ‘Colour Vision Deficiences’, 1997, C. R. Cavonius (Ed.),Vol. XIII: 3–20. In the later paper, attention is also given to a vague idea of George Palmer who said in 1777 that the retina may contain three types of fibers, corresponding to three physical types of light. Young’s work (1802) is, however, scientifically better based than Palmer’s. 11. Data in the literature for all these maxima vary significantly.

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How Do We See Colors? 12. J. Nathans, D. Thomas, D. S. Hogness, ‘Molecular Genetics of Human ColorVision: The Genes Encoding Blue, Green, and Red Pigments’, Science 1986, 232, 193–202; J. Nathans, ‘The Genes for Color Vision’, Scientific American 1989, 260 (2), 28; J. Nathans, S. L. Merbs, C.-H. Sung, C. J. Weitz, Y. Wang, ‘Molecular Genetics of Human Visual Pigments’, Ann. Rev. of Genetics 1992, 26, 403–424. 13. G. Jordan, J. D. Mollon,‘A Study of Women Heterozygous for Colour Deficiences’, Vision Research 1993, 33, 1495–1508. 14. D. H. Hubel, Eye, Brain, and Vision, Scientific American Library, New York, 1988 and 1995. This book is highly recommended as a clear text for interested nonspecialists written by one of the pioneers on the neurobiology of color vision. 15. Non-invasive brain-research methods other than PET, such as functional magnetic resonance imaging (MRI) and low-resolution electromagnetic tomography (LORETO) do not to date (1998) seem to have been applied for color vision research. 16. S. Zeki, M. Lamb,‘The Neurology of Kinetic Art’, Brain 1994, 117, 607–636. 17. The phenomenon of the so-called blindsight is related to such a bypass of area V1 to V5. Patients with a partial lesion of the visual cortex may see in a rudimentary sense (particularly motion), but they are not consciously aware of seeing. 18. S. Zeki, A Vision of the Brain, Blackwell, Oxford, 1993. 19. Gyri are the outward wrinkles of brain lobes. Today, it is known that the lingual and fusiform gyri of man are homologues of area V4 of macaque monkeys. 20. S. Zeki,‘A Century of Cerebral Achromatopsia’, Brain 1990, 113, 1721–1777. 21. J. C. Maxwell, ‘On the Theory of Compound Colours, and the Relations of the Colours of the Spectrum’, Phil. Trans. Royal Soc. London 1860, 150, 57–84. 22. L. M. Hurvich, Color-Vision, Sinauer, Sunderland, MA, 1981; L. M. Hurvich, D. Jameson, ‘Some Quantitative Aspects of an Opponent-Colors Theory’, I, II, and III; J. Opt. Soc. Amer. 1955, 45, 546–552, 602–616; ibid. 1956, 46, 405–415. 23. I. Abramov, J. Gordon, ‘Color Appearance on Seeing Red or Yellow, or Green, or Blue’, Ann. Rev. Psychology 1994, 45, 451–485. 24. It is,however,not related to the term ‘saturation’in the Munsell system discussed in Sect. 4.1. 25. M. A.Webster, J. D. Mollon,‘Changes in Colour Appearance Following Postreceptoral Adaptation’, Nature 1991, 349, 235–238; ‘Colour Constancy Influenced by Contrast Adaptation’, Nature 1985, 373, 694–698. 26. R. Nijhawan, ‘Visual Decomposition of Colour through Motion Extrapolation’, Nature 1997, 386, 66–69. 27. H. Zollinger, ‘Correlations between the Neurobiology of Colour Vision and the Psycholinguistics of Colour Naming’, Experientia 1979, 35, 1–8. 28. J. D. Mollon, p. 146 in T. Lamb and J. Bourriau, Ref. 8 in Chapt. 1. 29. Land described his investigations in several papers published between the late 1950s and late 1980s.A good introduction can be found in various articles in the Scientific American, e.g., in December 1977, p. 108-128. Zeki summarized it in his book (Ref. 18) on p. 246–255. 30. Similar observations were made earlier, in particular by Goethe in his work Zur Farbenlehre (see Ref. 60 in Chapt.7, Didactic Part, § 37). Goethe neglected, however, such a phenomenon, as he wrote:‘Es ist eine Gotteslästerung zu sagen, dass es einen optischen Betrug gebe’ (‘It is a blasphemy to say there is such a thing as an optical fraud’).

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How Do We See Colors? 31. M. S.Livingstone, ‘Art,Illusion,and the Visual System’, Scientific American 1988, 258 (1), 68–75. 32. K. von Frisch, ‘The Dance Language and Orientation of Bees’, Harvard University Press, Cambridge, MA, 1967. 33. J. Chittka, R. Menzel, ‘The Evolutionary Adaptation of Flower Colours and the Insect Polinators’ Colour Vision’, J. Compar. Physiol. A 1992, 171, 171–181. 34. Stomatopoda is an order of marine crustacea including the squillae and having gills on the abdominal appendages. 35. C. Neumeyer,‘Tetrachromatic Color Vision in Goldfish’, J. Comp. Physiol. A 1992, 171, 639–642. 36. G. H. Jacobs, ‘The Distribution and Nature of Colour Vision Among the Mammals’, Biol. Rev. 1993, 68, 413–471.

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6. How Do We Name Colors? 6.1. From Color Chemistry to Color Linguistics How does a scientist working ever since graduation in the fields of color chemistry, textile dyeing, and colorimetry come to get involved in linguistics and cognitive science? This chapter is an attempt to answer that question. Although experienced in dyeing and in technical colorimetry, I was ignorant of, but curious about, the biology of color vision. Swiss nationality is a bonus when it comes to picking up modern European languages, and so I knew four of these, as well as Latin and, last but not least, Japanese. These different strengths and weaknesses inexorably came to crossfertilize one another, and the fruit, in the 1960s, was in the form of a naive etymological idea: major color terms (whatever ‘major’ means in this context; see Sect. 4.2) like red,yellow,green,and blue (and also white and black) do not have any readily discernible etymology, but several ‘intermediate’ colors, orange and violet, for example, have names whose origin is easily comprehended. Such considerations led to this question: are color-terms something which might be called a ‘psychological response function’ of the complex sequence of optical, chemical, and neurobiological reactions which together comprise human color vision? It is advisable to approach this question with considerable caution, as there has been a classic instance of a false conclusion in this field. William E. Gladstone, better known as Queen Victoria’s longest-serving Prime Minister (1868–1874, 1880–1886, 1892–1894), was a scholar of the Greek language. Observing the rarity – to our minds – of color names in Homer’s Iliad and Odyssey, he developed a conviction that the preclassical Greeks must have had, at the very least, a less mature and less definite relationship to the concept of color than later cultures. Some later scholars (though not Gladstone himself) even considered that Homer as an individual, or preclassical Greeks in general, must have been color-blind. Subsequent detailed literary analysis of Homer’s work demonstrated that the rarity of color-terms is related to the general attitude of the Greeks towards color as a phenomenon. Personally, I received some insight into 123 Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

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this condition when I had the opportunity to cruise on a small sailing boat between the solitary islands of the Aegean. There, the atmosphere – land, sea, sky – is not characterized so much by (chromatic) colors, but by the quality of the light. Another helpful impression came after my co-worker André von Wattenwyl (also a chemist) and I had started linguistic fieldwork on color-terms in the 1970’s. He performed color naming tests with Kekchi Mayans (at that time the Hispanicized spelling of Kekchi was preferred) living in the rainforests of Guatemala. On one hand, he was impressed by the clear and distinct answers given by his test subjects and, on the other hand,by comments they made indicating that,while they considered color vision important, the names of the colors were not. This attitude is also reflected in a paper by other authors published in 1974 and entitled ‘We Don’t Talk Much about Colour Here: A Study of Colour Semantics on Bellona Island’ 1. Back to the etymological idea mentioned above! It later became clear to me that the ‘psychological response function’ approach for color vision was not a good one, particularly after Berlin and Kay had challenged the prevailing cultural relativism of color-term linguistics in their book Basic Color Terms: Their Universality and Evolution2. I was fortunate enough then to have an offer from the Department of Mechanical Engineering at Massachusetts Institute of Technology (MIT) for a one-year visiting professorship in dyeing technologies.This I accepted,last but not least in order to become acquainted with Roman Jakobson (1896–1982), then the world’s most renowned linguist, and Noam A. Chomsky and his associates in the Department of Linguistics. During my stay at MIT, I confess I spent more time studying linguistics than teaching dyeing technologies and becoming acquainted with mechanical engineering. That was my second sojourn at MIT. The first had been as a postdoctoral fellow in 1951/52. Both periods were characterized by a transition from relativism to universalism. In the 1950s, it had struck me that the scientifically interesting aspect of dye chemistry and technology was the physical organic chemistry approach, then completely neglected by practically all traditional dye experts. During my second stay, Chomsky’s generative grammar was in development. His central target at that time was not semantics, but universal grammar3. It was an excellent foundation for color-term linguistics. My conclusions, in summary, were that linguistics cannot explain the physiological mechanisms and pathways of color vision, but that color naming should correspond to established facts related to color vision. I made this 124

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claim in my first paper on color naming in 1972 and in later publications, and was gratified to receive support from other researchers, such as the neuroscientists Abramov and Gordon (1994; see Ref. 23 in Chapt. 5).

6.2. The Phenomenon of ‘Human Language’ Before discussing the linguistics of color naming, we should devote some thought to our present knowledge of human language in general. Only in the last three decades has it become clear that human language is something fundamentally different from the means of communication used by animals. All our languages – there are some 10,000 of them known on Earth – are based on a language of thought, in which humans are capable of using symbols. In neonates, and later on in early childhood, language develops spontaneously, requiring neither formal instruction, nor recognizable conscious effort. This conclusion is supported not only by several scientific investigations,but also by well-known parental experience.Only two examples will be referred to here. As mentioned by Steven Pinker in his book The Language Instinct5 (p. 264), psychologists have shown that four-day-old French babies suckle more intensely when hearing French than Russian, and suck more strongly when a tape changes from Russian to French than from French to Russian. The second example is the wellknown observation that children of elementary-school age easily learn foreign languages with no formal knowledge of grammar rules either in their mother tongue or in the foreign language. For high-school children, however, learning of a foreign language requires a lot of conscious effort. These examples lead us to the question of why Pinker considers language an instinct. The confinement of language to humans was clearly recognized by Charles Darwin in The Descent of Man (1871), where he claimed that human language differs ‘widely from all ordinary arts, for man has an instinctive tendency to speak, as we see in the babble of our young children, while no child has an instinctive tendency… [for other arts,like] …to brew, bake, or write’. It was Chomsky who, some eighty years later, realized that human language is characterized by two fundamental facts. Firstly, practically all sentences that a person says, hears, or reads are new combinations of words appearing for the first time in human history. The brain, therefore, must contain a ‘program’ that can build an unlimited number of sentences from a finite lexicon of words6. Secondly, children develop these complex repertoires rapidly, and essentially by themselves. In Chomsky’s opinion, this strongly suggests that children are innately equipped with such a ‘program’, which fits the grammars of all real languages7. There125

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fore, this ‘program’ is called ‘universal grammar’8. More recently, Pinker has demonstrated in The Language Instinct that evolutionary adaptation – and not only innate ‘hard-wiring’– is involved in the development of language. In 1998, evidence was found that a gene responsible for language is located in a small area of chromosome 7, and, in the same year, Pinker spread his ideas on language and included emotional reactions into them. His new book How the Brain Works will be discussed briefly in Chapt. 8. In this discussion of human language as an instinct, it is important to stress that there is, so far, no indication that this human capability developed from our animal ancestors in an evolutionary way. No other organism living today shows anything which may be an earlier form of human language. Therefore, it is likely that man is the first organism in which this type of language instinct is present9. Interestingly, indirect (albeit rather oblique) allusions to this distinctive quality of language turn up in biblical descriptions of the genesis of the world. Psalms 33: 6, for example, reads: ‘By the word of the Lord were the heavens made’. More clearly, St. John’s Gospel begins (1,1): ‘In the beginning was the Word, and the Word was with God, and the Word was God… In him was life; and the life was the light of man’. The term ‘word’ is used in the latter as a symbol for God’s power as expressed in his language. In Christianity, ‘Word’ (with a capital W) became a symbol for Jesus Christ and, later, for human beings. In the original Greek New Testament, St. John uses ‘logos’, which does mean ‘word’, but can also mean ‘reason’, ‘mind’, and ‘revelation’. After this brief review of the general background of human language, we shall now turn to color naming. The background should help us appreciate the subtle, complex, and intricate psychological processes which underlie color words. To begin with, here are two quotations by Ludwig Wittgenstein (1889–1951), who had a great interest in color. In his Remarks on Colour10, he wrote (§ II-76): ‘Indeed the pure colours do not even have commonly used names; that’s how unimportant they are to us’. Regarding color names, however, he also commented (§ III-52): ‘It is a fact that we can communicate with one another about the colours of things by means of six colour words. Also, we do not use the words ‘reddish-green’, yellowish-blue, and so on’. Doesn’t Wittgenstein’s first statement share a great deal of common ground with Newton’s remark on the immaterial existence of color, mentioned in Sect. 1.2.? 126

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Wittgenstein’s basic assumption is that the meaning of words is given by the manner of their use. The actual use of words can be approximated by ingenious ‘language-games’ which he developed.

6.3. Categorization of the Color Space by Color Naming The modern understanding of Newton’s classical experiment tells us that the only difference between the two ends of the visible spectrum is that of the wavelengths of the light (see Fig. 2.3). These are 400 and 700 nm for the light we see as violet and red, respectively. The same also applies for all light waves in between: for blue, green, yellow, and the other hues between them. In Sect. 4.1, we discussed how color is not merely a onedimensional phenomenon as suggested by the wavelength scale, but a three-dimensional one, as colors may be weak or strong, and they may be brilliant or dull. Blue, for example, may be a brilliant and light hue such as sky-blue, or it may be dull and dark like navy-blue. Therefore, color can be represented as a space (also called a solid) with continuous change in hue, brightness and saturation (see Sect. 4.1). In language, however, we introduce boundaries into this space; and boundaries may change, as we know from geography and human history. The boundaries in color space between different regions named using different words have arisen for two reasons. First, neither the physical chemistry of light-absorbing pigments,nor the neurophysiology of sensation and perception involve the processing of light in strictly separated procedures for each wavelength of the visible spectrum, but handle relatively large portions of it collectively (see Fig. 5.5). Second, the semantics of human language tell us that we organize hierarchies of terms for related subjects. The word ‘animal’, for example, is appropriate for an elephant, but also for a small insect. In the late nineteenth century, the classical categorization of life-forms into plants, the animal kingdom, and man needed updating into bacteria, fungi, (green) plants, and animals (with man now included in the animals). The basis of this change was cultural, as scientists had discovered microorganisms, not known before as such (but used by man since prehistoric times in cheese making, brewing etc.), and because there was good evidence that the human biology fitted within mammalian specifications. These hierarchies of terms also exist for color lexica and the cultural factors influencing color naming. Linguistics research into color terms began in 1858 with Gladstone’s study of Homer’s Greek. In the 140 years since, works have been published on 127

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almost three hundred languages11 in various parts of the world. Most of these investigations were based either on interviews with native speakers of a language, or on asking them to name the colors of samples of dyed wool, other yarns, or painted paper. The result was a bewildering variety of color-naming systems in different languages, which were found to contain anything between two and twenty or more major color terms. The only conclusion which could be drawn from these results was that, for people speaking a particular language, color naming was extremely dependent on their particular and individual cultural factors. This opinion was consistent with the relativism in linguistic anthropology during the first two thirds of the 20th century, which tended to assume a dominant relationship between each language and its cultural surroundings, thereby minimizing linguistic universals. It is, therefore, understandable that von Helmholtz, in his book on physiological optics (1911), wrote after his discussion of Newton’s division of the spectrum into seven colors that ‘these divisions are essentially capricious and largely the result of a mere love of calling things by name’. Comparisons of field-study results were a questionable foundation for the search for universals in color naming, because the color samples used varied between trials, very few of them being standardized or characterized colorimetrically. A major challenge to the prevailing relativism of all color terms was, however, posed in 1969 by Berlin and Kay2. They surveyed reports on color vocabularies in 78 languages already published in the literature, and conducted their own investigations into 20 languages, using test subjects from the San Francisco Bay area. The subjects first had to make a list of all color terms in their language.The investigators then decided which of the elicited terms were ‘basic’ color terms, defining these as those which are a) monolexemic (i.e., a term the meaning of which cannot be derived from one of its constituent parts, like ‘blue’, but not ‘skyblue’), b) not subsumed within any other color term (unlike ‘crimson’ as a kind of red), c) with an application not restricted to a narrow class of objects, and d) psychologically salient as demonstrated by their frequent and general use (unlike ‘magenta’ and ‘mauve’). After this evaluation by the investigators, each subject was then shown an array of 320 Munsell color chips in a rectangular arrangement of 32 hues from red through orange, yellow, green, and blue to purple, each at ten levels of brightness. The subject was then asked to indicate the chip which he or she considered to be the best representative of each of the basic color terms in his or her language. In addition, subjects were asked to name nine achromatic chips in a row ranging from black through gray to white. 128

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English speakers as subjects gave a list of eleven basic color terms: namely black, white, red, orange, yellow, green, blue, purple, brown, pink, and gray. On this basis, Berlin and Kay evaluated the literature data for the other 78 of the total of 98 languages and came to the conclusion that universal semantic categories do indeed exist in all the languages investigated. The range of basic color terms they display can be as limited as that of Dani, spoken in the Western Highlands of New Guinea (and some other languages as well), using only the contrast light-warm (white) and dark-cold (black) for the naming of both achromatic and chromatic colors. Berlin and Kay’s categorization starts with some languages which have only two basic color terms and ends with those with eleven categories,such as those mentioned for English (for examples,see Table 6.1).These basic color terms show a fairly regular cross-cultural order of appearance. Those languages with two terms only, meaning always white and black, are called Stage I languages. If a third term is present, it will be for red (Stage II), a fourth will label either green or yellow (Stage III), and a fifth the remaining one of these two (Stage IV). The sixth term is for blue (Stage V), and a seventh for brown (Stage VI). Terms for pink, purple, orange, and gray then follow, but in no particular order (Stage VII). The hierarchy of the basic colorterms shown in Fig. 6.1 and Table 6.1 gives examples for all these stages. The publication of Berlin and Kay’s book in 1969 aroused widespread interest among linguists all over the world, firstly because the linguistics of color terms was already the second most frequently studied word-field (after kinship terms) prior to 1969, but also because the universality and evolutionary claims of the authors were significantly at odds with earlier

VII Fig. 6.1. Berlin and Kay’s seven vocabulary stages and the 11 basic color terms of English

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How Do We Name Colors? Table 6.1. Selected Examples of Language in Berlin and Kay’s Categorization by Basic Color Terms Vocab- Language ulary stage

Linguistic phylum (country)

Foci of basic color words black

white

I

Dugerm Dani

Ndani (New Guinea)

*

*

II

Pomo

Hokan (California)

*

*

*

III

Hanunóo

Austronesian (Philippines)

*

*

*

*

IV

Tzeltal

Penutian (Mayan) (Mexico)

*

*

*

*

*

V

Mandarin Chinese

Sino-Tibetan (Northern China)

*

*

*

*

*

*

VI

Bari

Nilo-Saharan (Sudan)

*

*

*

*

*

*

*

VII

Arabic

Semitic (Lebanon)

*

*

*

*

*

*

*

*

Hebrew

Semitic (Israel)

*

*

*

*

*

*

*

*

Japanese

Altaic (Japan)

*

*

*

*

*

*

*

*

Hungarian

Altaic (Hungary)

*

*

*

*

*

*

*

*

Russian

Indo-European (Russia)

*

*

*

*

*

*

*

*

Zuni

Penutian (USA)

*

*

*

*

*

*

*

*

red

yellow or green

yellow blue brown pink and violet green orange gray

opinion. Early reviews of Berlin and Kay’s book offered the criticism that, for most of the twenty languages investigated by interview, the authors had only one test subject and that subject lived in an English-speaking environment. In 1993, Gage (see Ref. 4 in Chapt. 1, p. 79) wrote ‘most unsatisfying about Berlin and Kay’s approach to color language is the assumption that the subjects tested will respond in a ‘natural’ way to the presentation of small chips of colored plastic from the Munsell system used by the researchers’. That this criticism is valid cannot be denied, and it accords with an observation made by Kuschel and Monberg1 in their tests with sub130

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jects on Bellona Island: all analytical methods in the natural sciences, and even more so in the social sciences, simplify reality into one-dimensional test situations, carrying with them the risk of artificial noise factors. Saunders’critical discussion13,in addition to her own assessment, includes references to other criticisms. Berlin and Kay are committed to the deeply contested thesis of linking propositions between color vision, categorization, and biology. Their model of color-term categorization has recently received support from general brain research demonstrating that an important neural strategy of the brain is its ability to ‘cluster’, or categorize. Domany at the Weizmann Institute recently developed a new method,or algorithm,for performing categorization on computers, and found his algorithm to be analogous to human intuition. In summary, I think the following quotation from a 1971 review is relevant: ‘We are left […] with the discovery of Berlin and Kay […] which (we may hope) is similar […] to the achievement of Linnaeus14 who somehow was able to give the living world an evolutionary arrangement without knowing anything about evolution’. My personal opinion about the framework of Berlin and Kay’s claims is as follows. As discussed in the epilogue (Chapt. 8), there are arguments for assuming the existence of questions related to cognition and consciousness for which we intrinsically cannot know an answer. On that basis, no system of color categorization can solve all the questions raised by color naming tests. Color categorizations are, therefore, only models and theories in a strong sense. Models are, by definition, not reality: they have their limits as described very well by Bertold Brecht in his speech quoted in Sect. 2.1. Berlin and Kay’s system can be used as a model for the interpretation of empirical test results in color naming. Cases which do not fit that model system are particularly interesting,because they can encourage us to think about potential causes of such apparent failure, and such search may consider factors outside linguistics. Berlin and Kay’s book has indeed provoked a very considerable response over the last 29 years. It has been estimated that, up until 1993, more than two hundred research papers had been published in which problems related to this book were discussed. It may, therefore, be that color linguistics has today become the most frequently studied word-field15. 131

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It is neither possible nor appropriate to review even only a small number of these papers in this book. Therefore, I shall select major points of general importance and include a few specific references. Central to Berlin and Kay’s work is its categorization into two to eleven basic color terms. Subsequent work by many linguists and the two original authors themselves has demonstrated that 1) distinction between basic and non-basic is sometimes very difficult, 2) the original claim of a maximum of exactly eleven basic terms is questionable,3) some terms are ‘more basic’ than others, and 4) the evolutionary sequence may vary slightly. Examples are to be found in Russian, with two basic types of blue (goluboy and siniy, see below), analogies in Polish and Catalan (also two types of blue,but different in hue from the Russian ones),and in Hungarian (two types of red). The Japanese aoi (blue and green) is ‘more basic’ than midori (green)16. Corbett and Davies17 applied behavioral tests (reaction time, frequency of use, order of occurrence) to rank, by salience, the basic character of color terms to such apparently abnormal cases, and MacLaury’s vantage theory12 is also applicable to them. I shall return to this possibility when discussing that theory later in this section. The evolutionary sequence of basic color terms is not absolutely constant. Even in 1969, Berlin and Kay found twenty different cases, a number which had increased to thirty by the early 1990s. Even thirty is a small number, however, relative to the 2000 sequences which would be expected if they were random. Fuzzy set theory,which permits the assignment of varying degrees of membership to particular categories, has been applied to color categorization to resolve inconsistencies in some of the problems listed above (Kay and McDaniel 18). While this improved the (statistical) certainty of the results, it did not,in my opinion,contribute to our understanding of color naming19. We already know from colorimetry (Chapt. 4) that color is not a onedimensional phenomenon, conditional solely on the hue (or wavelength) scale, but a three-dimensional one. In particular, the color space includes the dimension of brightness.Although this fact had been mentioned occasionally by language researchers,its full significance only came to be properly appreciated in the 1990s. MacLaury20 and others have shown that brightness is a critical factor in the construction of color vocabularies,and it is likely that brightness discrimination is an evolutionary precursor of hue discrimination. The rarity of hue terms, and the importance of the 132

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quality of light in the works of Homer, mentioned in the introduction to this chapter,suggests such a development for classical Greek.The two basic blues in Russian (see above) also reflect differences in brightness: siniy is dark-blue, goluboy light-blue. The same effect is likely to be the cause of the analogous cases in Polish and Hungarian. Obviously, the two basic color-terms in various Stage-I languages are not black and white in the Western cultural sense, but, as mentioned earlier in this chapter for the case of New Guinean Dani, their meaning encompasses the brightness dimension in some manner. This was already acknowledged by Heider21 when she investigated Dani color-sample naming in 1972. A further phenomenon, which may also be related to brightness dimension, is known on a restricted lexical level in languages in which opponent hues are named with the same word. Terms which can mean both blue and yellow appear in a number of Slavic languages, and are derived from a Proto-Slavic term polvu. In this context, it should also be mentioned that flavus, used for yellowish hues in Latin, evolved into bleu, blue, and blau in French, English, and German, respectively22. This phenomenon of categorizing blue and yellow together is also observed in other linguistic communities, such as Ainu, the language of the indigenous people of Northern Japan, Daza, a Nilo-Saharan language of East Nigeria, and in the language of the Mechopdo people of Northern California. The Ainu also have combined terms for red and green, while the Chinese and Japanese (Kanji) character for green consists of a combination of the characters for fresh and for red. This Chinese character is a case of another type of connotation. Hanunóo, a Philippine language spoken on Mindoro, is a classical example of connotative meaning, as shown and carefully investigated by Conklin23 in the pre-Berlin-Kay period. It is a Stage-III language (basic terms for dark/cold, light/warm, red, and green), the first two terms alluding to succulence and desiccation, respectively. Despite some minor open questions in Berlin and Kay’s original work and subsequent investigations, it was clear, by and large, that a universal colorterm development pattern had been discovered in 1969. Kay and McDaniel18, therefore, concluded that the ‘semantics of basic color-terms in all languages directly reflect the existence of […] neural response categories’. I agree with this statement, except for the word ‘directly’. Even in 1978, I was adding words of caution to a review paper in Experientia:‘Results from 133

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linguistic investigations can corroborate mechanisms in neurobiology […] but they cannot be used […] for a new theory in biology’. Color sensations are formed in the retina and transmitted as signals to the brain. To communicate about the colors seen, we have to correlate these sensations with our more general experience. Only then is it possible to understand the meaning of color terms.This explains the observation by investigators into color naming in very remote cultures that natives ‘do not care much about color names’ (see the remark on the Bellona Islanders in Sect. 6.1). There are two recent approaches to color-term categorization: the theories of Wierzbicka24 and MacLaury12,20. Wierzbicka explains the indifferent mental attitude towards color naming seen in remote cultures by the hypothesis that color concepts are anchored in certain universals of human experience. She proposed that universals in our environment, such as day, night, fire, sun, vegetation, sky, ground, are models for color concepts, and that such universals are, therefore, reflected in languages. On this basis, she interpreted Berlin and Kay’s evolutionary sequence from universals originating in a culture’s environment, and not in the neural representation of color in the human brain. The culture of man includes the environment. But what is culture? It is the entirety of human inheritance beyond the merely genetic, i.e., all features of a civilization including its beliefs, its artistic and material products in the physical environment, and its social institutions, including its languages.Culture is based on learned and shared information in all those areas. The relationship between human vision and culture is a Niels Bohresque complementary problem, as discussed for the nature of light in Sect. 2.1. It can only be comprehended from two independent approaches, one starting from the physics, chemistry, and neurobiology of vision, and the other from culture to the language and meaning of color terms. This conclusion accords with Heider’s comment21 that the color space, a phenomenon of color physics, is ‘far from being a domain well suited to the study of the effects of language on thought’. The other development in the understanding of color-term categorization is MacLaury’s vantage theory. Taking as basis the fixed universals confirmed by Berlin and Kay , he adds mobile cognitive variables to them. The word ‘vantage’ comes from medieval French referring to an advantageous military position, as on high ground. In the context of color naming tests performed with the help of Berlin and Kay’s array of color chips, it refers 134

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to a point of view constructed with reference to the coordinates of these arrays. The best example of the application of this theory is the integration of brightness into the Berlin and Kay categorization. MacLaury’s experimental procedure is characterized by a combination of two techniques. Each subject has to name a random sequence of color chips and to map each of his color terms onto the array of 329 chips. Many researchers using the Berlin-Kay or analogous arrays of chips have observed that there are areas of the array where two or more terms overlap. MacLaury calls this phenomenon co-extension: color terms are coextensive if they share a significant portion of their total gamut of extension,even though the subject is of the opinion that the best examples (foci) of the two hues are widely separated.A characteristic of co-extension studies is the emphasis on similarity and distinctiveness. MacLaury calls his procedure ‘vantage theory’ because its appraisal reveals certain characteristic but hardly explicable results of Berlin and Kay’s and subsequent studies. These can be rationalized if the evolutionary emergence of Stages I to VII is based on vantage. An important factor in this development, besides hue,is a change in brightness.The finding that red and yellow appear more distinct than green and blue (Stages II to IV in Fig. 6.1) is related to the evolutionary emergence of the category warm before the category cool. This can be seen in the observation that, after Stage I color lexica, the term for warm (light) gets divided into two terms (white and red) before the term for cold (dark) splits further: warmth is more subject to extension or diversification than coldness. Earlier in this section, Hungarian, Russian, and Polish were mentioned as languages which, inconsistently with the original Berlin and Kay theory, each possess two words for one basic term. Hungarian uses piros and vörös for red, while Russian has goluboy and siniy for blue (Polish is similar). MacLaury’s appraisal (p. 426 of Ref. 12) of tests in Hungarian demonstrated that, in the case of one of the subjects, vörös was dominant and piros recessive, but for other subjects the opposite result was obtained. For the two Russian blues, the dominant and recessive roles of siniy and goluboy – or their equivalents in other languages – may also interchange significantly. Usually, siniy corresponds to dark blue, goluboy to light blue. It is impossible to determine which of the two blues is elemental on the basis of these inconsistent results. One Russian investigation into small children’s learning of color words, however, found that they used siniy before goluboy. MacLaury emphasized that this observation suggests that, on vantage theory precepts, siniy must be the basic color term. 135

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The MacLaury book appeared after I had submitted the manuscript for the present work. It was, therefore, unfortunately not possible to read it carefully and discuss his results here as extensively as I would have liked. Another short reference to vantage theory appears in the section on cultural influences on color naming (Sect. 6.5).

6.4. Color and Phonological Universals The subject of phonology is the sound pattern of a language including that of its phonemes, i.e., the units of sound in spoken language. In this section, I shall discuss a striking correlation between, on the one hand, the step-by-step development of the phonemic system of languages and, on the other, color perception as elucidated by the linguistic evolution of basic color terms. This correlation can be found in Roman Jakobson’s Kindersprache, a pioneering study of linguistic research published for the first time in 194125. In this monograph, Jakobson showed that the development of language in the child takes place in a regular sequence, one which is reversed in patients with aphasia (i.e., persons who lose speech because of mental illness).As Jakobson wrote,this sequence is ‘by its very nature closely related to those stratified phenomena which modern psychology uncovers in the different areas of the realm of the mind’. For the evolution of basic color terms and the brightness vs. hue problem, it is very interesting to note that Jakobson recognized close relationships between speech sounds and color perception: ‘Like visual sensations, speech sounds are, on the one hand, light or dark, and, on the other hand, chromatic or achromatic in different degrees. As the chromatism (abundance of sound) decreases, the opposition of lightness and darkness becomes more marked. Of all the vowels, A possesses the greatest chromatism and is the least affected by the light-dark opposition’. Jakobson demonstrated that the vowels u and i show a ‘minimally distinct chromatism’ which turns out to be the basic process of dark (U) and light (I) to which chromatism is added as the second dimension; it leads to a, which Jakobson, in later papers 26,27, called the most ‘compact’ sound. Jakobson observed analogous relationships for the consonants, although these are sounds ‘without pronounced chromatism’. That sounds are psychologically related to the perception of color was already accepted before Jakobson, particularly by Köhler and Stumpf. The specific understanding of the development of sounds as parallels to colors is,however,due solely to Jakobson.These relationships can be symbolized by triangles, the triangle of vowels having been used by Hellwag as early as 1781 (Fig. 6.2). 136

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Fig. 6.2. Principal coordinates of sound system, after Jakobson

In the context of our problems, the correlations between sounds and specific colors as analyzed by Jakobson are most interesting. The vowel A as the most chromatic and compact sound is symbolized by red, as various analyses of psychological sensations have demonstrated. The U-I axis is related to black and white. Blue lies between red and black, and corresponds to O; while AE and E are located on the A-I axis, their corresponding colors are pink and yellow. Analyzing children language, Jakobson also realized that ‘the less structured units in the development of the phonemic system are replaced by more and more structured units’ and ‘all laws of solidarity are explained by the stratification of more simple and undifferentiated oppositions by more refined and differentiated ones’. Analogous later investigations, such as Chastaing’s research into French, confirmed Jakobson’s essential results. From the above, it can be seen how Jakobson, in a certain sense, predicted the most important features of the linguistic evolution of color terms on the basis of analogies with sound developments. First come achromatic oppositions (white/black), followed by some form of opposition to the optimum chromaticity (red) and then by further refinement within this framework. This refinement was already implicit in Jakobson’s original work25, where he mentioned that a ‘combination of parallel phonological oppositions’ of two pairs of vowels or (in Czech) of two pairs of consonants might exist: A U

E I

K P

C T 137

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The horizontal dimension is the grave-acute opposition, the perpendicular dimension the compactness. It is tempting to assume a similar square relationship for the four basic chromatic colors red, green, yellow, and blue. However, artists have persuasively shown that, in their perceptions, the red/green and yellow/blue opponent pairs are not equivalent (see also the analogous conclusion from colorimetry, Fig. 4.8). The red/green opposition is a relation between a ‘fundamental’color (red) and its opponent color green; it signifies the perception of chromaticity contrast. Yellow and blue, however, are chromatically equivalent (but not achromatically, i.e., with respect to darkness); they signify the perception of the chromatic opposition. The black/white pair signifies the perception of the achromatic opposition. The weak chromatic character of green can be demonstrated well by the development of the meaning of pallidus, which meant yellowish green in Latin,but became just pale in English, Italian (pallido) and French (pâle). The feeling that green is normally not a color of high chromaticity is also evident in the German expression ‘giftiges Grün’ (poisonous green) for a very brilliant green, i.e., a hue considered unnatural. In summary then, Jakobson, on the basis of his investigations into child language and aphasia, predicted the results for color naming thirty years before Berlin and Kay carried out their cross-cultural linguistic study. Jakobson approached the problem from a direction which might be called the ontogenetic approach.It is interesting to refer here to a linguistic investigation into the phylogenetic evolution of the human senses. Williams28 investigated the metaphorical usage of adjectives related to the five senses. He found that the transposition of adjectives associated with one particular human sense to a metaphorical meaning in a different sense’s ‘sphere of influence’ occurs almost exclusively only in those directions shown in Fig. 6.3. Such transfers are called synesthesias. In the arrangement of the senses given in Fig. 6.3, metaphorical shifts generally take place only from left to right. Interesting examples in the context of color vision are warm (touch → color), full (dimension → color), austere (taste → color), bright (color → sound), and strident (sound → color). Williams demonstrated that this scheme applies not only to English, but also to Japanese. There is strong evidence that the directions of transfer shown in Fig. 6.3 parallel the biological evolution of the senses, or their phylogenetic development in animals and man. The hindbrain of early vertebrates process138

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Fig. 6.3. Metaphorical transfers of sensory adjectives to other senses. The visual sense is referred to in two areas, color and dimension (= spatial perception)

es tactile,gustatory,and vestibular experience,while the midbrain of higher vertebrates is specialized in processing olfactory and visual stimuli. The acoustic sense probably developed in tandem with the visual sense. This implies a sequence of sense development from tactile to gustatory, olfactory, and finally to acoustic and visual (or visual and acoustic). The fact that the visual and the acoustic senses cannot be ordered in a distinct sequence parallels their position in Williams’ scheme of metaphorical transfers. It is, therefore, striking, and probably not coincidental, that the metaphorical transfer sequence of sensory adjectives follows the phylogenetic development of the senses. An important metaphorical transfer is the touch-to-color synesthesia of the adjectives ‘cold’ and ‘warm’. We have already mentioned these qualifying terms in the preceding section in the context of the Stage-I language Dani and in Wierzbicka’s interpretation of the categorization of color-term evolution. Their metaphorical usage for white or light, and black or dark perceptions is a prompt for a scientist to seek a physics-based interpretation. As we know, the wavelengths of light in the visible range of the spectrum vary from 400 to 700 nm, and the wavelength of electromagnetic radiation is inversely proportional to its energy. The violet and blue colors have wavelengths near 400 nm, and so are higher in energy than those at the long-wavelength end of the visible spectrum. The ‘warm’ colors red, orange, and yellow, however, are not at the high-energy end, but on the low-energy side of the spectrum.Various authors mention this contradiction, but offer no explanation for it. The apparent contradiction is, however, understandable on the basis of molecular motion and the wave/particle complementarity of light. If ultraviolet light is absorbed by molecules, 139

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its (high) energy results in the cleaving of chemical bonds and the destruction of molecules, or in the promoting of certain electrons to higher energy levels. This irreversible bond cleavage is well known from sunburn in places where levels of ultraviolet light are intense, such as on high mountains. Visible light is energetically too weak for most bond cleavages, but not for elevating electrons to higher energy levels. This, as discussed in Sect. 3.4, is the cause of color by absorption (see the five electronic energy levels in Fig. 3.2). In the long-wavelength part of the visible spectrum, another type of electromagnetic energy absorption must be considered in addition to the promotion of electrons to excited states as the physical cause of color (discussed in Sect. 3.4). Long-wavelength visible light and, even more so, near-infrared light is transformed into stretching, rotational, and bending vibrations of chemical bonds between particular atoms in a molecule. We sense these vibrations physiologically as heat. Therefore, an infrared lamp makes us feel warm, while an ultraviolet lamp does not. For light of the visible spectrum, a related experiment was conducted (probably) for the first time by Benjamin Franklin (1706–1790), the writer and statesman who was also successful as a scientist. In 1761 he wrote to a friend that he wanted to find out how much sunlight each of the spectral colors absorbs. For this purpose, he asked a tailor for small square pieces of a wool fabric, dyed in black, navy blue, sky blue, green, yellow, red, and undyed (i.e., white). On a sunny winter’s day, he laid out these pieces on snow for some hours. Afterwards, he observed that the black piece had sunk deepest into the snow, the navy blue somewhat less, and the white piece not at all. All the others were in-between. As discussed in Sect. 4.1, a black surface absorbs all visible-light energy, navy blue all light at wavelengths longer than ca. 600 nm plus a significant portion of the rest of it, the sky-blue sample only wavelengths above 600 nm, and the other dyed samples only below 600 nm. The white wool does not absorb visible light at all, of course. A more elaborate experiment than Franklin’s would show that the physical warm/cold scale of colored samples starts with yellow, followed by orange, red, violet, and blue. Green is similar to blue, as a green object has two absorption maxima, one at low wavelengths (< ca. 450 nm, like yellow), and one at long wavelengths (> ca. 600 nm, like blue; see Fig. 4.1). Color terms, in general, can have very different metaphorical meanings in various languages.Color metaphors more often have pejorative rather than 140

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laudatory connotations, as is particularly the case for yellow.This is shown, for example, in a compilation of 15 different meanings in 14 languages from Korean to classical Greek. In my opinion, this case demonstrates that an understanding of the highly complex system of color metaphors, with the notable exception of Williams’work discussed above,is not possible.Color metaphors for sounds are closely related to the synesthesia between painting and music, the subject of Sect. 7.7.

6.5. Influence of Culture on Color Naming In Sect. 6.1, I briefly explained how, a quarter of a century ago, I hoped to be able to find a direct link between the neurobiology of color vision and color naming. This link is, to say the least, heavily disguised by additional factors. What are these factors? The work of many investigators, myself included, has shown them to be based on cultural influences. I shall now discuss some of these features,starting with a short description of the technique and some results from our own linguistic fieldwork together with examples from investigations by others. We carried out a series of standardized tests on university science students who were native speakers of German, French, English, Hebrew, and Japanese,on art students at German- and Hebrew-speaking art academies,and with Japanese children (12 to 14 years old) at three culturally distinct locations. A slightly different procedure was used with analphabetic adults in Guatemala and Honduras who were monolingual speakers of Kekchi and Misquito, respectively. Each of all these groups consisted of nineteen to fifty-five subjects, with the exception of English speakers (only eight). First, they were asked to write down a number of color terms, which they were to divide into two groups, a) the first group consisting of words considered absolutely necessary for a minimum color lexicon, and b) the second group including words considered to be of secondary importance.The total number of words allowed was arbitrarily set at twelve. Next, the subjects were shown and asked to name a set of 113 to 117 Munsell color samples. This set was made up of 20 Munsell system hues at three to four levels of brightness and three to four levels of saturation. Each sample had to be named within 20 seconds. The subject could describe a particular sample by using a word from his or her chosen color lexicon, or by using any other word(s).If the sample could not be described within twenty seconds, the corresponding space on the questionnaire was left blank. With analphabetic subjects, the test was only made orally without time limit. 141

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A summary of the interpretation of the first part of the test is given in Table 6.2 for the six languages German, French, English, Hebrew, Japanese (all with science students), and Kekchi (with analphabetic test subjects). To keep this table clear, the results with Misquito are not mentioned, and for all other languages, terms mentioned by less than 20% of the subjects are omitted. In every language except Kekchi, the terms for red, green, yellow, and blue were mentioned by over 85% of subjects. Achromatic colors were mentioned less frequently in some European languages (see meaning of ‘color’ in Sect.1.1). Terms for brown, purple, pink, orange, and gray also occurred, albeit with lower importance. These languages are, therefore, Stage-VII languages in the Berlin-Kay typology. In Kekchi, however, only five terms were selected in the naming test: sacc (white), qkecc (black), ccan (yellow), cacc (red), and rax (green and blue). On the basis of this list, Kekchi would clearly be classified as a Stage-IV language. Bearing in mind what was said in the preceding section on white-light-warm vs. black-dark-cool, it is interesting that Kekchi Maya consider their terms for achromatic hues less important than the three terms for chromatic colors.

Table 6.2. Color Terms Considered Necessary and Desirable in Six Languages. Capitals: terms mentioned by 86 to 100% of all interviewees; lower-case letters: mentioned by 20 to 85%. In parentheses: number of interviewees German30 (42)

French30 (31)

E n gl i s h30 (8)

Hebrew32 (19)

J a pan e s e33 (55)

K e kc h i31 (21)

weiss grau schwarz GELB

blanc gris noir JAUNE ocre ORANGE beige BRUN rouge carmin ROUGE rose mauve

white gray BLACK YELLOW

LAVAN afor SHAKHOR TSAHOV

SHIRO kai(-iro) KURO KI

sacc

ORANGE

katom

daidai

BROWN

khoom

cha(-iro)

RED pink mauve purple violet

ADOM varod

AKA pink murasaki

cacc

BLUE

KAKHOL

RAX

GREEN

JAROK

AO(I) kon MIDORI

orange beige braun ROT

violett BLAU GRÜN

142

VIOLET indigo BLEU turquoise VERT

qkecc ccan

tekhelet segol

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It is interesting to mention briefly that the five Kekchi terms for colors correspond to the five colors of the world in the belief of the classical Maya: rax was related to the Tree of the World or the center, cacc to East, ccan to South, qhecc to West, and sacc to North29. The second part, the Munsell chip naming test, was evaluated by plotting the frequency of occurrence of color terms. This is the percentage of subjects mentioning a specific term for a specific sample, over the twenty hues of specific saturation (chroma in the Munsell system) and brightness (value) levels.The certainty of determination was then calculated.This figure corresponds to the sum of all color-terms given to a specific sample, or to all twenty hues at specific levels of value or chroma (also expressed as the percentage of all participants in the respective language). We plotted the series of hues of a Munsell color circle (5R, 10R, 5YR … to 10RP) on constant value and chroma vs. frequency of occurrence. Figs. 6.4 – 6.634 are examples of such plots for the highest level of chroma and value for German and Japanese science students and for Kekchi. Analogous figures are shown for the tests in other languages,with the other social groups of test persons mentioned, and on other chroma and value levels. The certainty of determination, corresponding to the sum of all color terms men-

Fig. 6.4. Frequency of occurrence of color terms with Germanspeaking science students in Zurich

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Fig 6.5. Frequency of occurrence of color terms with Japanesespeaking science students in Tokyo

tioned by the subjects for a specific Munsell sample, can be plotted in the same manner. These three test series appear very different, a result which is not astonishing if we bear in mind the enormous cultural differences between the three groups of test subjects. I shall use them to draw some conclusions and comparisons between these ethnic groups.The results for Kekchi (Fig. 6.6) are characteristic for a language with a small color lexicon. The frequency peaks are well separated and the regions of overlap between peaks observed in a higher Berlin-Kay stage language are not found here.In addition, the certainty of determination is high, especially for rax (blue and green), and somewhat less so for the terms cacc (red) and ccan (yellow). The phenomenon that a term for a certain color may change during the development of a language into a term of the corresponding opponent color was mentioned in Sect. 6.3. It is likely that this effect is present in a weak form in Kekchi color naming: the small move for rax on the left-hand side of the diagram indicates that, respectively, two and one Kekchi called the slightly yellowish red 10R and the orange 5YR rax. Another interesting detail is that 52% of the test group used the Spanish word morado for violet samples although they did not mention this word in the color-lexicon test, and none of these Maya were able to speak Span-

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ish. The term qkecc (black) occurs frequently over the entire hue range, with the exception of the green region, but only for the series of the twenty hues at a relatively impure level (chroma = 4; value = 5).

Fig. 6.6. Frequency of occurrence of color terms with Kekchispeaking Maya in Chahal (Alta Verapaz, Guatemala)

As Kekchi Maya used the word rax for blue and green, we ran an additional test with all 21 subjects. They were asked to arrange 20 Munsell chips, selected from all chromatic colors, into groups of similar hues. The majority of subjects (13) formed five to seven groups. This suggests that they did not classify the color samples on the basis of their color vocabularies, but, rather, by using their color perceptions. In particular, they separated green hues from blue hues.Only two put green and blue chips into the same group. In 1999, Roberson and coworkers21 discovered another type of color distinction in the Berinmo language,spoken in New Guinea,namely two terms not and wor within the yellow chips; this result is not expected on the basis of classical Berlin-Kay categorization. In my opinion, it would be useful to apply MacLaury’s vantage theory to see if brightness differences cause the not/wor boundary. A comparison of the certainty of determination in several languages, age groups, professional backgrounds, and interests is helpful for understanding.A superficial look at Fig. 6.4–6.6 already indicates that the certainty of determination is highest for Kekchi, medium for German, and low for Japanese. The numerical figures for the certainties of determination in these 145

How Do We Name Colors? Table 6.3. Certainties of Determination in Color Tests with Various Groups of Subjects (data from Iijima et al.35, von Wattenwyl and Zollinger31,32, and Zollinger30,33) Language with Berlin-Kay Stage

Locationa)

Characterization of subjects profession, ageb), sex (% male)b)

Certainty of determination

German German German French English Hebrew Hebrew Hebrew Japanese Japanese Japanese Japanese Misquito Kekchi

ETH, Zurich KGS, Zurich KGS, Zurich University, Lausanne ETH, Zurich Technion, Haifa Weizmann Institute, Rehovot Bezalel Academy, Jerusalem TKD,Tokyo Public school,Yonezawa Public school,Tokyo IJS, Düsseldorf Ahuas and Waxma, Honduras Chahal, Guatemala

Science students, 18–25, 80% Junior art students, 17–19, 50% Senior art students, 18–21, 50% Science students, 18–25, 70% Science students (graduates), 23–28, 87% Science students, 19–26, 70% Employees, 20–60, ?% Art students, 19–25, 50% Science students, 18–25, 90% Pupils, 13–15, 40% Pupils, 13–15, 40% Pupils, 13–15, 40% Illiterate men, 25–50, 100% Illiterate men, 25-50, 100%

57.2 ± 31.1 20.0 ± 21.5 14.0 ± 11.0 73.4 ± 28.4 60.6 ± 35.6 56.6 ± 29.6 29.1 ± 24.1 33.6 ± 32.4 43.2 ± 26.8 ca. 27b) ca. 30b) ca. 38b) ca. 40b) ca. 94b)

VII VII VII VII VII VII VII VII VII VII VII VII VII IV

a

) ETH: Eidgenössische Technische Hochschule (Swiss Federal Institute of Technology), KGS: Kunstgewerbeschule (School of Arts),TKD:Tokyo Kogyo Daigaku (Tokyo Institute of Technology), IJS: International Japanese School b ) Estimated

three tests are given in Table 6.3 together with eleven tests with other groups of subjects. First of all, the high standard deviations of certainties in Table 6.3 suggest that important personal differences are probably making themselves apparent, despite the relatively high homogeneity of the subjects within the group. Comparison of the mean values suggests potential causes for these large variations in color naming, found not only in these investigations, but also in several by other authors, irrespective of the test methods applied. The large difference observed between Misquito (ca. 40) and Kekchi (ca. 96) is interesting, because the habitat, sex, age, and level of literacy of the test groups from both Mesoamerican tribes were similar. Very different, however, is the diversity of their color vocabulary. In the corresponding test, Misquitos mentioned seventeen color-terms, including loan words (blu, kafe), whereas Kekchi had only five (see above). The color-sample naming diagrams for Misquito subjects are also completely different from Fig. 6.6 with its fairly clear separation of color-term areas. Misquito diagrams essentially resemble Fig. 6.4 and 6.5 (German and Japanese, respectively). 146

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An explanation for these differences may be found in the statement which I quoted earlier in this chapter for a native of Bellona Island (Solomon Islands, Pacific): ‘We don’t talk much about color here’. The Bellonese language is very difficult to classify in the Berlin-Kay system. Formally it is in Stage II, but as the investigators (Kuschel and Monberg1) stated, it has innumerable ‘color words’, way beyond Western systems, and thus is much more sophisticated36. The position of Kekchi Maya is probably similar to that of Bellona Islanders. In their traditional culture, similarity in all domains is of primary importance, from family life, food, and shelter to social units (villages, tribes),and the senses,including color. Societal change from such cultures to more and more modern societies, which have developed new techniques, materials, and production methods, is characterized by differentiation, leading in color vision to more elaborate color terms. For a Kekchi, it is easy to make a choice between the five colors he has in his language. Only in the unfamiliar situation of color naming in a test, the purpose of which he does not understand, does he use the loan word morado.He is,however,as capable of differentiating hues as we are.This is clearly shown from our color-grouping tests with these Mayan subjects. Science students, test subjects with fairly comparable backgrounds of schooling, professional interests, and age, were studied for five different mother-tongues. The results for native speakers of German, French, English, and Hebrew cannot be differentiated further, although the probability is that French has a higher certainty of determination of 60–70%, as revealed by statistical tests. The certainty of determination of Japanese students is, however, clearly lower (probability: >90%). Drawing on my own experience of Japanese culture, I assume that the tasks in these tests are more difficult for Japanese students than for their Western counterparts. Japanese etiquette requires very subtle and intricate forms of addressing the person to whom one is speaking and is much more important (and difficult) for a Japanese in all situations; this applies also to color naming. Therefore,we compared color naming of Japanese in different cultural surroundings. As a sufficiently large group of Japanese science students is difficult to find outside Japan, we conducted the test with schoolchildren at three locations: in Yonezawa, a town of 100,000 inhabitants in Yamagata Prefecture ca. 250 km north-east of Tokyo, in Suginami-ku (Metropolitan Tokyo), and at the International Japanese School in Düsseldorf, Federal Republic of Germany, where some 400 children, 6 to 15 years of age, obtain 147

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their tuition using the Japanese curriculum.The children had started their first courses in English as a foreign language three to twelve months before we made our tests at these three locations. Those in Düsseldorf did not learn German in school. The certainty of determination increases in the sequence Yonezawa (27) → Tokyo (30) → Düsseldorf (38). Is this effect a function of increasing Western influence? It does indeed seem to be so, because this conclusion can be corroborated by several comparative results in the color-vocabulary test performed with these children. The following numbers refer, in the sequence Yonezawa → Tokyo → Düsseldorf, to the percentage of children who listed the given terms as absolutely necessary: dai-dai (literally = bitter orange) orange-iro (orange-colored)

26 → 6 → 4 4 → 15 → 15

momo-iro (peach-colored) pink

6 → 0 → 0 18 → 40 → 28

The two pairs of color terms are considered synonymous. The first terms of the pairs are genuinely Japanese, the second terms are loan words. The loan words show a clear increase in frequency of occurrence between Yonezawa and Tokyo, but in Düsseldorf the loan word pink is not mentioned as often as in Tokyo. We assume that the reason for this is the fact that English is the dominant foreign language in Tokyo, but in Düsseldorf, of course, German is dominant. The term pink, however, has no German counterpart, and many Germans do not know what pink means. There are other investigations which demonstrate the usurping of colorterms in the traditional language of a population by terms borrowed from English, as shown, for example, by Davies, Corbett, and co-workers37 for Setswana, the main language of Botswana. There, borrowed terms were used mainly by children and by people who had been to school. Setswana is a language which does not fit the original Berlin and Kay scheme,because a term for brown exists although the blue/green separation has not yet taken place. Kay38 described languages spoken on Pacific islands which, measured by older persons’ color terms, had to be classified as Stage V, but by those of the younger generation as Stage VI and even Stage VII. The lower certainty of determination obtained in the tests with the relatively older staff of the Weizmann Institute of Science in comparison with 148

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that for Hebrew science students at the Technion may be due either to the higher average age of the Weizmann employees or to the inhomogeneity of the test group. An unexpected and astonishing result was obtained when we conducted our color tests with art students. Swiss and Israeli art students experience significantly more difficulties in naming a random sequence of color samples within 20 seconds per sample than science students do. On closer examination of the words that art students used to describe these samples, however, the reason for this difference becomes obvious. The color vocabulary showed that art students have a much more elaborate color lexicon. They used modifying additional words for a basic color term more frequently, and, in particular, these ‘modifiers’ cover a much broader spectrum of words. Investigating this aspect in more detail for German-speaking art and science students, we found that 44.0 (±24.4)% of all sample names used by junior art students contained modifiers, against 23.0 (±16.2)% and 26.0 (±18.5)% for advanced art students and science students, respectively. Statistically, there is a significant difference at the 95% confidence level between junior art students and each of the other two groups. It seems reasonable to explain these differences in color vocabulary and certainty of determination between science and art students, both German- and Hebrew-speaking, as follows. Science students have, in general, no dominant personal relationship to color, and, therefore, are not particularly concerned with color names. Art students, in contrast, are inclined rather to describe color impressions, and not only the perception of a physical sensation. It is impossible to convey this impression perfectly within twenty seconds. Thinking along such lines, we might speculate that the decrease in the tendency to modify color words that we saw in the comparison of junior and advanced art students in Zurich might be due to the longer schooling of advanced students. They have learned to use modifiers in a professional way,whereas junior art students use such words intuitively. An analogous case in the art of writing was described by Thomas Mann (1875–1955,Nobel Prize for Literature 1929) in his novella Tristan for a writer: ‘Der Wahrheit die Ehre zu geben, so war dies mit dem ‘Zuströmen’ ganz einfach nicht der Fall, und Gott wusste, aus was für eitlen Gründen Herr Spinell es behauptete. Die Worte schienen ihm durchaus nicht zuzuströmen; für einen, dessen bürgerlicher Beruf das Schreiben ist, kam er jämmerlich 149

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langsam von der Stelle, und wer ihn sah, musste zu der Anschauung gelangen, dass ein Schriftsteller ein Mann ist, dem das Schreiben schwerer fällt als anderen Leuten’ 39. None of our investigations was planned to include the study of gender differences. There have, however, been many such investigations on the use of color-terms, particularly with English-speaking subjects. In a recent investigation on gender differences in China, Yang40 demonstrated that the results obtained in Gansu (northwest China) are similar to those found in investigations in Western countries. College students were asked to name 120 printed color samples (unfortunately not described colorimetrically). Women used fewer basic color terms than men, but more of the elaborate ones. Simpson and Tarrant 41 found that British subjects of either sex used more elaborate terms with increasing age. The discussion of the further development of Berlin and Kay’s basic color terms (Sect. 6.3 ) mentioned the possibility of the formation of new basic color terms. Gellatly42 has stated that ‘color terms evolve to communicate culturally important distinctions’. Terms for bright hues intermediate between blue and green are a good example of one such development.As can be seen in Table 6.2, French students mentioned ‘turquoise’ and Japanese ‘kon’. The historical development of terms for ‘turquoise’hues is interesting.Bolton and Crisp43 made an extensive study of color terms in folk-tales from 40 cultures. They found a significant, positive association between the relative salience of color categories in folk-tales and the Berlin and Kay evolutionary sequence, and confirmed the hypothesis that cultural complexity is associated with the size of the basic color lexicon. Cultural complexity stimulates the development of the basic color lexicon in several ways, among them through the production of a larger variety of objects exhibiting previously uncommon colors,and by extending the range over which trade in these objects occurs. Bolton and Crisp also found, in those cultures where the respective data were available, that technological specialization correlates significantly with the number of color terms. In the 40 cultures investigated (eight of them European), no term for turquoise hues ever occurs except in the Native American Taos culture of New Mexico.Taos folk-tale heritage contains eight references to turquoise hues. The explanation is a simple accident of geology: turquoise stones are frequently found in this part of America and are the most important gems in this culture. Folk-tales are relatively ancient ingredients of a culture, and, to our knowledge, there is no comparable cross-cultural or monocultural 150

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study for the use of color terms in contemporary literature. The word ‘turquoise’ comes from dialects of Italian: ‘turchino’ (= Turkish) was frequently used in previous centuries to describe textile goods in such shades imported from East Mediterranean countries like Turkey. There is, however, one broad investigation by Thurow44, who collected color terms used by over 300 university students in Freiburg, Germany. In the blue-green region of the color solid,‘grün’ and ‘blau’ were mentioned most frequently (376 and 293 times, respectively), followed by ‘hellgrün’ (27), ‘dunkelgrün’ (17), ‘hellblau’ (17), ‘türkis’ (14), and ‘dunkelblau’ (12). Unlike at the period when the folk-tales originated (at least 100 years ago, probably much more), therefore, ‘türkis’ is indeed used in German at the present time. What is the most probable reason for this? It is most likely due to the development of synthetic dyes and their use in dyeing textile fibers, plastics, and other materials. Dyeing of turquoise shades with good fastness properties has only been possible for about 55 years, since the invention of the synthetic dyestuff copper phthalocyanine and its introduction in the dyeing industry. The Scottish Dyes Corporation discovered this particular dye in the 1930s and produced it commercially a few years later. Today, copper phthalocyanine and many of its derivatives are produced by most dyestuff companies in Europe, America, and Asia, with suitable brands available for practically all types of textile fibers.Turquoise became a fashion shade. Should ‘türkis’/‘turquoise’ now be promoted to the status of a basic color term? Since we know today that the definition of this term is more difficult than supposed by Berlin and Kay in 1969, I think we assume a ‘yes’ rather too early. Thurow’s results, and the analogous findings in the colorvocabulary test with French-speaking students (Table 6.2), however, offer sufficient evidence for broader use of this color term since the times when our cultures’ diverse folk-tales were taking shape. An appraisal of ‘turquoise’ or ‘türkis’ using the technique developed by MacLaury in his vantage theory might give us an answer. Purple is another color term culturally important in several respects. Robes dyed in purple were the most highly esteemed textiles of the Assyrian and Babylonian kings, the Minoans of Crete, King Solomon and Israelite high priests, and Roman emperors like Nero, Diocletian, Constantine the Great, and Justinian. The exclusive character of these robes for nobles and for ceremonial use is well attested by the dye’s name Royal Purple (also called Tyrian Purple or Ancient Purple) and by corresponding passages in the Old Testament of the Bible, from the time of the Second Book 151

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of Moses (Exodus 26:1, 31). The social, political, and religious prestige of Purple is the consequence of its economic value. By weight it was always worth more than gold, due to the fact that precursor compounds of Purple are found only in tiny quantities from glandular secretions of several molluscan species (Murex brandaris, Phyllonotus trunculus, Stramonita haemastoma, Fig. 6.7) occurring in the Mediterranean, but also in the Atlantic and Pacific Oceans, in locations such as the west coast of Central America and Australia.In Europe and the Near East in antiquity,the Phoenician coast, in particular the city of Tyre, was the center of the Purple industry, as is documented in the Bible (Second Book of Chronicles 2:7 and Ezekiel 27:7). After ca. 1200 A. D., and particularly with the decline of the Eastern Roman Empire (1453, conquest of Constantinople by the Turks), the Purple indus-

Fig. 6.7. The three dominant molluscan species used for the Ancient Purple industry : Murex brandaris, Phyllonotus trunculus, Stramonita haemastoma (from left to right). Scale in cm. Photo: D. Darom (published by Spanier 45, p. 194) (reproduced by permission of Prof. E. Spanier, University of Haifa)

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try in the Eastern Mediterranean towns declined. There was, however, still demand for prestigious robes in that shade of Purple. For instance, just at that time (1464), Pope Paul II decreed that the gowns of cardinals should be dyed in purple shades. As this was no longer technologically possible, the shades chosen for these gowns were not purple but became an ever more brilliant red. This can be seen in paintings of cardinals since the 16th cen-

Fig. 6.8. El Greco (Domenikos Theotokopoulos, 1541–1614): Portrait of a Cardinal, probably Cardinal Don Fernando Niño de Guevara (1541–1609) (Photograph © 1992,The Metropolitan Museum of Art) (reproduced by permission of The Metropolitan Museum of Art)

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tury (Fig. 6.8) and also in the fabric used for cardinals’ gowns today (Fig. 6.9). Purified Antique Purple is, however, much bluer, as shown in Fig. 6.10. Best historical evidence for the claim that Antique Purple has a bluish violet hue comes from mosaics in the church of San Vitale in Ravenna (Figs. 6.11 and 6.12). These show the Emperor Justinian I (482–565 A. D.) and Empress Theodora with their entourages. Their gowns are violet, and, because (inorganic) mosaic stones hardly bleach or change color even after 1500 years, we can assume that this was indeed the purple hue used when the Phoenician industry was still active. In another mosaic in San Vitale, Jesus Christ also wears a gown of the same color. Fig. 6.9. Sample of fabric used in 1986 by Vatican tailors for gowns of cardinals (reproduced from Textilveredelung 1989, 24, 211)

Later, however, purple hues in mosaics were shown a bluish red. A good example can be found in the Chera monastery (now Kâriye Camii) in Istanbul where a mosaic from ca. 1320 A. D. shows a red wool strand presented to Mary46.In an inscription it is stated explicitly that the wool has a purple color.On the other hand,Goethe in his Farbenlehre (see Ref. 60 in Chapt. 7,Didactic Part, § 792) mentions in the discussion of red hues that he sometimes calls a brilliant carmine purple, ‘ob wir gleichwohl wissen, dass der Purpur der Alten sich mehr nach der blauen Seite hinzog’ (‘…although we know nevertheless that the purple of the Ancients was on the blue side’). According to Greek legend,Ancient Purple was discovered accidentally by the Phoenician god Melkarth, patron of Tyre, when he, the nymph Tyros, and his dog were walking along the shore of the Mediterranean. Playing at the shore, the dog bit into a sea-snail, and afterwards its mouth was stained a purple shade. Melkarth collected these sea-snails and dyed a robe for the nymph.

Fig. 6.10. 3.1% Dyeing of pure 6,6’-dibromoindigo (Ancient Purple) on wool (reproduced from Textilveredelung 1989, 24, 211)

The hypobranchial gland of the three molluscan species mentioned above contains minute amounts of an organic derivative of the dye. Its chemical structure was elucidated in 1909 by the Austrian chemist P. Friedländer, who had to extract the dye from 12,000 snails and isolated only 1.4 g of the pure compound, i.e., 0.1 mg dye per snail. Structural elucidation showed it to be 6,6’-dibromoindigo, and thus a compound closely related to indigo (see formulae 10 and 9, respectively, in Sect. 3.3). Archaeological evidence for the Phoenician purple industry has been found all along the present-day coasts of Lebanon and Israel. On beaches near various ancient towns are hills in the sand which contain tons of opened shells of these snails, residues from the dye extraction. Chemical analyses have identified Ancient Purple on potsherds found near

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the towns of Sarepta (today Sarafand, Lebanon) and Tel Akko (Israel, see Fig. 6.13). Our knowledge of the technology of this industry is due mainly to a detailed description made by Pliny the Elder in the first century A. D.

Fig 6.11. Mosaic of Emperor Justinian I (6th century A. D., Church of San Vitale in Ravenna) (reproduced by permission of Edizioni Salbaroli, Ravenna)

In Hebrew, there are two terms for purple dyeing: argaman for a reddish purple and tekhelet for bluish purple. The dyeing shown in Fig. 6.10 is indeed a bluish purple. Therefore, one might ask if another dye is necessary for the reddish argaman. In my opinion, it is impossible to obtain argaman shades with pure Ancient Purple. However, the dyeing process as used in antiquity by the Phoenicians is still not completely clear to us, despite various investigations. The most modern analytical techniques have been applied by Koren47, who used high-performance liquid chromatography to analyze the dye from an ancient potsherd found in Tel Kabri (Northern Israel) and compared the results with dye extracted and freshly isolated at a nearby beach. Both samples consisted of ca. 36% 6,6´-dibromoindigo, 11% 6-bromoindigo, 2% indigo, and 1% of a reddish dye which was probably indirubin, a dye closely related structurally to indigo and its bromo derivative. If a dyed textile material contained much more than 1% indirubin, it is likely that a reddish purple would be obtained. It 155

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Fig. 6.12. Mosaic of Empress Theodora (6th century A. D., Church of San Vitale in Ravenna) (reproduced by permission of Edizioni Salbaroli, Ravenna)

is, however, very unlikely that indirubin could be obtained from purpleproducing mollusks, either directly or by chemical rearrangement from indigo, with the methods available at the time of the Phoenicians. In my opinion,a red dye from other sources might at that period have been added to a dyeing with Ancient Purple, were an argaman shade desired. The analytical method mentioned above was also used by Koren to detect Ancient Purple in an ancient fabric found at the rock-fortress of Masada, site in 70 A. D. of the last stand of Jewish resistance to Roman rule before its defenders’ mass suicide in preference to final surrender to the Romans in 73 A. D. Fig. 6.13. Potsherd from Tel Akko, showing a residue of Ancient Purple (from Nira Karmon, ‘The Purple Dye Industry in Antiquity’ in ‘Colors from Nature,Natural Colors in Ancient Times’,Eds.C.Sorek and E. Ayalon; reproduced by permission of the Center for Maritime Studies, Haifa University)

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In conclusion, all these investigations show that color terms are not simple reproductions of color perception because language cannot be disentangled from several other aspects of culture.

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References and Notes 11. R. Kuschel, T. Monberg, ‘ ‘We Don’t Talk Much about Colour Here’: A Study of Colour Semantics on Bellona Island’, Man 1974, 9, 213–242. A very instructive investigation into color naming in a culture far removed from Western civilization. 12. B. Berlin, P. Kay, Basic Color Terms: Their Universality and Evolution, University of California Press, Berkeley and Los Angeles, 1969 and 1991. The text of the second edition (1991) is unchanged, but a bibliography is added. 13. This was also the case for linguists outside Chomsky’s school at that time and later, as shown for example by Marshall Blonsky when he edited the book On Signs in 19854: The title of his introduction is ‘The Agony of Semiotics: Reassessing the Discipline. A Crisis of Theory’. Semiotics is the science of signs, i.e., the scientific study of both verbal and averbal systems of communication, whereas semantics is related to verbal communication only. The section on basic principles of semiotics,written by Sebeok (p.45 ff.in Blonsky’s book) is a useful introduction for non-specialists. 14. M. Blonsky, Ed.,‘On Signs’, Blackwell, Oxford, 1985. 15. S. Pinker, The Language Instinct,William Morrow, New York, 1994 (German edition, Kindler Verlag, Munich, 1996). An excellent treatment of modern linguistics for non-specialists. See also the interview with Pinker, in The Third Culture, Ed. J. Brockman, Simon & Schuster, New York, 1995, p. 223–228. 16. Such a program is called a ‘discrete combinatorial system’ (see Ref. 7).Its essence had already been summed up by von Helmholtz,who stated that language makes infinite use of finite media. 17. N. Chomsky,‘Linguistics and Cognitive Science: Problems and Mysteries’, in The Chomskyan Turn, Ed. A. Kasher, Blackwell, Cambridge, MA, 1991. 18. There are, however, some linguists, e.g., R. Fonts, who claim that the beginnings of the use of grammar can be found in the behavior of trained chimpanzees. 19. The postulate of a universal grammar (but not the innate capability of learning language) is, however, doubted by several authors, notably by Searle (1992) and Roth (1994/1997); see Refs. 2, 3, 4, and 6 in Chapt. 8. 10. L. Wittgenstein, Remarks on Colour, Ed. G. E. M. Anscombe, University of California Press, Berkeley and Los Angeles, 1977; also published by Blackwell, Oxford. Both include the original German version, written but not published in 1950–1951. 11. The index in MacLaury’s book12 (p. 602) includes 297 languages. 12. R. E. MacLaury, Color and Cognition in Mesoamerica. Constructing Categories as Vantages. University of Texas Press, Austin, TX, 1997. 13. B. Saunders, ‘Disinterring Basic Color Terms: A Study in the Mystique of Cognitivism’, History of the Human Sciences 1995, 8, 19–38. The word ‘disinterring’ reflects the judgment of the author on Berlin and Kay’s model for color terms. An additional paper in this context is by B. A. C. Saunders, J. van Brakel, ‘Are There Nontrivial Constraints on Colour Categorization?’, Behavioral and Brain Sciences, 1997, 20, 167–228. The paper includes, after the target article, so-called Open Peer Commentaries of 39 authors (including Berlin and Kay) and the Author’s Response.

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How Do We Name Colors? 14. Carl von Linné (Linnaeus, 1707–1778) developed the system of naming plants used worldwide to the present day. 15. Kay, Berlin, and Merrifield, in collaboration with the Summer Institute of Linguistics,initiated the World Color Survey,a systematic evaluation of basic-colorterm usage, which is planned to include field data from some 2500 speakers of 111 languages. MacLaury’s book12 contains results from interviews with 900 speakers of 116 Mesoamerican languages. 16. The word midori appears in Japanese literature for the first time in an eighth century Manyô poem (Nara Period) as ‘midori-go’. There midori means fresh, go is child, therefore midori-go = infant. 17. G. G. Corbett, I. R. L. Davies, ‘Linguistic and Behavioral Measures for Ranking Basic Colour Terms’, Studies in Language 1995, 19, 301–357. 18. P. Kay, C. K. McDaniel, ‘The Linguistic Significance of the Meanings of Basic Color Terms’, Language 1978, 54, 610–646. 19. My statement is related to Karl Popper’s remarks in his autobiography Unended Quest (p. 24, see Ref. 6 in Chapt. 2):‘The quest for precision is analogous to the quest for certainty, and both should be abandoned […] One should never try to be more precise than the problems demand […] It is always undesirable to make an effort to increase precision for its own sake – especially linguistic precision – since this leads usually to loss of clarity’. 20. R. E.MacLaury,‘From Brightness to Hue’, Current Anthropology 1992,33,137–187. 21. E. R. Heider, ‘Universals in Color Naming and Memory’, Journal of Experimental Psychology 1972, 93, 10–20. Berinmo, another language from the same area of New Guinea,was investigated more recently by J.Davidoff,I.Davies,D.Roberson,‘Colour Categories in a Stone-Age Tribe’, Nature 1999, 398, 203–204. 22. For further examples from Latin, see Umberto Eco’s contribution (p. 158 ff.) to Blonsky’s book4. 23. H. C. Conklin,‘Hanunóo Color Categories’, South-Western Journal of Anthropology 1995, 11, 339–344. 24. A.Wierzbicka,‘The Meaning of Color Terms: Semantics, Culture, and Cognition’, Cognitive Linguistics 1990, 1, 99–150. 25. R.Jakobson,Kindersprache.Aphasie und allgemeine Lautgesetze,published 1941, reprinted by Suhrkamp Verlag, Frankfurt a. M. 1969; in English by Mouton, The Hague, 1968. 26. R. Jakobson, C. G. M. Fant, M. Halle, Preliminaries to Speech Analysis, MIT Press, Cambridge, MA, 1951. 27. R. Jakobson, M. Halle,‘Phonology and Phonetics’, in Fundamentals of Language, Eds. R. Jakobson and M. Halle, Mouton, The Hague, 1956. 28. J. M. Williams, ‘Synaesthetic Adjectives: A Possible Law of Semantic Change’, Language 1976, 52, 461-478. 29. M. D. Coe, Breaking the Maya Code, Thames and Hudson, London, 1992. 30. H. Zollinger, ‘A Linguistic Approach to the Cognition of Colour Vision in Man’, Folia Linguistica 1976, IX-1–4, 265–293. 31. A. von Wattenwyl, H. Zollinger,‘The Color Lexica of Two American Indian Languages, Quechi and Misquito’, International Journal of American Linguistics 1978, 44, 56–68. 32. A. von Wattenwyl, H. Zollinger,‘Color Naming by Art Students and Science Students’, Semiotica 1981, 35, 303–315.

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How Do We Name Colors? 33. H. Zollinger, ‘Influence of Cultural Factors on Color Naming by Japanese Children’, Vision Research 1988, 28, 1379–1382. 34. As samples 10G to 5B and 10PB are not available on chroma level 10 (as all others), lower-level samples were used for those hues and the results extrapolated to level 10 (dashed part of curves). ↑ = location of unique yellow, green, and blue. 35. T. Iijima, W. Wenning, H. Zollinger, ‘Cultural Factors of Color Naming in Japanese’, Anthropological Linguistics 1982, 24, 245–262. 36. The authors doubt whether natives react in a ‘natural way’ to color chips, which, of course, have no relation to their daily lives. This criticism holds, of course, for most such investigations. 37. G. Laws, I. R. L. Davies, G. G. Corbett, R. Jerret, D. Jerret, ‘Colour Terms in Setswana’, Language Science 1995, 17, 49–64. 38. P. Kay, ‘Synchronic Variability and Diachronic Change in Basic Color Terms’, Language in Society 1975, 4, 257–270. 39. ‘If the truth were told, this about the rush of words was quite simply wide of the fact. And God knows what sort of vanity it was made Herr Spinell put it down. For his words did not come in a rush; they came with such pathetic slowness, considering the man was a writer by trade, you would have drawn the conclusion, watching him, that a writer is one to whom writing comes harder than to anybody else’. 40. Y. Yang,‘Sex- and Level-Related Differences in the Chinese Color Lexicon’, Word 1996, 47, 207–220. 41. J. Simpson, A. W. S. Tarrant, ‘Sex- and Age-Related Differences in Colour Vocabularies’, Language and Speech 1991, 10, 57–62. 42. A. Gellatly,‘Linguistic and Cultural Influences on the Perception and Cognition of Colour, and on the Investigation of Them’, Mind & Language 1995, 10,199–225. 43. R. Bolton, D. Crisp,‘Color Terms in Folk Tales: A Cross-Cultural Study’, Behavior Science Research 1979, 14, 231–253. 44. M. Thurow, personal communication, 1979. 45. E.Spanier (Ed.), The Royal Purple and the Biblical Blue,Keter Publishing House, Jerusalem, 1987. 46. For a reproduction in color, see J. Gage’s book (Ref. 4 in Chapt. 1), p. 55. 47. Z. C. Koren,‘High-Performance Liquid Chromatographic Analysis of an Ancient Purple Dyeing Vat from Israel’, Israel J. of Chem. 1995, 35, 117–124.

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7 . Color in Art and in Other Cultural Activities 7.1. Color in European Art from Antiquity to Gothic The definition of culture has already been given in Sect. 6.3, in the context of color naming. Color, however, finds its most important cultural role in visual art. That topic is the main theme of this chapter, supplemented with a few sections on color in other cultural activities. In a book claimed by its author to represent an attempt at a multidisciplinary approach to color, it is of course a conditio sine qua non to include a chapter on color in art.All historians and philosophers of art discuss color in their books: is it, therefore, possible that a scientist who has worked for many decades on the scientific and technological aspects of color may also add some meaningful remarks on art? That judgment must rest with the reader. Naturally, I have visited several museums of art for the specific purpose of this book, and have studied some of the (extremely voluminous) literature on art. It is appropriate to comment that I found the most useful and interesting ideas in John Gage’s book Colour and Culture (Ref. 4 in Chapt. 1), presumably not least because Gage included many cross-links (to use a term from chemistry) to scientific and technological problems associated with colors. We know relatively little about color in classical Greek and Roman art. Until the early nineteenth century it was wrongly assumed that antique buildings and sculptures had not been colored.In the medieval period too, stone sculptures were mostly painted. Nineteenth-century misinterpretation of the significance of color in Homer’s works has already been mentioned with respect to that period in Sects. 6.1 and 6.3. Greek literature, however, boasts various writings, poetic and prose, in which color is discussed. Of key significance are the color theories of Empedocles and Democritus (fifth century B. C.), adopted some hundred years later by Plato (in the Timaeus) and Aristotle. For all these early philosophers,color arose from the contrasts between black and white,or dark and light, and from the four classical elements: earth, air, fire, and water. The first chromatic colors mentioned after black and white were red and 161 Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

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ωχηρον (ochron, probably a light yellowish green). It is interesting that these four colors match the first four color names found in Berlin and Kay’s Stages I to III (see Sect. 6.3). They also correspond to the colors of the human body in the theory of Hippocrates. He assigned red to blood, white to phlegm, and yellow and black to two types of bile. Aristotle expanded the color scale to seven in his work Περι αισθησεωζ και αισθηετων,Peri aistheseos kai aistheton (On Sense and Sensible Objects). He placed five colors – red, violet, green, dark blue, and gray – between black and white, stating that they are formed by mixing dark and light in the form of derivatives of either black or yellow, which he called a sweet white. It may be that Aristotle had in his mind an analogy between these seven colors and the seven tones of the diatonic musical octave, already well-known during his lifetime. As mentioned in Sect. 4.2, Newton also speculated for some time about a tentative correlation of his seven colors to the octave, but eventually abandoned the idea. This and other works by Aristotle had a profound influence not only on the Romans, but also much later, in the Gothic period and even during the Renaissance. The most important artist with a particular interest in colors was Apelles, who lived in Greece in the fourth century B. C. Unfortunately, little is known of his life, and none of his paintings have survived. Chiefly famous for his portraits, his works were still important standard pieces during Roman times.Almost two millennia later, Albrecht Dürer was hailed as the altero Apello. Other interesting critical commentaries on color in art come from Pliny the Elder.He lived in Pompeii,perishing there during the eruption of Vesuvius in 79 A. D. As is well-known, many of the wall paintings in Pompeii survived the eruption, and so we may reasonably conclude that Pliny’s critiques concerning the colors popular during his lifetime were influenced by pre-eruption Pompeii.In his book Historia naturalis, he complains that contemporary painters use too many ‘colores floridi’ instead of the traditional ‘colores austeri’.Only the latter,in his opinion,meet the Roman ideal of austeritas. These are white, black, red (rubica), and ochre, whilst the colores floridi are purple, cinnabar, and other brilliant colors, mostly imported from the East (Arabia, India etc.). There was at that time in the Roman Empire a general antagonism between ‘atticus’ and ‘asiaticus’. Its principle battlefield was rhetoric, but it also found expression in painting, characterized there as conflict between simplicity and directness vs. softness and ornamental richness. 162

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Later in this chapter we discuss the dichotomy of color and form during the Renaissance and in later centuries. In this context, it is worth mentioning that prominent representatives of classical Antiquity – Plato, Aristotle, Plutarch, and Philostratus among them – had already expressed various and contradictory opinions on this matter1. The fourth and fifth centuries A. D. witnessed the advent of two very important developments in the use of light and color in art. The first of these was mosaic-work on walls and in vaults, the second was the widespread use of gold and silver leaf,both in mosaics and elsewhere.The word ‘mosaic’ comes from the Latin ‘opus musivum’, suggestive of an etymological relationship to the Muses. Mosaics had been important means of artistic interior decoration in both Greek and Roman culture, but almost exclusively as floor mosaics. The Greeks originally used natural pebbles for their mosaics, but, after about 200 B. C., increasing use was made of regularly cut stones (i.e.,with at least one flat surface). The Romans first named these tesserae, and introduced the use of artificially colored terracotta and glass squares to increase the variety of colors. Unlike classical Greek and Roman mosaics, which adorned both temples and secular buildings, mosaics of the early Christian period were mostly to be found in churches and other religious buildings. Their primary purpose was to give a visual representation of the message of salvation. Symbolically, this message was represented by the use of brilliant colors, not easy to find in natural materials such as the pebbles used in classical mosaics, and so techniques to increase brilliance needed to be developed. One such was to sandwich gold leaf between two tesserae made of glass; another was to vary the angles of the glass surface. As a result of this, an observer standing or sitting in a room decorated with wall mosaics would, on shifting position slightly, see sparkling and shimmering from different parts of the mosaic2. It would,therefore,seem reasonable that an artist of the time would ascribe only secondary importance to the question of which color to choose for a specific purpose, religious or otherwise. The main aim would be to give the observer an impression of intense luminosity. Red was very often used for this purpose,most probably because of its relationship to the brilliance of fire, and also because cinnabar – a very brilliant red – was fairly easy to obtain. The latter reason also applies to blue, also widely used for these purposes. 163

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Red and blue, though, were also selected to draw a distinction between light and darkness. The vault mosaic in the atrium of the entrance hall of San Marco3, i.e., art work of the late medieval period (thirteenth century), provides an excellent example of this. There are certain similarities between these early Christian mosaics and the pointillist technique used by Neo-Impressionist painters during the late nineteenth century (see Sect. 7.2). It is unlikely that the old mosaic artists had such optical effects in mind though, at least for floor mosaics, as their viewing distance is inevitably the height of the observer. Two of the many early medieval mosaics (ca. 540 A. D.) which can be seen in Ravenna are reproduced in Figs. 6.11 and 6.12 in Sect. 6.5 (demonstrating the color of Ancient Purple).They are also excellent examples of mosaics of the early Christian period.The church of San Vitale – not very impressive as a building to a modern-day visitor – contains a huge number of magnificent mosaics. Those reproduced in Chapt. 6 depict Emperor Justinian and Empress Theodora. A third mosaic exhibiting several technical and artistic similarities to those of Justinian and Theodora is the picture of Jesus Christ. All these principal subjects wear purple gowns. My dominant impression when I saw these three Ravenna mosaics (on a cloudy day, sadly) was of the combination of Christian faith, classical Roman and Byzantine style,and also the impressive artistic technique that must have gone into giving the principals such lively expressive faces. The mosaic of Empress Theodora in Ravenna is also interesting from another point of view. The hem of her gown bears a decoration depicting the three Persian Magi, and this offers a good example of textile manufacturers’ keen interest in coloring their fabrics. At that time, textile printing did not exist: all pictorial representations on fabrics had to be accomplished by weaving with different-colored or gold thread incorporated into the main woven network of textile fiber (mainly wool and silk) or by embroidery. Tapestry had already been known for a very long time, and is described in the Bible (Exodus 39,1–3) for the gowns of the officiants and the fabrics used in the Tabernacle. Aristotle remarked that nobody had better knowledge than the clothiers of the harmony and contrast effects obtainable with colors. Allusions to elaborate textile fabrics were often used for explaining symbols of the Christian religion; the Commentary of the Apocalypse by Beatus of Liébana (ca. 1109 A. D.), for example, states that the mystery of the Trinity can be understood by comparing it with the weaving of white wool, where warp, weft, and thrum are combined, and where a brilliant atmosphere is produced by the use of vermilion4, 164

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green, yellow and scarlet, together with red and black threads. It must also be emphasized that Beatus warned against using too many different colors. Textile art reached another pinnacle in sixteenth- and seventeenth-century France, in the manufacture of Gobelins5 and other tapestries. It is rather difficult to fully appreciate old Gobelins today,because,when hung,they have tended to fade even when not exposed to direct sunlight. There is no doubt that the colors of newly made Gobelins were much brighter and more intense then than now. That the various colors used did not fade at the same speed can easily be seen from the blue sections of a Gobelin, which are generally in better color than the remainder. This is due to the fact that the blue threads were dyed with indigo, which has a higher lightfastness than the majority of natural dyes used at the time of Gobelin manufacturing. Another Middle Ages application of color which – like textile coloration – we would not today classify as art in the usual sense, but as applied art, is the colored decoration of books. The rock on which the long and extensive tradition of colored pictures in books is founded is the Bible, Book of Books in the Christian and (Old Testament) Jewish faiths. As several excellent examples demonstrate, the goal not only of the Bible, but of all religious books, is that of enlightenment – a major function of the text of the book. This is reinforced by the inspiring pictures on the front page and in the text, as well as by the large illuminated initial letters of the first word of a new paragraph or chapter. In the centuries when books were handwritten, color was particularly important for all these types of ornamentation. At this stage of book culture, and also after Gutenberg had produced the first printed book (the Bible) in 1455, the text of practically all books was written using black ink, because of its better readability. This black text is not visually inspiring, and so colored ornaments and pictures performed an important function in maintaining the reader’s interest. Isn’t this factor still important today? Coloration in books is also known as miniature painting – not because these paintings are usually small, but because, in the Middle Ages, red lead (Latin: minium) was used for initials,margin adornments,and other ornamentation. Egyptian scrolls and Roman codices were copiously decorated with illustrations, a tradition adopted in Christianity in the monastic scriptoria, both in Western Europe and in the Byzantine Empire.The center of Byzantine book painting was Constantinople, where the influence of Coptic, 165

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Syrian, and Armenian miniature painting was powerfully felt. Book painting in Constantinople degenerated after crusaders sacked the city in 1204, disappearing completely after the conquest by the Turks (1453). In Western Europe, Islamic culture continued to influence Christian book illustration in northern Spain between the tenth and twelfth centuries. The following four reproductions demonstrate the development of the art of book illustration between 600 and 1500 A. D. In the middle of the first millennium,Benedictine monasteries in England and Ireland developed a new style with notable Syrian influence.An example from the seventh century is the Evangeliar of Durrow (Ireland), which contains a symbolic picture of the evangelist Matthew (Fig. 7.1). From Ireland, book painting spread to the Continent with the Irish missionaries. The picture of the evangelist Luke (Fig. 7.2) in the Ada manuscript, written in the Carolinian period, demonstrates the changes in style which had taken place in the two centuries since the writing of the Evangeliar of Durrow. Fig. 7.1. ‘The man – symbol of Matthew’ in the Evangeliar of Durrow (reproduced by permission of The Board of Trinity College Dublin)

In the Early and High Gothic, book painting crept into secular chronicles, poetry,and other books,as well as into ‘books of hours’. An excellent example from the Early Gothic is the Manessische Liederhandschrift (Codex Manesse), a collection of minnesongs (amatory lyric poems) in Middle High German, written in Zurich by several mostly unknown writers and dedicated in memory of knight Rüdiger Manesse (d. 1304) and his son Johannes (d. 1297), both of whom lived in Zurich and were collectors of old minnesong books. Consisting of 426 folio pages on parchment, the Codex was begun in about 1300–1303 by minnesinger Johannes Hadlaub, a citizen of Zurich, and continued by others until 1340. It contains songs, each of them portrayed on a full-page miniature, by 140 poets. This Codex is by far the most important collection of Middle Age minnesongs,and also contains abundant information about life, culture, politics, and religion in the fourteenth century. An example of a page is given in Fig. 7.3. It introduces Bruno von Hornberg, whose lady comes on horseback to his castle and shackles his hands with golden strings.The painter took this scene from two lines of von Hornberg’s minnesong: Miner frowen minne strike, hant gebunden mir den lip (My lady has shackled my body with the bonds of love). The coat of arms of von Hornberg depicts two black horns above a black mountain (Berg) with three peaks.

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Fig. 7.2. The evangelist Luke in the Ada manuscript (ca. 800 A. D.) (reproduced by permission of the Stadtbibliothek,Trier)

Other examples of Swiss book painting are the so-called Chronicles of Berne and Lucerne, written by the historians Diebold Schilling the Elder (d.1485) and his nephew Diebold Schilling the Younger (1460–1520),respectively. Both chronicles contain over 400 large miniatures. The first picture of the Lucerne chronicle is reproduced in Fig. 7.4. and shows Diebold Schil167

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Fig. 7.3. Introductory miniature for Bruno von Hornberg in the Manessische Liederhandschrift,Fol.251r (reproduced by permission of the Universitätsbibliothek of Heidelberg)

ling the Younger presenting his chronicle to the Council of the City and State of Lucerne in 1513. The Swiss Confederation was founded in 1291, initially consisting of the three states of Uri, Schwyz, and Unterwalden. In 1332, Lucerne joined the Confederation, followed by Zurich (1351), Glarus and Zug (1352), and Berne (1353). Eighteen other states followed later. The 168

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Fig. 7.4. Diebold Schilling the Younger presenting his chronicle to the Council of Lucerne (1513) (reproduced by permission of the Korporations-Verwaltung der Stadt Luzern)

council-room windows shown in Fig. 7.4 display (from left to right) the coats of arms of the States of Lucerne, Uri, Schwyz, Unterwalden, Zurich, Berne, and Glarus. Schilling’s chronicle is the most important original source for the history of the Swiss Confederation in the first two centuries of its existence. The Lucerne chronicle was written 58 years after the invention of book printing, thus marking the end of the very long tradition of the art of book writing. In printed books, polychrome decoration of a style similar to that used in hand-written ones was not technically possible for many centuries. In the first decade of the sixteenth century, the chiaroscuro woodcut was invented. It was a significant improvement on classical woodcuts, as it allowed shadows to be drawn using parallel lines, a technique widely used until the early twentieth century for showing relief. By 1494, Augsburg book printer Erhard Ratdolt was already printing using a black line block followed by individual color blocks in red,gold,blue,and olive green. 169

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It was possible to obtain extremely vivid images using the chiaroscuro technique, and it was frequently used by German painters such as Albrecht Dürer, Lukas Cranach the Elder, and Niklaus Manuel Deutsch. In most pictures, however, they used only one color in addition to the black outlining. Despite this technique, book illustration was to lose the importance it had enjoyed before the invention of book printing. In a book on color, we should also stress the dominant role of color in stained glass windows, made either from colored or from painted glass. Very few glass windows have survived from the eleventh century or before. French early Gothic architecture is characterized by larger window openings in the walls, and so colored glass windows appear earlier in French churches and cathedrals than in other parts of Western Europe. The richest – and almost completely preserved – glass windows are those of Notre Dame Cathedral in Chartres, dating from the late twelfth and, mainly, the early thirteenth century. The rich, iridescent colors of the glass were obtained at that time by adding metal oxides to the glass before melting. The famous blues came from cobalt oxide, ruby red from copper oxide, green from iron oxide or copper oxide, purple from manganese oxide, and yellow and brown from sulfur or soot. Faces, drapery folds, and other details were applied by brush on the inner surface of the glass,using a mixture of glass filings and oxides with vinegar or urine. In the early Gothic, glass windows played the part of the symbolic manifestation of the light of the Christian dispensation, and their colors were attributes for the impression of divine light. Early Gothic cathedrals in France are relatively dark compared with their later counterparts in France and England, such as York Minster. Notre Dame Cathedral in Chartres impresses first of all by its large number of stained glass windows in a variety of window types, and by the richness of the colors in all the windows. There are three great Rose windows located over the Royal Portal and the North and South Porches of the transept.Each of these,as well as the countless other windows, is part of a complex iconographic pattern. Fig. 7.5 represents the North Rose, with five lancets below it. The North and South Rose windows are based on Old Testament prophecies of the coming of Christ. The outer semicircles of the North Rose show the twelve minor prophets (the four major prophets are the subjects of the South Rose). In the twelve squares are kings of Judah as ancestors of Christ. The Virgin 170

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Fig. 7.5. North Rose window of Notre Dame Cathedral in Chartres (early thirteenth century) (reproduced by permission of Editions Houvet, Chartres)

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and Child are in the center, surrounded by eight angels and four white doves. In the central one of the five lancet windows is St. Anne, legendary mother of Mary, with her child. In the other four lancets are four priests and kings of the Old Testament (from left to right: Melchizedek,David,Solomon, and Aaron), all of whom are trampling smaller devil figures. The spandrels to left and right below the rose are decorated with heraldry related to Queen Blanche of Castile, mother of St. Louis, regent of France from 1226 to 1234, and 1248 to 1252, and donor of the North Rose window. Our second reproduction (Fig. 7.6) is given for two reasons: firstly, it is an example of a circular representation on an enlarged scale which allows details to be studied, and secondly, it is one or two decades older than the North Rose window of Fig. 7.5. It is part of the Passion window, one of the three lancets of the West facade. This facade was built in the mid-twelfth

Fig. 7.6. Jesus washing his disciples’ feet. Detail from the passion window (Late twelfth century; Notre Dame Cathedral, Chartres) (reproduced by permission of Editions Houvet, Chartres)

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century, surviving an extensive fire in 1194. The medallion in Fig. 7.6 shows Jesus washing the disciples’ feet. Beginning in early fourteenth century Paris, the manufacture of colored glass experienced a number of considerable advances,a development with significant influence on the style of the High Gothic period. Bearing in mind the discussions in this book on various aspects of complementary colors (Sects. 4.1, 4.2, 5.4, 6.4, 7.2, 7.3, and 7.6), it is interesting to note the recommendations in a treatise by Antonio di Pisa (ca. 1400): [In glass painting] ‘take care in painting a person that, if you choose a green color for his dress, the gown is in red or purple and that the cape of the gown is white or yellow’. In the second half of the thirteenth century, glass painting spread to Germany, Austria, Great Britain, Bohemia, and Switzerland. From the fifteenth century onwards, smaller, framed glass paintings and medallion-like sections were produced,fitted into regular windows in sacred and secular buildings. Monumental glass painting, however, lost its importance during the Renaissance and disappeared altogether in the Baroque and Rococo periods. A revival of painted-glass windows has been seen since the late nineteenth century, and so it seems appropriate here to mention the work of one of the most famous glass painters of the twentieth century: Marc Chagall (1887–1985). His best-known glass paintings are the twelve windows in the synagogue of the Hadassah hospital near Jerusalem, depicting the twelve tribes of Israel. Chagall used the four unique hues of yellow, red, blue, and green almost exclusively. It is impossible to reproduce on paper the totality of the impression created by the four sets of three windows viewed from the center of the square room of this synagogue. The same applies for Chagall’s five windows in the chancel of the Fraumünster church in Zurich, a late Romanesque building for which he made the stained glass pieces in 1969–1970. There too, Chagall concentrated on the four unique hues. Fortunately, the Fraumünster Church does have one additional modern stained window in the transverse aisle which can be reproduced,and which is worth mentioning because it is a work by Augusto Giacometti (1877–1947)6. Among Swiss artists, he is accredited with the deepest and most intensive relation to color in oil and glass work. Growing up in the Bregaglia,a remote Swiss alpine valley,he was a born colorist with a strong interest in the early Renaissance painter Fra Angelico (1401–1455), but also in William Turner and in art-historian John Ruskin’s (1819–1900) theoretical treatment of aspects of color. Giacometti is also regarded as the artist who, in 1899, painted the first abstract art picture (see Fig. 7.12 in Sect. 7.3). 173

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His Fraumünster window is shown in Fig. 7.7. In the upper middle are God the Father and Jesus Christ, below them two groupings of five prophets, and, in the lowest row, the four evangelists. On either side are five angels in prayer. The diversity of color is much broader than in Chagall’s work.

Fig. 7.7. Augusto Giacometti. Das himmlische Paradies (The Heavenly Paradise,1930/45;Fraumünster church,Zurich) (reproduced by permission of the Kirchenpflege Fraumünster, Zurich)

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Returning to Chagall, it seems appropriate to read how he interpreted his Fraumünster windows in his inaugural address in 1970: ‘Comme dans ma vie intérieure l’esprit et le monde de la Bible occupent une grande place, j’ai essayé, dans ce travail, de l’exprimer. Il est essentiel de représenter les éléments du monde qui ne sont pas visibles et non de reproduire la nature dans tous ses aspects. En dépit des difficultés de notre monde, j’ai retenu l’amour de la vie intérieure dans laquelle j’ai été élevé et l’espérance de l’homme dans l’amour. Dans notre vie il y a une seule couleur, comme sur une palette d’artiste, qui donne le sens de la vie et de l’Art. C’est la couleur de l’amour7.’ This address, as well as Giacometti’s work, demonstrates a spiritual continuity between Gothic and modern art in their Fraumünster works. It, therefore,seems more appropriate to include Chagall and Giacometti here, rather than in the later section on contemporary art.

7.2. From Renaissance to Neo-Impressionism In a sense it is arbitrary to dissect the development of art into eras encompassing several different artistic periods, rather than treating each period individually. Color in art is the subject of only one chapter in this color book though, and grouping seems more appropriate. This applies particularly for the four centuries between the Renaissance and Neo-Impressionism, as these cover the most important developments regarding color in art. The two keystones of color in art both relate to its complete integration into artistic expression: firstly during the Renaissance and secondly in the Neo-Impressionist school,before abstract art developed and tried to separate form and color entirely. The relationship between form and color in art is a problem debated since antiquity. This was very actively so in Italy from 1390 to the end of the Renaissance,under the title of the disegno versus colore debate.One – but probably not the only – cause was technical in nature: the increasing availability of more brilliant color pigments and the changeover from painting in fresco (on plastered walls) and egg tempera (on panels) to oil painting on canvas. This technique had developed in northern Europe, particularly in the Netherlands, and reached Italy only in 1475, introduced to Venice by immigrant Flemish painters. Oil-based paints have the advantage of being viscous and slow-drying, giving glossy and highly refractive surfaces. The paint layer can be thick or thin, transparent or opaque. Oil-on-canvas allows soft color changes to be executed more easily, whereas in tempera 175

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painting such changes were made with the help of short strokes and dabs, similar to the technique used four centuries later by the pointillists (see end of this section). The works of Carlo Crivelli (born in Venice between 1430 and 1435, died there before 1500) provide examples of this. Italy’s long pre-Renaissance tradition in painting and other arts furnished two important books on painting. In 1390 the Tuscan painter Cennino di Drea (also called Cennino Cennini) published Il libro dell’Arte, and in 1435–1436 Leon Battista Alberti produced the Latin book De pictura, also published in a shorter Italian version (Della Pittura). These two treatises on painting differ from one another in several respects. Cennino’s book is clearly the work of a practicing painter, telling the reader how to paint. Alberti also had (some) experience in painting, but was also interested in theory, his book achieving its greatest fame for the fact that it offers probably the first description of single-point perspective. To obtain as many shades of a color as possible, Cennino recommended using the most concentrated form of a (chromatic) color for the deepest shades. Hence, for a given colored object, this concentrated form would be used for the most deeply shadowed areas of the object. For areas not in shadow, and for lighter objects of the same color, white would be added to the color. Alberti, on the other hand, would freely add either white or black to the concentrated form of a color to change its chromatic intensity8. Following Cennino’s system, the most intense colors will be seen in the shaded areas of a painting. This is not the case, however, if Alberti’s recommendations are followed. Cennino described in detail his technique for painting a face. Firstly, the skin parts should be fixed using a pale green earth pigment, then a pink flesh tone hatched or painted onto this foundation in such a way that the green still shows through in half-tone. Finally, a little of a brown-green shadow color should be added. For painting the faces of young and old subjects, he recommended different types of colors : the pale yolk of a town hen’s egg and the darker yolk of the eggs of country hens, respectively. Finally,Cennino drew attention to the use of cangianti effects,used to enliven surfaces by the addition of white or yellow strokes to light tones, and black,violet,or dark green to darker tones.This technique was widely used by painters from the fourteenth to sixteenth centuries, and remains in use today for richly dyed textile fabrics: heavy silk fabrics, basically dyed in dark hues, may contain some threads in brilliant yellow or red hues. If a 176

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lady in such an evening dress moves, these brilliant threads give an iridescent effect, which is called ‘changeant’ in French. Alberti developed some hypotheses concerning primary colors: ‘Fia colore di fuoco el rosso, dell aere cilestrino, dell acqua el verde, et la terra bigia et cenericcia …. l’biancho e ’l nero non sono veri colori … Et truovasi certa amicitia de’ colori, che l’uno giunto con l’altro li porgie dignità et gratia…’ 9. Alberti’s observation that some combinations of colors may enhance one another when placed side by side was later named the color-chord effect, and was to become the basis of the theory of complementary colors (see Sect. 4.2). Alberti’s technique of adding either white or black to the chromatic color to manage light and shade is the origin of the chiaroscuro (light/dark) technique which developed in the sixteenth century Italian Renaissance. Leonardo da Vinci (1452–1519) found another method for painting a magically dark atmosphere. Called sfumato (smoke-like), it combines soft shadows with finely modeled forms. This is only one of the many achievements of Leonardo, probably the most multi-talented genius who has ever lived. As well as painting the Mona Lisa, the most famous painting in existence, this true ‘uomo universale’ accomplished great works as an artist, architect, scientist, and engineer. Most of his work in design and engineering is preserved only in sketches, but there are some 7000 of them. They also contain this observation about colors: ‘la pittura è composizione di luce e di tenebre insieme mista con le diverse qualità di tutti i colori semplice e composti’10. Let us return to Cennino. He came to Florence in 1467 at the age of fifteen and stayed there for fourteen years, then spending another fourteen in Milan. At the peak of his career he returned to Florence (1500–1506), and then moved to Rome for four years. Finally, he lived for two years at the castle of Cloux in the Loire valley where he was invited by King François I of France as ‘premier paintre, architecte et méchanicien du Roi’. We can only speculate about the influence Cennino and Alberti’s two books had on fifteenth-century Italian painters. Literacy and dissemination of information were much more restricted in those days, and so it is not surprising that direct indications of their influence are very few in number or entirely absent. The main center for painting in the fifteenth century was clearly Florence. Slowly but steadily though, Venice was also emerging as a center for art; 177

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its position on the Adriatic and its profitable trade with the eastern Mediterranean, Arabia, and India made precious colored pigments available to Venetian painters. First among these was ultramarine blue (Azzurrum ultramarinum), which is obtained from Lapis lazuli, a rare aluminium-silicate mineral containing some sulfide11. It came from present-day Afghanistan and the northern Hindu Kush. Marco Polo brought it to Venice for the first time following his travels in the Far East (1298). It seems likely that the late fifteenth and early sixteenth century development of the Venetian school of painting was influenced more strongly by this commercial consideration than by the books of Cennino and Alberti. The availability in Venice of this brilliant ultramarine blue was obviously a reason why Titian so very often used it, and it was a powerful factor behind him becoming such a master of coloration, as already mentioned in the introductory chapter. Bacchus and Ariadne (Fig. 1.1) is a painting from his early period (1523). The ultramarine used for the sky is the purest found in any painting examined to date at the National Gallery in London12.Yet it is not only that radiant blue which catches the eye, but also the brilliance of the other pigments used. It is the overall harmony of all the colors which is impressive, as seen, for example, in the yellow of the fabric in the left foreground. In the highlighting, it also enhances the brownish skin in the center, and returns in the greenish brown of the leaves of the first tree in the right corner, where the brownish shadow gives way to the sunny green tree behind. That lower diagonal is paralleled by the main diagonal from Ariadne to Bacchus. It is full of movement, from Ariadne’s feet to the flattering cloak of Bacchus. Ariadne’s blue mantle slices through the red of her ribbon and makes a bridge to the blue sky. The whole picture is a typical example of Titian’s sure-footed approach in bringing colors as the most important element into the composition of a painting – a style new in 1523. Titian was the first to endow colors with such a high intrinsic value. From about 1530 to 1545 though, he reverted temporarily to Mannerism, a phase of style between late Renaissance and early Baroque, characterized by frequently affected movements and unquiet colors. Its main practitioner was El Greco in Spain (see Fig. 6.8).After 1545, however, the glow of colors returned into Titian’s painting style. His portraits display a superior knowledge of human nature, and the colors in his later paintings seem to break away from the objects depicted. In late fifteenth and early sixteenth century Florence, the leading artists were clearly Leonardo and Michelangelo Buonarroti (1475–1564). The lat178

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ter grew up and found fame as a sculptor, painter, architect, and poet in Florence, his residence until 1534, albeit interspersed by several sojourns (some lasting several years) in Rome. The last 30 years of his life he spent in Rome. For this book, it is his paintings that are important. From Michelangelo’s paintings of people, it is easy to see that he was a sculptor first and foremost.As a painter, he is known mainly for his monumental fresco wall and vault pictures; he did not like oil painting. Most famous is his work on the vault of the Sistine Chapel in Rome, which in recent years has been very carefully restored, bringing back the brilliance of the colors as Michelangelo originally painted them between 1508 and 151213 , and maybe throwing doubt on some conclusions to be found in the extensive earlier literature. Comparison with Titian’s contemporary work, such as Bacchus and Ariadne (cf. Fig. 1.1), highlights Titian’s use of a larger number of brilliant colors in harmonies and contrasts, whereas Michelangelo’s work, at least for this non-specialist, creates dramatic, even mysterious, impressions by the use of a smaller number of colors. As several art scholars have already remarked, consideration of Michelangelo’s restored paintings is likely to diminish further the significance of the disegno versus colore debate. That quarrel started because of Titian’s method of working. He began his paintings directly on canvas, with a brushful of color applied in heavy strokes, and never hesitating to put a second layer in another color on top of it later if he wanted to change something. This modus operandi was in marked contrast to those of other painters who first drew one or more rough sketches of the whole subject of interest or of important parts of it.This can be seen very well in many of Leonardo’s previously mentioned sketches. In the sixteenth century, discussion of these two modes of painting developed into something like intellectual duels or swordfights, particularly in the art academies of Florence and Venice. Slowly but steadily, Titian’s method gained ground in Italy and elsewhere in Europe, as can be seen in the works of Baroque painters like Peter Paul Rubens (1577–1640), Diego Velázquez (1599–1660), and Rembrandt (Harmenszoon van Rijn, 1606–1669). The dispute flared up once more in the seventeenth century, not in Italy, however,but in France,in altercations between the so-called Rubenists and 179

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the Poussinists,disciples of French painter Nicolas Poussin (1593/ 94–1665). Even though Poussin lived in Rome for 39 years,and is known to have worked with a rich palette of different colors14, he considered color to be secondary in painting. His style was more formal and less vivid than that of Rubens. It is well-documented in many developing movements, both in art and in science, that the followers have more extreme opinions than the masters. This seems to apply for the French Poussinists, as can be seen in a 1672 statement by Charles Le Brun, President of the Académie Française and advisor and functionary to King Louis XIV for all royal matters relating to art. He said that while pictorial representation without colors can endure, color without drawing does not. It seems that the Poussinists’ opinion was strongly influenced by the rationalism of Descartes, for whom the form of objects was ‘claire et distinct’ , but their color was not. The relationship between drawing and color once more became a cause of dispute in early-nineteenth-century France. Disagreement this time was between painters Jean Auguste Dominique Ingres (1780–1867) and Eugène Delacroix (1798–1863). Ingres considered drawing ‘la seule honnêteté dans l’art’ (the only honesty in art), describing color as ‘la dame d’atours puisqu’elle ne fait que rendre plus aimables les véritables perfections de l’art’ (color is the court lady, since it does no more than render more charming the true perfections of art). Delacroix was a many-faceted painter, well acquainted with complementary colors and with color contrast. According to his close colleague Charles Blanc, author of influential art books and editor of an art journal, Delacroix possessed total mastery of the mathematical rules of colors. On the other hand, Delacroix himself wrote that he had a ‘horror of the common run of scientists […] they elbow one another in the antechamber of the sanctuary where nature hides her secrets, and are always waiting for someone more able than themselves to open the door a finger’s breadth for them…’15. Delacroix created many monumental wall and vault paintings in Paris during the 1840s and 1850s (the Louvre, Palais Bourbon, Palais Luxembourg, libraries of the French government etc.).All are characterized by voluptuous combination of colors. He was also known for the importance he attached to the number and arrangement of the colors on his palette. He used to work on several paintings at the same time, but had a separate palette for each painting, with the colors in different arrangements16. From my personal point of view, J. M. William Turner (1775–1851) was the nineteenth-century painter most fluent in using colors to express an aston180

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ishingly diverse spectrum of moods. Favoring his natural surroundings for the subjects of his paintings, he once remarked that ‘I did not paint … to be understood’,revealing his awareness that his viewers’reactions might be very different from his own. He was highly prolific, as can be seen from the fact that after his death his estate included more than 19000 drawings and sketches. Besides a large number of oil paintings, he painted many watercolors on travels in France, the Swiss Alps, Italy, and Germany. Turner’s style is characterized by its painstakingly conscientious combination of light-and-shade effects with subtle use either of complementary colors or of variation in one dominant (often primary) color. Paramount, however, is his fascination with light effects, particularly transitory ones. His paintings featuring clouded skies, rainbows, snow storms, sunrises and sunsets, smoke and fire provide the greatest examples of these. Two Turner paintings are reproduced in Figs. 7.8 and 7.9. ‘Rain, Steam and Speed – The Great Western Railway’ contrasts with ‘The ‘Fighting Téméraire’ Tugged to her Last Berth To Be Broken up’.

Fig. 7.8. Turner. Rain, Steam and Speed – The Great Western Railway (1844; National Gallery, London) (reproduced by courtesy of the Trustees, The National Gallery, London)

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Fig. 7.9. Turner. The ‘Fighting Téméraire ’ Tugged to her Last Berth To Be Broken up (1838; National Gallery, London) (reproduced by courtesy of the Trustees,The National Gallery, London)

Both paintings must be seen in the context of the growth of British industry in the first half of the nineteenth century.The railway painting,though, is future-oriented,while that of the Téméraire is preoccupied with the past. The railway emerges from a rainy background (the past), thanks to steam it gains speed on the bridge, where it is held safely by the iron tracks. Only the engine can be seen clearly, with its chimney dominating; the following wagons must be imagined. Turner was the only really great artist to depict these technological wonders in the first decades of their existence. In the maritime picture, it is also a chimney, that of the tugboat, which dominates. The tugboat sails in front of the veteran warship ‘Fighting Téméraire’, still a sailing ship at the time of Turner’s painting, but decommissioned from the navy. Built in 1798, she had fought in 1805 in Nelson’s defeat of the French at Trafalgar17, the victory which was to ensure British naval supremacy for over a century. It is likely that Turner’s choice of sunset for his setting of the Téméraire was not merely for coloristic reasons, but intended as symbolic,as a nod to the technical revolution in shipbuilding at that time. It is said that Turner called the Téméraire ‘my darling’.

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Both pictures also demonstrate the rarity of sharp contrasts of color or light/dark tones in Turner’s paintings. The sharply delineated chimneys are the exceptions. These exceptions were probably intentional, in order to emphasize their importance in the ensembles of the whole pictorial composition. The railway was painted six years after the Téméraire. This sequence provides one good example of the ever decreasing representation of real objects in Turner’s later works. This development in Turner’s work was one of the major influences on Impressionism, which became the dominant style in France in the second half of the nineteenth century. Claude Monet (1840–1926) was the first to be impressed by the atmospheric virtuosity of Turner’s art18. However, it was only in the work of certain representatives of Impressionism that color became separated from objects. There are, however, important differences between Turner – he might be called a progressive representative of Romanticism and a precursor of the Impressionist artists – and the true Impressionists.As already mentioned, sharp color and tone contrasts are found only occasionally in Turner’s work. Practically all Impressionist painting, however, makes use of polar contrasts of chromatic colors, mainly complementary colors. Although Turner and the Impressionists were mainly outdoor painters, there is an interesting difference between them. As mentioned, Turner often depicted atmospheres (in a very general sense), and so tended to concentrate on the far distance, neglecting the foreground. The Impressionists, however, gave most of their attention to objects in the immediate foreground and used the background as an eye-catcher very rarely. Impressionist painters very often depicted the same scene several times. This is particularly characteristic of Monet. In his work he evolved series of many – dozens and more – pictures of particular scenes: showing haystacks, poplars, and, finally, scenes in his garden in Giverny, particularly the waterlilies in the pond. He did not limit his chosen themes to works of nature, but also made extensive use of such human creations as the west front of Rouen Cathedral and the Japanese bridge over the pond in his garden. The individual paintings from any of these series are not preliminary sketches which he wanted to improve to his satisfaction, but detailed studies in the changes of appearance of a given object resulting from changes in the light. It is, therefore, inappropriate to view the haystacks as unimaginative subjects for an artist, as can clearly be seen on comparing various 183

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paintings of the haystacks series with one another. In this context, Monet’s deep admiration of Turner’s atmospheric virtuosity is not surprising . His pictures of waterlilies were so famous at the beginning of the twentieth century that Prime Minister Clemenceau, with whom Monet had been on friendly terms for many years, suggested to him that he present a series to the French nation. He did so at the age of 78 on the occasion of the 1918 Armistice. His donation consisted of a quadriptychon and a triptychon, which were put on public display at the Museé de l’Orangerie in Paris after Monet’s death in 1926. Each section of the triptychon and the two middle sections of the quadriptychon are 200 x 425 cm in area, the side sections of the quadriptychon 200 x 200 cm. Other large waterlily paintings can be found in several places around the world: in Nantes, Zurich, Richmond, Los Angeles, and Tokyo, for example. As an example of Monet’s oeuvre, I have not chosen a painting from one of the series mentioned, but one of the two pictures of his stepdaughter, who modeled outdoors for him (Fig. 7.10). This painting shows characteristics not typical merely of Monet, but of Impressionists in general: the sunny open-air atmosphere with a few white clouds in the blue sky, a very colorful foreground with green/red and green/brown/orange contrasts in the plants in the sunny part of the meadow, with the only outlined contrasts in the shaded part of the meadow. The diagonal from lower left to upper right indicates contrasts more subtly, from the dark green grass to the reddish orange shadows of the white dress billowing along the diagonal to the yellowish green umbrella. Eleven years earlier, Monet painted a similar picture which he called La promenade. La femme à l’ombreille. The Dutch painter Vincent van Gogh (1853–1890) moved to Paris in 1886, associating with Impressionist circles. Two years later he departed Paris for Arles in Provence. His experiences in southern France inaugurated a completely new artistic style in his paintings of landscapes and townscapes, and also in his interior scenes and portraiture. Characteristic is the vivid and very strong mode of expression, the colors intensified and applied in very broad brushstrokes.He also made extensive use of extremely sharp complementary color effects20. Still another style to develop from Impressionism was that founded by Camille Pissarro (1830–1903), Georges Seurat (1859–1891), and his disciple Paul Signac (1863–1935). Seurat called it Divisionism, but also used the terms ‘peinture optique’ and ‘chromo-luminairisme’. Today this style is 184

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Fig. 7.10. Monet (1840–1926), Femme à l’ombrelle tournée vers la gauche (Open Air Study, Woman Facing Left; 1886, Museé d’Orsay, Paris) (© SPADEM 1986;reproduced by permission of the Réunion des Musées Nationaux, Paris)

frequently called Pointillism21 and is regarded as part of Neo-Impressionism. Pointillism is essentially based on a technique in which the whole picture is composed of small dabs of paint, so-called ‘taches’, in different – but predominantly brilliant – colors.Pointillist artists totally rejected any sub185

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tractive mixing of palette pigments. Descriptions of the palettes used by Seurat and Signac can be found in the literature; in 1885 Seurat was using one made up only of red, yellow, blue, and white, yet shortly before his death he was favoring one with three rows, the first consisting of eleven unmixed brilliant chromatic colors in spectral sequence, the second with the same pigments but mixed with white, and the third comprising eleven portions of white as reservoirs for mixing white with chromatic colors in other proportions. Black did not exist on this palette (most Impressionists used very little or no black). Seurat applied taches as dots, Signac as rectangular strokes, while Italian painter Giovanni Segantini (1858–1899), who worked mainly in the Engadine Valley in the Swiss Alps, preferred straight parallel lines of about a centimeter in length. Adjacent spots are never the same; there is a contrast either in tone or between complementary colors. Pointillists claimed that this technique made the surface of the paintings livelier, and that its use of juxtaposed complementary colors enhanced them all, rather than yielding the gray which, as had long been known, would have been the result of mixing them in the traditional manner. Seurat’s Une baignade à Asnières (Fig. 7.11) is interesting because he painted it in 1883–1884, shortly before he began to apply his new technique. In 1887 he incorporated some Pointillist features into it, using the technique to darken the water to the right of the three boys and lighten it behind them. The first and abiding impression of this painting is of the light: full but soft, pearly light in a generally quite hazy atmosphere. The blueness of the Seine is light, but it is not the same blue as the lighter sky. In a sense, it could quite legitimately be said that the general atmosphere of this picture is cool and fresh, but personally I can also feel the hot summer’s day, no refreshing breeze, typical of the climate of the Ile-de-France. The middle boy is lethargic and tired because of the heat (look at his face), the two others are glad to be in the cooling water. The heat is also reflected (almost literally) in the orange-tinged buildings in the background. The complementary contrast of the bluish river and these buildings may symbolize the cool/warm contrast in the painting. The coolness of blue can also be seen in the dark shadows of the trees on the other bank of the river. The tonal gradation interrupting the local colors is a technique often used by Delacroix. Seurat’s key work in the Pointillist style, however, is Un dimanche aprèsmidi sur l’île de la Grande Jatte, on which he worked for two years, and for 186

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which he made several preliminary drawings and color studies in oils22. The whole picture consists of an immense number of dots; even the rectangular border is painted using this technique, its hue echoing that of the adjoining parts of the painting’s subjects.Unfortunately,Seurat used a particular synthetic yellow pigment for an important part of the picture. Relatively new at the time, it has turned brown over the last hundred years, and so the work no longer reflects Seurat’s original intentions.

Fig. 7.11. Seurat. Une baignade à Asnières (Bathers in Asnières 1884–1887;National Gallery,London) (reproduced by courtesy of the Trustees,The National Gallery, London)

Viewers of the original of this picture (and all other Pointillist works) will soon become aware that the colors change depending upon the viewing distance. At a short distance, the dots and their colors are individually identifiable. From further away, however, the juxtaposition of neighboring complementary color dots results in light and dark grays,as colorimetric principles predict. It is the same effect that we discussed in the chapter on color processing in the brain (cf. Sect. 5.4), observed at the end of the Engadine Ski Marathon, when the ten thousand or more skiers come in closer to the spectators on the shore of the frozen Lake of Sils. Seurat claimed that his technique was developed on scientific principles. Color187

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imetrically and neuroscientifically, the phenomenon is partially understood today, but debate still continues as to whether his claim was justified or whether this basis was added a posteriori. In his writings, Seurat makes reference to the scientific work of Chevreul, the leading midnineteenth-century French color scientist. Seurat only met him in 1884, however, when he and Signac visited the grand old master in his ninetyeighth year. There is no doubt though, that of all of the Impressionists and Pointillists, it was not Seurat, but Pissarro and Signac who were most concerned with color theory, including work done after Chevreul. The problem of the mutual relationship between color science and color in art,both in general and for Neo-Impressionist art specifically, was investigated by Gage (see Ref. 4 in Chapt. 1), while Signac is the subject of a monograph by Ratliff 24. I agree with Gage who sees an ‘irreconcilable antagonism between … the artisan and the intellectual conceptions of the history of art’, and also with Mollon25 who, in a rather critical review of Ratliff ’s book, wrote ‘But why should contrast – of color, lightness or texture – be so pleasurable to the eye? To this day, visual scientists have no secure answer; and we should be ready to admit it’. I close this section with a Gage quote on Pointillism (see Ref. 4 in Chapt. 1, p. 176), which makes a good link to twentieth-century art, the next section of this book.‘So far from making the beginnings of a scientific aesthetic, the optical concerns of the Neo-Impressionists signaled its demise, and helped to usher in that disdain for the methods and discoveries of the natural sciences which has had important consequences for the painterly study of color in the twentieth century.’

7.3. Color in Twentieth-Century Art It is not altogether inappropriate to see a major change in the position of color in art at the turn of the century. Neo-Impressionism, particularly Pointillism, came to a halt in the last years of the nineteenth century and, as we shall see, some important artists of the first years of the twentieth century became interested in an art devoid of chromatic color. This view, however, oversimplifies the historical development. Besides the German Expressionists, at least two French artists were perceptually sensitive to the role of color in art, and together these bridge the apparent gap between color in Impressionism and color in modern art. The Frenchmen are Paul Cézanne (1839–1906), whose main activity was during the nine188

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teenth century, and Henri Matisse (1869–1954), both from the French Impressionist tradition. As mentioned in the preceding section,van Gogh’s work after leaving Paris and the Impressionists was characterized by very sharp color contrasts. Cézanne, however, did not use the whole chromatic spectrum, or complementary pairs of hues,but focused on a nuanced range of neighboring colors. His deep relationship with color is reflected in his statement that the true nature of man is in the innermost recesses and that colors are the expression of this depth at the surface when they rise from the roots of the world. Matisse went a step further as indicated in one of his notes: ‘I cannot copy nature in a servile way, I am forced to interpret nature and submit it to the spirit of the picture’(English translation,quoted from Gage,Ref. 4 in Chapt. 1, p. 211). The best example of this attitude is the well-known picture of his atelier (1911). He painted its wall originally in a bluish gray which was close to its actual color, but later he painted it over in a vivid red and called it ‘L’atelier rouge’. There is also a pink version of the picture. The German Expressionists sought to surpass mere perception of reality and express their emotional responses to objects, scenes, and events, which they did using glaring colors applied over large areas and deformations of forms. Expressionism began with the group ‘Die Brücke’ in Dresden (1905–1913), one of its founding members being Ernst Ludwig Kirchner (1880–1938).It is interesting to compare an aspect of Monet’s work with some of Kirchner’s later paintings of snowy landscapes, created during the years he lived in Davos in the Swiss Alps (1917–1938). In Kirchner’s works the snow in shadowed areas may be reddish,pink,or blue,but appears only occasionally in the complementary colors favored for shadow in Monet’s paintings, like that of his stepdaughter (Fig. 7.10) or the studies of the West facade of Rouen Cathedral. In contrast to Monet’s work, color in the work of Kirchner and other Expressionists is set free from reality. In the painting Wildboden im Schnee ([The hamlet of] Wildboden in the snow, 1924, Kirchner Museum, Davos), we see greenish and bluish snow in the shade, orange-brown snow in the sunshine. The walls of the wooden houses are pink and green, many of the fir trees violet, with a few green ones and one orange. As well as these developments championed by Cézanne, Matisse, and Kirchner in the use of color in art, an idea expressed by Cézanne and concerning form was to be important for twentieth century art. In his later 189

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Fig. 7.12. Augusto Giacometti. Abstraktion nach einer Glasmalerei im Musée de Cluny (1899, Kunstmuseum Chur, Switzerland)

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years he said that the goal of the representation of nature in art is creation based on simple geometrical bodies like cylinders,spheres,and cones.This statement makes him a predecessor of abstract art. The first painter to paint non–representational works using only color was the Swiss Augusto Giacometti, whose work in stained glass we discussed in Sect. 7.1. At the age of 22, during a sojourn in Paris (1899), he created his Abstraction after a glass painting in the Musée de Cluny (Fig. 7.12) in pastel on paper26. He continued with abstract color studies until the 1920s.

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Non-representational painting of forms, but without color, first appeared in 1907/8, championed by Pablo Picasso (1881–1973) and Georges Braque (1882–1963). Based on Cézanne’s ideas mentioned above, this was the beginning of Cubism. Shortly afterwards (1913), the Russian painter Kasimir Malevitch (1878–1935) claimed that, in fine art, total perception and not mere duplication (of nature) should have supremacy. Therefore, he named that basic art Suprematism. It is not my intention here to hold a general discussion of these and related movements in twentieth-century art history, but only to look at their relationship to color. Interesting statements on the position of color can be found in both Malevitch and Braque. In his book on Suprematism, Malevitch wrote, ‘In the logically consistent development of Suprematism color will also disappear. A black and white phase will set in, based on squares of black and white, to which later as color red will be added.’28 Isn’t it remarkable how this statement chimes with Berlin and Kay’s much later color-term evolutionary theories? In an interview with Vallier30, Braque commented on the development of the rediscovery of color in Cubism. He said that, at the beginning (1907), color played no role at all either for him or for Picasso, and only later did it strike them ‘that color has an independent effect on the form, … as it were something like music.’ In his later collages, he was able to separate color from form, and ‘used it as a definite form for showing that color is independent of the object.’28 During his Cubist period (1908–1914), Braque used mainly black and brown on a white background, sometimes with a little blue. After the First World War, he returned to the representational art of still lifes, yet with a coloration restricted to only a few hues. It is well-known that Picasso’s early work was colorful. First he preferred blue. This so-called ‘période bleue’ (1901–1904) was interpreted by the psychologist Jung as a nekyia,a mythic ride to hell. Afterwards,he used a vivid range of colors in the ‘période rosée’ before inventing Cubism with the ‘Demoiselles d’Avignon’ in 1907. This painting is characterized by split forms and colors. Further developments in his style led first to complete abstraction, but after his Cubist period he once more used a broader palette of colors,as shown,for example,in ‘The Three Musicians’(1921)31.Cubist planes are still much in evidence, but now they are in red, yellow, and blue on black, white, and a variety of browns. His famous work ‘Guernica’, which Picasso painted for the pavilion of the Spanish Republic at the 1937 191

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World Exhibition in Paris, and in which he depicted the mass murder of Guernica’s population in the Spanish Civil War, clearly belongs to his PostCubist period. It is polychromatic, but, like ‘The Three Musicians’, on a dark background, a feature typical of many Spanish painters over several centuries. An early commentary on Picasso’s style can be found in Kandinsky’s book Das Geistige in der Kunst (On the Spiritual in Art32, p. 52). ‘Picasso scheut vor keinem Mittel zurück, und wenn ihn die Farbe im Problem der rein zeichnerischen Form stört, so wirft er sie über Bord und malt ein Bild in Braun und Weiss. Diese Probleme sind auch seine Hauptkraft. Matisse – Farbe. Picasso – Form. Zwei grosse Weisungen auf ein grosses Ziel.’33 Wassily Kandinsky (1866–1944) actually painted his first non-representional picture as early as 1910. His style is characterized by geometrically confined monocolored planes and by guiding lines. Their combination produces impressions of space and motion which work together harmoniously. This effect reflects Kandinsky’s interest in color psychology, which is also a major theme of his book.The subject of Das Geistige in der Kunst32 is a modern color theory based on the experience of a practicing artist with a strong interest in psychology and in synesthetic phenomena. In a footnote he says ‘Alle diese Behauptungen sind Resultate empirisch-seelischer Empfindung und sind auf keiner positiven Wissenschaft basiert .’34 As a natural scientist, I cannot but support Kandinsky’s attitude as expressed in this modest footnote (isn’t it true that very important statements are often only mentioned in footnotes?). As I shall discuss briefly in the epilogue of this book, there are aspects of the phenomenon of color which do belong to science,but others belong only to Kandinsky’s ‘seelische Empfindungen’. For me it seems clear that it is inappropriate to try to explain some (not all) aspects of one side of color using the arguments of the other side and vice versa35. The style of the slightly younger Swiss painter Paul Klee (1879–1940) is related to that of Kandinsky. His paintings are often characterized by subdued colors, fantastic figures, and forms which, although abstractions, are still representational. His figures and forms often depict dreamlike processes, or they show a cheerful and charming irony. Klee liked to depict moving motives, combined with arrows or letters. Like Kandinsky, Klee was interested in color theory, and was also a teacher at the Bauhaus. Founded in Weimar by Walter Gropius in 1919, the Bau192

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haus was a college for the fine arts under the leadership of architecture and combining arts and crafts. During its existence (later in Dessau and Berlin), the Bauhaus had a very profound influence on the development of arts in Germany and beyond. Color theory was taught by Klee and Kandinsky, and by the Swiss artist Johannes Itten, undergraduate (Vorlehre) Dean. Itten’s textbook Kunst der Farbe36 is based on his activities at the Bauhaus. Kandinsky, Klee, and Itten were also much interested in music and its relationship to color (see Sect. 7.7). The Bauhaus was closed in 1933 when the Nazis came to power in Germany. This very brief (and hence superficial) discussion of the developments in abstract art pioneered by such artists as Giacometti, Braque, Picasso, Kandinsky, and Klee does serve to demonstrate that these trailblazers either never wholly severed contacts with representational art, or later resumed contact,albeit subliminally37.The style of Dutch painter Piet Mondrian (correctly Mondriaan,1872–1944),however,progressed in steps from Cubism (in 1911) to ever greater degrees of abstractness. He reduced the elements of his pictures into an extreme state in his Compositie met twee lijnen (Composition with two lines, 1931), from which no further abstraction seemed possible. This picture consists of two straight black lines, one horizontal and one perpendicular, crossing on a large white square. Most of his paintings, however, are not reduced to as few elements as that, but use color for some of the rectangular spaces (Fig. 7.13). Mondrian exclusively used the primary colors red, yellow, and blue – always in unique hues38. He had a very strong aversion to green, and after 1930 – as far as I know – never used it. In 1940, he moved from Europe to New York. In his last paintings, he experimented with a chromatic fragmentation of the horizontal and perpendicular black lines into a framework of rectangular sequences in white, black, gray, red, yellow, and blue. These he called Broadway Boogie-Woogie and Victory Boogie-Woogie (1943/44, unfinished). The dynamics and rhythmics of these paintings are rooted in Mondrian’s taste for jazz dancing; the title of the second work is undoubtedly a reference to the then foreseeable victory of the Allied Forces in the Second World War. Commenting on Mondrian’s work, Albrecht 27 sees a relationship between Mondrian’s colors and early stages of consciousness, as expressed in the preference for primary colors (including black and white as primary colors) encountered in so-called primitive cultures and in folk art. Obviously, the individual in these cultures has no urge to distinguish himself from the general populace by a distinct choice of colors. 193

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Fig. 7.13. Mondrian. Compositie met rood, geel en blauw (Composition with Red, Yellow and Blue; ca. 1935, Kunsthaus Zurich) (reproduced by permission of Alfred Roth)

Mondrian’s paintings give the observer an unequivocally two-dimensional impression. Swiss artist Richard Paul Lohse (1902–1988) further developed constructivistic abstract art into visual three-dimensionality,but not by the means of perspective39. Looking at his paintings, first impressions are of an interesting assembly of many planes in a multitude of series of strong colors, not just the three primary colors of Mondrian’s works. Colors in Lohse’s pictures are arranged systematically in rectangles. Looking more closely at a Lohse work, it becomes evident that neither the geometry – the sequence of squares and rectangles – nor the color sequences are at all arbitrary. His work lends itself well to deciphering, becoming easier to understand in this process. Fig. 7.14, a painting entitled Fifteen Systematic Color Series with Vertical Concentration towards the Bottom, demonstrates the construction of Lohse’s paintings. The first row has fifteen squares in different chromatic colors, a chromatic color circle begins with violet in the first square and progresses through the odd-numbered 194

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squares to yellow. In the other direction, the sequence goes back from blue to yellow analogously. From top to bottom, there are fifteen fields, in each of which (except the last) the height is decreased by 6.7% relative to the preceding one, but the system of colors is different from that in the horizontal sequences.

Fig. 7.14. Richard Paul Lohse. Fünfzehn ineinandergehende systematische Farbreihen mit vertikaler Verdichtung nach unten 1955/1966 (Fifteen Interpenetrating Systematic Colour Series with Vertical Condensation towards Bottom 1955/1966) (reproduced by permission of the Richard Paul Lohse-Stiftung, Zurich)

Lohse explained his operational methods in several publications. Firstly, he differentiates between modular and serial orders. Fig. 7.14 is an example of serial order; modular orders are circular systems. Both offer the interested viewer a three-dimensional impression. Study of Lohse’s explanations of his method and of his art works allows the reader to appreciate how much intellectual input went into his art in general and into each specific painting. Lohse once summarized the crucial points of his method as ‘Anonymity of means, unlimited structural laws, relativity of dimensions, equilibrium of quantities, possibility of extensions, flexibility of system determine the future expression’ (translation of a quotation given in Ref. 27, p. 137). I was privileged in knowing him personally, and what most impressed me about him was the steadfast integrity of his personality. In our scientific and industrial age, his was a vision of a world both rational and human, striv195

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ing in his work to combine the logic of artificial forms with the sensuousness of colors. Artwork by Mondrian and Lohse is clearly non-representational,but it does not attain complete segregation of form and color. In my opinion that is unattainable, because it is impossible to conceive of color without form in visual art. Developments in the use of color in non-representational art were influenced significantly by the paintings of Robert Delaunay (1885–1941) and by the book Interaction of Color by former Bauhaus teacher Josef Albers (1888–1976), who emigrated to the United States. Delaunay’s first abstract painting Disque simultané (1911, see Ref. 26) was followed in the 1930s by series like Jeux de disques multicolores and Rhythms. As a teacher at various American Colleges and at Yale University, Albers instructed many art students in color theory. His book became widely known, but his teachings were not continued by others, as discussed by Gage in his book (Ref. 4 in Chapt. 1). Are there any further significant developments in the second half of the twentieth century? It is not easy to give a definite answer: we stand too close to it, unable to see the wood for the trees. As a chemist who has performed many experiments over several decades,I know only too well that,of a hundred or a thousand experiments, ten or so turn into viable projects, and perhaps one of these becomes a success. With this perspective on things, I say yes, there is activity in the art of color, and it is to be found in experiments in art. The work of the American Morris Louis (1912–1962) is a typical example of such an experiment. He allowed layers or small streams of viscous colored resins to flow downwards over unsized canvas, changing the slopes of the plane of canvas, but not otherwise correcting the random flow42. Another representative of American minimal art is Frank Stella43. While we are discussing the segregation of color and form,we should briefly mention kinetic art. This emphasizes motion as the major theme of art, and restrains color and form as much as possible. The dominant representative of the first phase of kinetic art was American artist Alexander Calder (1898–1976),who built constructions of slender bars of metal hanging from the ceiling of the room or, later, from open-air structures. Black and white leaf-like metal plates were attached onto bars, keeping them in equilibrium.Originally these mobiles were moved by motors,later by wind only, introducing the element of chance and unpredictability. In his later works, however, he gave up the complete elimination of color and incorporated colored plates into his mobiles. The second phase of kinetic art is characterized by the Swiss Jean Tinguely (1925–1991). As well as chromat196

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ic colors, he also almost completely annihilated form, by making it utterly insignificant. He assembled useless machineries from rusty scrap iron containing various moving parts driven by an invisible motor, so-called métamatiques. A major factor was the noise made by many of his works – a synesthesia of sound and visual motion? The neuroscientist Zeki (Ref. 16 in Chapt. 5) became interested in kinetic art when he discovered that the area V5 in the cortex is sensitive to visual motion. I doubt, however, that kinetic art may be a good starting point for studying relationships between brain activity and aesthetics of visual arts (see also Ref. 17 in Chapt. 8). I shall close this section with a quotation on modern art. Friedrich Dürrenmatt (1921–1990) is considered in German-speaking countries to be the most prominent Swiss playwright and essayist of the twentieth century. In a 1985 essay on reality he wrote: ‘Malerei … ist dem Jetzt verhaftet und damit zeit- und bewegungslos … Der Surrealismus [versuchte] die Wirklichkeit darzustellen, indem er sie übersteigerte, der Kubismus, indem er sie geometrisierte und dabei schon in einen Gegensatz zu sich selbst geriet… (der Kubismus ist planimetrisch, seine Stereometrie ist vorgetäuscht), … ‘die Abstrakte’ flüchtete sich ins rein Logische und damit in Sicherheit’44.

7.4. Color in the Art of Non-European Cultures: The Case of Japan In Sect. 6.4, we discussed various cases of the influence of culture on color naming in non-European languages. In the light of these, it is not surprising to find that the role of color in non-European art may be very different from its counterpart in our culture. The analysis and interpretation of color in any foreign culture is, however, much more complex and difficult than a study purely of color naming in that culture. In intercultural color linguistics, the predominant common factor is the biology of color vision. However, for color in art, the relevance of biology is peripheral at best. For investigations into color in the art of a specific culture,one should have not only a fair knowledge of the relevant language, but also of the general history, geography, sociology and – of course – the history of art in that country or population. The consequence for this book of these prerequisites is that Japanese art is the only non-European case study included45. 197

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The discovery of ancient tombs on Kyushu, the most south-westerly of the four large islands which make up Japan, provided evidence of the use, for ornamental purposes, of indigo for blue and iron oxide for brownish red. It suggests a connection with similar tombs in Northern Korea, which could be dated to ca. 100 B. C. Furthermore, white, yellow, green, and purple fragments of ornaments were found. These additional colors, as well as some geometrical figures not known from Korea, may be interpreted as influence from a third culture. A number of other archaeological excavations have demonstrated that white, black, and red were already important symbolic colors in Japan two thousand years ago. This tradition is still alive today, its primary relevance being to Shintoism, the original religion of the Japanese. Girls in the service of Shinto shrines and of the Imperial Court wear a white outer garment of silk and a brilliant red undergarment.Today also,gifts are wrapped with red and white ribbons. White symbolizes purity and red honesty. The influence of Korea on Japan is not particularly astonishing; the shortest distance between them is only about 150 km.The ancient Japanese knew that Korea was under the influence of China in many ways, and that China had a very old and highly developed culture. Around 400 A. D., the Japanese adopted the Chinese script. As is well known,Chinese writing is based around individual characters,each of which means a complete word.It was less well suited to the Japanese language than to Chinese, and the Japanese later developed an additional new system of characters for syllables (it has two variants: haga-kana and hira-kana).This remains in use,together with the old Chinese characters (kanji). Politically and intellectually, the most important personality Japan ever produced was Prince Shôtoku,regent to Empress Suiko from 593 to 621 A. D. He was the author of the first constitution (Seventeen Articles), and, on his initiative, Japan instituted diplomatic and cultural relations with China, while making it clear that Japan wanted to learn from China, but not become a part of the Chinese Empire.This position is reflected in the manner in which the first ambassador of the Chinese Emperor Xang (605–616) was received by Queen Suiko and Prince Shôtoku in Japan. The ambassador had to wait several months until all members of the Japanese court had their appropriate ceremonial dress. The result was perfect; the ambassador reported back to Emperor Yang that Japan was a very civilized country. The extremely valuable silk fabrics brought as gifts to Japan by the ambassador were highly appreciated by the Japanese, as they already possessed great sensitivity towards beauty in general and towards luxurious 198

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textiles. Prince Shôtoku also introduced Buddhism to Japan as its second religion. Shintoism and Buddhism remain the official religions of Japan46 to this day. Japan was attacked by China only in the thirteenth century, when the Mongolian Yuan dynasty ruled China. Kublai Khan (1216–1294), grandson of the great Genghis Khan, conqueror of much of Asia and part of Europe, wished to extend his sovereignty over the Japanese islands. In 1274 he sent an expeditionary force to Hakata Bay, in the north of Kyushu Island. The Mongol invaders were defeated. In 1281, Kublai Khan sent a second force of 100,000 men, again to Hakata Bay. The battle ended in an overwhelming victory for the Japanese, due to the timely arrival of a violent typhoon proclaimed to be a kamikaze, or divine wind. This secured the political independence of Japan from the mainland until the present. First contacts with Europeans took place in 1543 with Portuguese sailors and in 1600 with a vessel of the Dutch East India Company. In 1549, the first Christian missionary came to Japan. Japanese merchant ships sailed to Macao and Thailand, as well as to Mexico. In 1639, however, the country was closed to all foreigners except the Chinese, Koreans, and Dutch. The Dutch, however, were allowed to stay only on a small island in the Bay of Nagasaki on Kyushu Island.The ban was ended only in 1854,by the Treaty of Kanagawa with the United States. Soon similar treaties were signed with Great Britain, Russia, the Netherlands, and other countries. Emperor Meiji (1852–1912), who ascended the throne in 1867, paved the way for Japan’s internal modernization and its entry into the family of modern nations. The Japanese talent for assimilating foreign culture and creatively modifying it has already been mentioned for the case of Japanese script. It can also be observed in the art resulting from Buddhism’s co-development with Shintoism. The oldest wall paintings in Japan are in the Horyû-ji temple near Nara. Unfortunately, a fire in 1949 partly destroyed these late-seventh-century paintings. A slightly less ancient painting is the picture of the Buddhist goddess Kichijô-ten in the Yakushji temple in Nara (Fig. 7.15). This popular goddess, characterized by open-mindedness and extravagance, is luxuriously dressed in silk in the style of the Chinese T’ang dynasty, her hairstyle very elaborate.In her left hand is the so-called kichijô-ka (pomegranate of blissful happiness). The picture is painted on hemp. 199

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Fig. 7.15. Picture of Kichijô-ten (eighth century; Yakushji temple, Nara)

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In the following three centuries, Japanese culture became more independent as, from the beginning of the tenth century, Japan entered an almost three-hundred-year period of isolation from the Asian mainland. This is evident in a comparison of the picture of Kichijô-ten (above) with that of Fudô Myô-ô (blue Fudô, Fig. 7.16). This is a very large painting (203 × 148 cm) on silk, probably created at the turn of the tenth to eleventh centuries by the priest Benchô in Nara. Fudô Myô-ô is the central Buddhist

Fig. 7.16. Fudô Myô-ô and two errand-boys (tenth/eleventh centuries; Shôren-in, Kyoto)

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deity of the group of five kings of light (Go Dai Myô-ô), usually representing fierce rage which conquers devils. The painting shows him in a typical posture. Fudô’s blue body and green loincloth form an effective contrast with the yellow and red flames. The deep and rich colors give the impression of greater vigor than in the painting of Kichijô-ten. Another example of the use of colors at that time, but at a more everyday and secular level, are the jûni-hitoe costumes worn from the ninth to eleventh centuries (Fig. 7.17). Jûni-hitoe means twelve layers, from the multicolored layers seen at the neckline.

Fig. 7.17. Jûni-hitoe costume (ninth to eleventh centuries) (reproduced by permission of the Sugino Costume Museum, Sugino Women’s University,Tokyo)

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The portrait of the Zen priest Ikkyû Sôjun (1394–1481; Fig. 7.18) testifies first of all to the development of Buddhism in Japan during the preceding two centuries. The Zen sect of Buddhism, founded in India in the sixth century, was brought to Japan via China in the thirteenth century, developing further there as a typically Japanese institution. The philosophy of Zen Buddhism has as one of its goals the elimination of all impressions and expressions which are not absolutely essential. This is demonstrated very impressively in various branches of Japanese art, notably ink brush painting (sumie) and calligraphy, i.e., two types of black-and-white art. The portrait of Sôjun (Fig. 7.18) is not achromatic, but color is clearly secondary to draftsmanship47.

Fig. 7.18. Portrait of the Zen Priest Ikkyû Sôjun (fifteenth century; National Museum,Tokyo) (reproduced by permission of the National Museum,Tokyo)

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In the context of Zen philosophy,chanoyu and architecture should be mentioned, last but not least because of their almost ascetic but well thoughtout use of colors. Chanoyu is the tea ceremony, where one learns to appreciate an artistic and very quiet atmosphere with the aid of the delicate aroma of powdered green tea in a small teahouse accommodating not more than four or five persons.Tea bowls,a caddy,a whisk,and a spoon are needed, the latter pair made of bamboo. The bowls and the caddy are valuable art objects, colored in exquisite, rather dark shades. By far the best example, I feel, of Zen-influenced architecture is the Katsura Imperial Villa and its large garden, built near Kyoto in 1620–1624. The buildings are very simple one-floor wooden constructions with a large variety of views in a diversely laid out garden with many bushes, trees, ponds, and small bridges. They are planned very carefully and impress by virtue of the simplicity of villa and garden. Concurrently with this movement to austerity, however, color remained important in other fields of Japanese art. From the tenth century onwards, many charming, colorful, and sometimes amusing landscape scenes were produced on folding screens,first in palaces and later also in townspeople’s houses. Another wide and very traditional field of symbolic color application was and still is the theater. There are three types of Japanese theater: bugaku, nô, and kabuki. All were developed in Japan. Bugaku is non-dramatic and based on dance and music; it is related to the court and to the aristocracy. Since the fifteenth century, Nô-gaku has been the genre for cultivated audiences, originally samurai (knights) and daimyo (lords). The nô play lasts several hours and has only a few male players; female roles are also played by men. The colors of nô costumes have very strict symbolic meanings, well known to the connoisseur audience. Kabuki is a popular entertainment, originating in seventeenth-century Kyoto, where it was first performed in the partly dried up bed of the Kamo river. The artists at that time were, therefore, disdainfully referred to as ‘river-bed players’. The style of the plays is realistic and – in most cases – dramatic, with many players. Here too, protagonists are characterized first of all by the colors of their costumes, although these are less elaborate than those of nô plays. Very typical of Japan are ukiyo-e (pictures of the floating world), woodcuts which record the life, fashions, and entertainments of Japan’s people. The word ukiyo-e appears for the first time in a 1682 Japanese novel; the intent is to capture the shifts of joys and sorrows in everyday human exis204

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tence. Originally, ukiyo-e were black and white only. Partly colored (with minium, or red lead) ones later appeared, and in the early eighteenth century, hand-coloring was adopted, using watercolor dissolved and applied with gelatin. In 1765, color printing with up to twenty separate print blocks was introduced; the technique was not yet in use anywhere else at that time. During the first flowering of ukiyo-e (end of the eighteenth century), subjects were mainly portraits or scenes in family houses (see Fig. 7.19).

Fig.7.19. Harunobu Suzuki (1725–1770). Contemplation of the full moon (Color print, about 1769; Rietberg Museum, Collection Julius Müller,Zurich) (from Palette 1965, 19, pp.2–13; reproduced by kind permission of the publishers)

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In the nineteenth century,town scenes and landscapes – but always including people – became more common. The pinnacle of this phase was the work of Hokusai Katsushika (1760–1849) and Hiroshige Andô (1797–1858)48. Hokusai is well known for his series of 46 woodcuts Fukaku sanjurokkei (The 36 Views of Mount Fuji)49. He was also a very talented painter in oils – of people, animals, and landscapes – as well as of the canopies of the floats carried aloft by four or more men in parades. Hiroshige’s main work is the series of 55 pictures Tôkaidô Gojûsan Tsugi (The 53 Stations of the Eastern Sea Road, first edition 1833–34)49. The Tôkaidô is one of two main roads joining Tokyo and Kyoto, the new and the old capitals of Japan. Almost 500 km long (including 24 km by boat), the Tôkaidô was very well organized, with its 53 stations all well equipped both for riders and foot-travelers. Hiroshige’s drawings show all four seasons. He is at his best in depicting the beauty of a landscape, travelers in specific situations and in his careful drawing of details.The beauty of these works effortlessly captures the typically Japanese sense of the deep sadness of life. Two very different ukiyo-e by Hiroshige are reproduced here. Fig. 7.20 shows the coast near the nineteenth station (Ejiri at Suruga Bay). It is a calm day, as seen in the blueness of the water. The careful depiction of the sailboats in the harbor is typical of Hiroshige. Fig. 7.21 is one of those pictures from the series showing travelers and local peasants. Here, we see them in a rainstorm on a mountain path near the 46th station (Shôno, near Ise Bay). The forces of nature are expressed in the bending bamboo, as well as in the men’s stances. Hiroshige’s representation of landscapes anticipates the relationship of the French Impressionists to light,color,and atmosphere,as seen fifty or more years later in the works of van Gogh, Cézanne, and Monet. Particularly well-known, although not directly related to Hiroshige’s ukiyo-e, are Monet’s many paintings of the Japanese bridge in his garden in Giverny. It is, hence, not astonishing that during 1888–91 a journal with the title Le Japon artistique was in print in Paris. In the introduction to the first issue, its German-born editor Samuel Bing wrote that ‘The Japanese are poets moved and inspired by the great spectacle of nature … They believe that nothing in the worlds of creation is unsuited to the high ideal of art. Even a single, small blade of grass’. Today, of course, Japan is not the same as it was before the opening of the country to foreign influence in the mid-nineteenth century. To me, how206

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Fig. 7.20. Hiroshige Andô. Ejiri: Distant view of Miho (1833)

ever, it seems that the quintessential Japanese talent on which Prince Shôkotu relied in the sixth century – adopting new developments in such a way as not to destroy valuable indigenous qualities – remains present as an enduring fundamental of Japanese culture.As far as colors in Japan are concerned, the high esteem enjoyed by black and white art parallels the position of these colors in color naming tests (cf. Sect. 6.5). Several books on color in Japanese art, written by Japanese authors but published in European languages, are available nowadays50.

7.5. Color in Psychology Colors play a significant role in various branches of psychology: a few examples are color preferences and dislikes, colors’ symbolic role, and 207

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Fig. 7.21. Hiroshige Andô. Shôno: A Shower (1833)

color in analytical psychology and in color tests. Indeed, the color space has been described as a microcosmos of psychology, reflecting our immediate surroundings and also the proper macrocosmos. This fact is presented in various ways in religions and philosophical movements such as Middle Ages European mysticism and Asian Buddhism. The latter tradition refutes any dichotomy between micro- and macrocosmos. There is a vast literature on this subject, but it is often inconsistent with respect to the evaluation of data. Some of these difficulties – for colors as symbols, for example – are the result of colors not being clearly determined using colorimetric methods. To a significant degree, this section is based on my own appraisal51 of two volumes of the Collected Works52 of psychologist Carl Gustav Jung (1875–1961), who made reference to color in the contexts of analytical psy208

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chology and of alchemy. His investigations into mandalas, symbols, alchemy, dreams, and medieval visions contain much information on colors which I reviewed in 1985. Jung published his results for the first time in the 1940s, and it is very gratifying to see that his conclusions have been corroborated by neurobiological, linguistic, and related work on color vision accomplished only after 1955. As discussed earlier, there is a certain dichotomy between achromatic and chromatic colors. It shows up in various aspects of our relation to color, not just in their purely physical differentiation as illustrated in Fig. 4.1. Color circles, for example, do not usually contain achromatic colors (see Figs. 4.5–4.7). In his book Psychology and Alchemy52 (Fig. 49 in Vol. 12), Jung used a circular scheme to represent the four functions of consciousness. He placed thought in the light upper half of the circle, feeling in the dark bottom, sensation on the left, and intuition on the right. One of Jung’s patients described to him a dream in which the patient saw two circles, with a common center, but perpendicular to each other. One circle contained the chromatic colors in the usual sequence, and the other the achromatic ones (in part in symbolic form as black birds). This dream reminded Jung of a vision of Guillaume de Digulleville, the fourteenth-century prior of a French monastery. In this vision, Guillaume was guided by an angel through several parts of Paradise. The angel told him about the Trinity, and Guillaume admitted that he had never understood the Trinity in a satisfactory way. The angel explained to him that there is an analogy with three primary colors, green, red, and gold, as they are combined in moiré silk fabrics or in peacock feathers. The gold represents God, the red is Jesus Christ, because he shed his blood on the cross, and the green is the Holy Spirit, because it is the color that nurtures and refreshes (‘la couleur qui verdoie et qui réconforte’). Jung was not surprised that there was no blue, as he had observed fairly often in his experience with patients that they tended to make colored circular drawings of the Mandala type (see below) with no blue present. Since Berlin and Kay’s pioneering work in 1969 on the evolution of color terms (see Sect. 6.3), we have known that terms for blue generally evolve only after white, black, red, green, and yellow. Jung put forward an analogy to the dream with the two perpendicular color circles mentioned above and conjectured that blue is present in the vertical axis of the achromatic colors: ‘We would conjecture that blue, standing for the vertical, means height and depth (the blue sky above, the blue sea below), … Hence, the vertical would correspond to the unconscious … Guillaume was so absorbed in the Trinity and threefold aspect of the roy that he quite forgot the reyne’ (in the Middle Ages, the symbolic color of 209

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the Virgin Mary was blue). Many psychologists have described patients’ dreams in which colors were important. We become aware of colors in dreams only when they have highly symbolic significance for our emotions or passions. It is remarkable that, if more than one color is recognized in a dream, there are often four of them, in a circular arrangement as shown in the example below. Jung discovered that there is a close relationship with the mystic diagrams called mandalas: circles and polygons with four (or multiples of four) corners. Mandala is a Sanskrit word and means circle. In the context of religious practice and in psychology, it is a symbol representing the effort to reunify the self: the ‘mandala circle’. It denotes circular or polygonal images which are drawn, painted, modeled, or danced. They are found, for instance, in Tibetan Buddhism, their purpose being to assist meditation and concentration. The Bible also contains dreams with fourfold mandala-like situations (e.g., Ezekiel 1 and Daniel 7). In alchemy, the quadratura circuli is a mandala motif. As dance figures, they occur in Dervish monasteries, while to modern individuals they can appear spontaneously in dreams, but also in certain states of conflict, such as under conditions of psychological dissociation or disorientation53. Psychological and cultural research offers a means of understanding the functional meaning of such mandalas. Fig. 7.22 presents a mandala drawn by one of Jung’s patients. It shows a color circle with eight fields and sixteen color steps.Several mandalas,both modern and ancient, are based on the four unique hues or include them in some fashion or other. The symbolic use of colors was already important in painting during Antiquity. Heraclitus mentions melanosis (blackening), leucosis (whitening), iosis (redness), and xanthosis (yellowing). In the Middle Ages, these four colors were important for the four phases of the alchemical process of spiritual transmutation of matter. This process starts with the black prima materia which forms the white (i.e., silvery) mercury, then the red sulfur (some compounds of sulfur with metals are red), and culminates with the yellow (golden) philosopher’s stone.In the fifteenth and sixteenth centuries, xanthosis is often replaced by viriditas (greenness). In this sequence we again see a striking similarity to the evolution of basic color terms as described by Berlin and Kay (see Sect. 6.3). For the dichotomy between a state of a triangle and a quadrilateral phase in alchemy, Jung52 210

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Fig. 7.22. Mandala drawn by a patient of C. G. Jung54 (reproduced by permission of Linder Niedieck AG, Zurich)

(Vol. 12, p. 151, footnote) quotes from a book by alchemist Michael Maier (1568–1622), published posthumously in 1687: ‘Similarly the philosophers maintain that the quadrangle is to be reduced to a triangle, that is, to body, spirit and soul. These three appear in three colors which precede the redness: the body or earth, in Saturnine blackness; the spirit in lunar whiteness, like water; and the soul, or air, in solar yellow. Then the triangle will be perfect, but in turn it must change into a circle, that is into unchangeable redness’. For alchemists, the mysterious transforming substance was round and quadrangular, i.e., an entity of four elements. The final goal of the alchemists was to extract the original divine spirit from the chaos of elements. The extracted spirit was called the quinta essentia. A popular subject of modern color psychology are studies on color preferences. Several such investigations have been performed, with adults and children from various cultures serving as test subjects. The overall result is inconclusive, partly due to the use of color samples which had not been clearly defined colorimetrically, but also to the fact that color preferences are determined by several cultural and individual parameters. A relatively new investigation by McManus et al.55, conducted in a critical and reliable manner using English test subjects, showed that there are various 211

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groups. The largest (60%) group considers blue its most preferred color and yellow the least (among eight main colors).A smaller group places red first and green last.Where does it come from that blue should be the most preferred color? Is this result related to the high symbolic value of blue? It is difficult to give an answer. Despite the fact that results on color preferences in different cultural surroundings are often contradictory56, the possibility that response to color might be a meaningful source of information for personality description has often been – and still is – investigated. The most noteworthy techniques involving color include the use of colored blots in the Rorschach technique,developed by the Swiss psychiatrist Hermann Rorschach in 1921. Another Swiss, Max Lüscher, developed the specific use of forced-choice color preference procedures from the late 1940s onward. Also important is the use of the interaction of color and form in terms of projective principles first described by Max Pfister, forming the basis of the Color Pyramid Test of American psychologist K. Warner Schaie. Among the many more recent approaches, I should mention the so-called Syntonics therapy developed by American J. Liberman58, which is based on a combination of light and color. It is well-known that the color of packaging has a significant influence on sales of consumer goods, particularly for food and soft drinks. Color optimization for the packaging of stimulants which may cause addiction and damage to health is sadly not rare59. For pharmaceutical products,the color of tablets has a placebo effect,most notably among children.Sedative medicaments have most effect if the tablets are blue. Pink has a stimulating effect, while yellow is more effective than red or green for patients suffering from depression. Almost all research into color preference performed over several decades is highly empirical and has not yet led to any real understanding. This discussion of color preference is accordingly rather brief.

7.6. Goethe ’s Zur Farbenlehre It is an extremely interesting fact that Goethe, unquestionably the greatest writer of German literature and one of the giants of world literature, wrote a book on colors60. It was even his most voluminous single work. Goethe’s own opinion of Zur Farbenlehre61, however, far outweighs this superficial 212

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aspect. He considered it his most important work, although referring to it as a ‘befremdliches Unternehmen’ (strange enterprise) in which he invested ‘die Mühe eines halben Lebens’ (the toil of half a lifetime). Goethe’s interests and creativity spanned a wide range: besides his activities in literature and the natural sciences, he had a consuming interest in color and painting. He also carried out various administrative functions in a theater, in museums, and at the University of Jena. These activities were, in part, the consequence of an invitation from Carl August, Duke of Sachsen-Weimar, to join his court in 1775. Goethe began his work on the fundamentals of color during his first journey to Italy (1786–1788), where he was impressed by Italian artists’ use of colors. He then became aware that these painters, while well aware of rules for drawing, had none for coloration.After returning to Weimar, he decided to study natural sciences and their treatment of color. During these studies, he became acquainted with Newton’s work. Before discussing Goethe’s first reactions to Newton’s Opticks and his papers of 1672, I should provide a brief chronology of Goethe’s own publications on color. In 1791 and 1792 he published two Beiträge zur Optik63 (Contributions to Optics). These contain observations from viewing objects and specially constructed displays through prisms. Goethe had it in mind to continue this series of contributions, but never did, apart from some verse epigrams in 1797. In 1805 he started work on the manuscript for the Farbenlehre60, intending that it should consist of four parts.He first wrote the Didactic Part (Entwurf einer Farbenlehre (Exposition of a Color Doctrine61)), which begins by presenting color phenomena, divided into sections for physiological, physical, and chemical colors. The second half of the Didactic Part is an appraisal of those phenomena and includes his conclusions on the psychological and aesthetic effects of color; the so-called sinnlich-sittliche Wirkung (sensory and ethical effect). The second part is the Polemic Part, sub-titled Enthüllung der Theorie Newton’s (Newton’s Theory Unveiled). The third, Historical Part (Materialien zur Geschichte der Farbenlehre (Material on the History of Color Doctrine)) is a review and discussion about work on color from Antiquity. It is comprehensive, although Goethe had planned an even more exhaustive review. The fourth, highly fragmentary part begins with colored illustrations by Goethe and an essay by Thomas Seebeck, a physicist who worked with him.He describes some photochemical effects, a subject not seriously taken up again until chemists did so in the twentieth century. Zur Farbenlehre was published in 1810 by Cotta in Tübingen. The planned fourth part was never complet213

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ed,but Goethe wrote no fewer than 66 poems on color theory between 1790 and 1827. Schöne64 collected these and commented upon them in his book. Goethe devoted some ten pages at the end of the historical (third) part of Farbenlehre to a section entitled Konfession des Verfassers (Confession of the Author). Here he described his response after becoming acquainted in detail with Newton’s papers. He borrowed prisms from Geheimrat Büttner (then of Göttingen, later of Jena), but hesitated to reproduce Newton’s experiments for a long time; Büttner twice asked him to send the prisms back, and Goethe had it in mind to do so without using them. Eventually, an errand-boy sent by Büttner already waiting, Goethe did bring himself to look quickly through the prisms.He neglected to darken the room,however, and so was unable to see any refracted spectral colors on the white wall in broad daylight.‘Es bedurfte keiner langen Überlegung, so erkannte ich, dass eine Grenze notwendig sei, um Farben hervorzubringen, und ich sprach wie durch einen Instinkt sogleich vor mich laut aus, dass die Newtonische Lehre falsch ist’65. He did not send the prisms back immediately, but he used them to observe in more detail the colors produced at boundaries. A local physicist told him that these boundary phenomena were well-known and could be explained on the basis of Newton’s work. Goethe mentions that anatomists, chemists, philosophers, and others supported his conclusions, but not physicists. He entered into correspondence with eighteenth-century Germany’s most famous physicist, Georg Christoph Lichtenberg (1742–1799) at Göttingen University, but Lichtenberg finally discontinued correspondence ‘als ich [Goethe] das ekelhafte Newtonische Weiss mit Gewalt verfolgte’ (‘when I pursued the disgusting Newtonian white with vehemence’). In the last pages of the Konfession, Goethe documented in various ways his displeasure at his ideas’ lack of acceptance by various groups, the ‘Beschränktheit der wissenschaftlichen Gilden’ (‘the narrow-mindedness of the scientific guilds’), the rejection of the idea that physical investigations without a mathematical basis are valid.Yet Goethe was also pleased that the Duchess of Sachsen-Weimar had attended one of his lectures with demonstrations of color phenomena. This short summary of the Konfession clearly shows that Goethe had a deep aversion to Newton’s work.It is likely that this negative attitude dated from his reading Newton’s publications for the first time, i.e., relatively soon after his return from Italy. What were the origins of Goethe’s vehement rejection of Newton’s experimental results and conclusions? To answer this question, I shall start with one of Goethe’s poems,written in 1827 (from Zahme Xenien,part VI,1827)66: 214

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Warnung eigentlich und symbolisch zu nehmen

Warning, to be taken in a true and a symbolic sense

Freunde, flieht die dunkle Kammer,

Friends, avoid the darkened chamber, Where they show you twisted light, Where they bow to images Distorted and impoverished. Superstitious worshippers We’ve had enough of them. Confine delusions, fraud and phantoms To your teachers’ heads.

Wo man euch das Licht verzwickt Und mit kümmerlichstem Jammer Sich verschrobnen Bildern bückt. Abergläubische Verehrer Gabs die Jahre her genug, In den Köpfen eurer Lehrer Lasst Gespenst und Wahn und Trug. Wenn der Blick an heitern Tagen Sich zur Himmelsbläue lenkt, Beim Sirok der Sonnenwagen Purpurrot sich niedersenkt: Da gebt der Natur die Ehre, Froh, an Aug und Herz gesund, Und erkennt der Farbenlehre Allgemeinen, ewigen Grund.

When on clear and sunny days You behold the heaven’s blue, When you see Sciroccos paint The setting chariot purple red: Give nature credit for it, Cheerful, sound of eye and heart, Take note, She is the common And eternal cause of what you see as color. (Kindly translated by Herbert Deinert, see the Preface)

This poem expresses Goethe’s intuition-based opinion of Newton’s experimental method and uncritical followers, as well as a flowery description of Goethe’s own ideas on light and color. On an intuitive level, Goethe was incapable of accepting Newton’s work, and was fundamentally dissatisfied with the analytical and inductive techniques Newton used. In Goethe’s opinion, Erscheinungen des Lichts (light phenomena) should be observed only on a purely phenomenological basis, directly and under natural (i.e., undispersed) conditions. For understanding nature, we should rely only on our senses’ immediate perceptions, not on theoretical assumptions or analysis. Therefore, he considered Newton’s ‘experimentum crucis’ – the recombination by a second prism of all the 215

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spectral colors into colorless (white) light – to be false. Even for us in the twentieth century, it is horribly counterintuitive to regard a white plain as a combination of all colors. In this respect, it should be mentioned that the difference between additive mixing of lights, forming white (Newton’s experiment) and subtractive mixing of dyes and pigments, known to painters and textile colorists for centuries, was explained only by Helmholtz in the mid-nineteenth century, but once more on a physical basis. Goethe’s opposing view was based on the observation that all chromatic hues are lighter than black, but darker than white. He sometimes called chromatic hues Halblichter (half-lights), and mixing all these different ‘half-lights’ does not eliminate darkening due to individual colors.We can see from these hypotheses that Goethe was not far away from understanding subtractive mixing sixty years before Helmholtz. So much for the relationship of Goethe’s Farbenlehre to the physics of color. I shall return to physics in the concluding remarks of this section. Before that, however, Goethe’s contribution to color vision should be discussed. After observing the colors at the edges of the prism during his unsuccessful reproduction of Newton’s experiment, he became interested not only in the opposition of white and black, but also in that of light and shadow, and its relationship to complementary colors. I quoted his nice description of such a case in Sect. 4.2. In the Didactic Part of his book, Goethe discussed several color-vision phenomena based on the two processes of successive contrast and simultaneous contrast. There is no doubt that he was the first to investigate these effects carefully and in detail. He realized that the contrasting colors produced were the complementary hues of a color circle: ‘…die [im Farbenkreis] diametral einander entgegengesetzten Farben [sind] diejenigen, welche sich im Auge wechselweise fordern. So fordert Gelb das Violette, Orange das Blaue, Purpur das Grüne, und umgekehrt’67. These quotations come from the chapter on what Goethe called ‘psychological colors’, arising out of interactions within the visual system resulting from the temporal succession and spatial distribution of illumination and color samples. In that chapter, he also mentions the appearance of a specific color not only on light impinging on a specific place on the retina, but also in the region surrounding that spot, which becomes sensitive to the corresponding complementary color. These ideas of Goethe share a very great deal of common ground with modern neurobiological and psychophysical findings in color vision (see Sects. 5.4 and 5.5). Goethe’s very first reaction to Newton’s investigations, 216

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‘the disgusting Newtonian white’, is also reflected in our present knowledge of neurons in certain areas of the cortex which are sensitive to luminance, but not to color. These might very well be responsible for our intuitive feeling that white simply just cannot be a combination of all colors of the spectrum. The relationship between Newton and Goethe has been discussed in several publications since the early nineteenth century68. In my opinion, Goethe’s emotionally-based criticism of Newton is the result of the fact that Newton’s investigations were purely physics-based, while Goethe was well aware that no mere quantitative study of the spectrum can in any sense explain the sensation or perception of color. Goethe’s own conclusions, on the other hand, are based on visual observation, and so are the province of color vision. In 1672, optical experimentation required human vision. Developments in physics over the next three centuries, however, have clearly made this redundant; today, Newton’s experiments with light refracted by one or two prisms could easily be performed without visual examination, by measuring the wavelengths of the spectrum of light passing through the prism(s). Newton even mentioned unequivocally that he was well aware that it was also possible to ‘see’ colors under entirely different circumstances (see quotations in Sect. 1.2)69. Goethe’s objection to Newton’s experiments and scientific conclusions are, therefore,clearly not appropriate.Devotees of Goethe have done him a disservice in lashing out at Newton, although these reactions have fortunately decreased in intensity over the last two centuries. Central to Goethe’s lasting contribution to our knowledge of color is his work on color vision. In particular, his observations of simultaneous and successive color-contrast phenomena are early examples of what today is called psychophysical investigation.Goethe’s above-mentioned remark on reaction in surrounding zones in the retina very closely reflects neurobiological results obtained only during the last three decades. This is also the case for his refusal to accept that white could be a combination of all chromatic colors, as we now know that there are – some – separate visual pathways in the brain for information on luminance and on chromaticity. In addition,Goethe’s ideas about the ‘sinnlich-sittliche Wirkung der Farben’ (sensory and ethical effect of colors) covered areas of interest to artists. Goethe used this title for the last chapter of the Didactic Part. In § 916–918, 217

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he distinguishes between three types of colors: a symbolic, an allegorical, and a mystic type. For Goethe, the symbolic color coincides entirely with nature. An allegorical color is more arbitrary as the allegory must first be communicated to us before we can know the sense of that choice of color (green as a sign of hope, for example). The mystic interpretation of colors is present in color schemes and in geometrical arrangements. Goethe, astonishingly, mentions not the color circle, but the triangle and his basic concept of yellow and blue which, by diverging, undergo what he called ‘Steigerung’ (enhancement) into red. This central hypothesis in Farbenlehre seems to be based on an undefinable level of the subconscious mind. It is astonishing, though, how well this ‘Steigerung’ into red corresponds to the dominance of red among the chromatic hues in Berlin and Kay’s evolution of basic color terms, a theory postulated 160 years after the Farbenlehre (see Sect. 6.3). The idea of ‘Steigerung’ was not generally accepted by artists; they were primarily interested in Goethe’s contrast observations. The German painter Philipp Otto Runge developed his color sphere of 1809 (see Sect. 4.2) in collaboration with Goethe. The nineteenth-century painter most interested in Goethe’s work was probably Turner, whose paintings very often include light/dark effects in combination with subtle changes in chromatic hues. The two paintings reproduced earlier (Figs. 7.8 and 7.9) provide good examples of this. Turner testified to his interest in Goethe’s work in the title he gave in 1843 to the second of his paintings on the Deluge: Light and Colour (Goethe’s Theory). Moses Writing the Book of Genesis. Turner also said, however, that he considered Goethe’s hypothesis of the formation of red by ‘Steigerung’ from yellow and blue as ‘absurd’. The German romantic philosophers Hegel, Schelling, and Schopenhauer were also interested in the Farbenlehre,Schopenhauer writing a book Über das Sehen und die Farben in 1816. In Germany, artists and practitioners of chromotherapy studied the Farbenlehre more commonly in the second half of the nineteenth century. While the French Impressionists were not, as far as I am aware, influenced by Goethe, the work of the German Expressionist Kirchner, discussed in Sect. 7.3, was in part influenced by his interest in Farbenlehre. Kandinsky wrote his book Über das Geistige in der Kunst32 in 1912 as a presentation of expressionist art theory, but discussed Goethe’s work only incidentally.He only became interested in it later,when in contact with Rudolf Steiner, founder of the anthroposophic movement in Germany and later in Switzerland70.

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The Bauhaus in Weimar and Dessau does not seem to have developed any regular and reliable color-teaching school, despite the fact that several of its instructors (Itten, Klee, Kandinsky, Hölzel), some of them contemporaries, were active artists with keen interests in color. From Klee’s diaries, we know that Goethe served as his primary theoretical guide and ideal model, and remained a lifelong influence although in later years Klee was to turn more towards music as a guide. Josef Albers (1888–1976) found expressive color combinations by grouping various hues in a triangle made up of nine smaller triangles. In the German edition of his book Interaction of Color 41, but not in the American original, he called the basic (large) triangle the rarely published,‘but wise triangle of Goethe’, obviously in analogy to the triangle mentioned previously in this section. Using this technique, Albers claimed he found psychologically expressive color chords, some of them lucid, some serious, some melancholic. The way in which he viewed his book is interesting. ‘This book’, he said, ‘… does not follow an academic conception of theory and practice. It reverses this order and places practice before theory, which, after all, is the conclusion of practice.’ Finally, I think we should bear in mind Goethe’s credo of ‘Ganzheit’ (totality) in nature as expressed in the second of the Beiträge zur Optik63, Der Versuch als Vermittler (the experiment as medium): ‘In der lebendigen Natur geschieht nichts,was nicht in einer Verbindung mit dem Ganzen stehe, und wenn uns die Erfahrungen nur isoliert erscheinen, wenn wir die Versuche nur als isolierte Fakta anzusehen haben,so wird dadurch nicht gesagt, dass sie isoliert seien, es ist nur die Frage: wie finden wir die Verbindung dieser Phänomene, dieser Begebenheit’71. This quotation demonstrates the difficulty encountered in assimilating the evident richness of Goethe’s Farbenlehre. It is simply not possible to see it in its completeness from the perspective of a single discipline, whether a natural science or one of the humanities. Goethe’s statement, of course, is wholly reflected in his poem reproduced above. Goethe was light-years removed from Newton’s analytical thinking. Today, we know that the method applied by Newton was to become extremely successful in the following three centuries, and that painstaking collecting of small ‘building bricks’ of research makes it possible in many, but not all, cases to reconstruct and understand the whole building made up of those small bricks.

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It is interesting to compare some remarks made by these two geniuses in their later years. Newton’s even relates to the small ‘building bricks’: ‘I do know what I may appear to the world, but to myself I seem to have been only like a boy playing on the seashore and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay undiscovered before me.’ In a conversation with his discussion partner and secretary Johann Peter Eckermann on February 19, 1829, Goethe said, ‘Auf alles, was ich als Poet geleistet habe, bilde ich mir gar nichts ein. Es haben treffliche Dichter mit mir gelebt, es lebten noch trefflichere vor mir, und es werden ihrer nach mir sein. Dass ich aber in meinem Jahrhundert in der schwierigen Wissenschaft der Farbenlehre der einzige bin, der das Rechte weiss, darauf tue ich mir etwas zugute, und ich habe daher ein Bewusstsein der Superiorität über viele’72. In the notes he made of his dialogues with Goethe, Eckermann mentions that on the date of January 22, 1831, fourteen months before Goethe’s death, they discussed his last will. Goethe said that, for a second edition of Zur Farbenlehre, the Polemic Part could be omitted if the publisher did not wish to print more than a certain number of pages.He added though that he did not at all disavow his rather sharp dissection of the Newtonian theorems – an action necessary at the time and of a value which would endure – but that he basically considered all polemic activity as contrary to his true nature. No separate second edition of the Farbenlehre was published, but it was later included in most editions of Goethe’s collected works. All five post1945 German editions which I know contain the Farbenlehre, but only one of them includes the Polemic Part; this is the so-called Leopoldina Edition, published in the German Democratic Republic (1947–1987). No reasons for the omission are given in any of the other four editions published in the Federal Republic of Germany (3) and in Switzerland (1).

7.7. Sound – Color Synesthesia Personally, I am fascinated by the synesthesia between color vision and the hearing of sounds,or more particularly between color in art and music. Of course, this synesthesia might have been discussed in or immediately after the sections on art in this chapter. But I have the feeling that these pages on color – sound synesthesia are enhanced by appreciation of the complexities of color in psychology and of Goethe’s Farbenlehre.

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First, some comparisons from physics. Even though Newton did not really follow up his early idea that there might be some kind of physical analogy between the visible spectrum and the octave of the musical scale,many people continued to speculate on the matter, particularly after it became generally known that both sound and light are wave phenomena. Both are manifestations of (physical) energy, which proceeds from a source into space. However, they are very different in nature: sounds are vibrations of matter, molecules of nitrogen and oxygen in air, H2O molecules in liquid water, various molecules in solid matter. The basis of sounds lies in the laws of classical mechanics. Light, however, is a manifestation of electromagnetic waves. As discussed in Sect.2.1,their energy is not based on movements of molecules, and can be described only by quantum mechanics. This basic difference between sound and light is also reflected in the neuroscience relating to the corresponding senses in living organisms. In the ears of mammals, including humans, sounds are registered by very small hair bundles in the cochlea, a coiled tube of fluid in the inner ear. Sounds cause wave-like movements of these hairs, and these are perceived by the brain. Light, however, induces a photochemical reaction in the visual cells of the retina,as discussed in Sect. 5.3.This reaction can be understood only on a quantum-chemical basis. Therefore, the physics and chemistry of sound and light – up to the processes in the internal ear and the retina, respectively – do not really arouse any expectations of synesthesia between the auditory and visual senses. The corresponding mutual interactions of brain processes remain mysterious, but various observations clearly suggest that auditory perception works analytically,as the brain is able to differentiate tones or instruments simultaneously, whereas in color vision we integrate light stimuli of two or more wavelengths into one color. Yet, as already discussed in Sect. 6.4, the metaphoric usage of sensory adjective terms does suggest the existence of such synesthesias. In the following paragraphs, it is shown that culturally more important synesthesias do exist between visual art and music. It is interesting that these synesthesias are mainly from hearing to vision. The timbre of some instruments is associated with specific colors, of which the best known example is that of the trumpet. The English philosopher John Locke (1632–1704), to give just one instance, associated this with red or scarlet. Kandinsky, who played the cello, considered the timbre of his instrument to be a deep blue. Besides these timbre-color pairs, however, few instruments are more or less unambiguously associated with a sin221

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gle color. The association of keys with colors is also disputable. C major is fairly often called a white key,F major green.The reasons here may be other than synesthesias in their proper sense: C major only uses the white keys (on the piano), while F-tuned horns are often used for pastoral and hunting scenes in operas, leading to the association F major – horns – nature. In Western culture, synesthesia ideas can be found from classical Antiquity onwards73. The Greeks regarded color as one of the properties of timbre and pieces of music, also believing that the harmonies of musical chords had their counterparts in colors.From the above remarks on sound and light waves, however, it is clear to the modern observer that such correlations would be unlikely, firstly because we hear about ten octaves, i.e., ten times the 2:1 wavelength ratio,but can see less than one ‘octave’ of light, and secondly because only our auditory sense is able to analyze mixtures of vibrational stimuli. At least from our twentieth-century point of view,therefore,it is not amazing that earlier attempts to construct color instruments for purposes analogous to music instruments were unsuccessful. One such was the ‘clavecin oculaire’ built by the French Jesuit Louis-Bertrand Castel in the 1730s. This contained twelve colored plates, which were exposed by the action of a mechanical handling device in a manner comparable to the way the keys of a piano are struck. By the end of the nineteenth century, however, great advances had taken place in color- and sound-effect technology, thanks to electrical engineering and theater stage-lighting. In 1915 the first performance of Alexander Scriabin’s symphony Prometheus – a Poem of Fire was held in New York. This work included a composition for a conventional orchestra, accompanied with color projections. Earlier plans to perform this work in Moscow and London had been unsuccessful. Arnold Schönberg also composed a work, Die glückliche Hand, in which music is combined with colored lights,and other composers made similar experiments. They are all forgotten today,although lightshows retain an enduring appeal in popular music – what are the reasons: superseded by technology? Not everything new has artistic value? An exceptionally good counterexample is, however, Händel’s Fireworks Music. Great thinkers whose primary interests lay in color searched for a common basis of color and music. Goethe emphasized first of all the differences between color and sound (Farbenlehre, Didactic Part, § 748): ‘Vergleichen lassen sich Farbe und Ton untereinander auf keine Weise, aber beide lassen sich auf eine höhere Formel beziehen, aus einer höheren Formel beide, jedoch jedes für sich, ableiten’74. 222

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In addition,he postulated that the thorough-bass75,a well-established fundamental of music theory, should also be considered for painting. This suggestion was taken up once more by Kandinsky in Über das Geistige in der Kunst’32 where he writes,‘Diese prophetische Äusserung Goethes ist ein Vorgefühl der Lage, in welcher sich heute die Malerei befindet. Diese Lage ist der Ausgangspunkt des Weges, auf welchem die Malerei durch Hilfe ihrer Mittel zur Kunst im abstrakten Sinne heranwachsen wird und wo sie schliesslich die rein malerische Komposition erreichen wird’76. Later in his book (p. 85), he speaks of a future grammar of painting based not on the laws of physics as in Cubism, but on laws of inner necessity ‘which may properly be described as spiritual’ (‘…die man ruhig als seelisch bezeichnen kann’). During the years when non-representational art was developing,Paul Klee, as well as Kandinsky, also came to the conclusion that abstract painting is related more strongly to music than to classical painting styles. A violinist, Klee saw that the music most relevant to study by modern painters was that of the baroque school.‘The problem of abstraction had been solved in music by the end of the eighteenth century, but it has now only begun in the fine arts’, he observed in 1928, during his time teaching at the Bauhaus (1921/24; 1926–31). In the Paul Klee Foundation at the Kunstmuseum in Berne, a drawing from Klee’s notes for his Bauhaus course includes a novel type of color sphere which he called The Canon of Color Totality. From youth onwards, Klee’s diaries are full of remarks about music and composers. First he revered Bach and Beethoven above all composers, but turned to Mozart in the 1910s, considering the Jupiter Symphony (KV 551) the ‘highest attainment in art’, while his favorite opera was Don Giovanni. In parallel with his painting and teaching, he was dedicated in his exploration of music theory and compositions of the seventeenth and eighteenth centuries. These explorations culminated in a large painting which he called Ad Parnassum (1932, Kunstmuseum, Berne). The title is an allusion to the important treatise on music theory Gradus ad Parnassum (stairway to Mount Parnassus) formulated by Johann Josef Fux (1660–1741),composer for the Imperial Court and later Imperial Conductor in Vienna. His book was first published in Latin (1725) and later translated into German, Italian, French, and English (1742–1791). It is documented that Haydn, Beethoven, and Leopold and Wolfgang Amadeus Mozart all learned the techniques of contrapuntal composition from Fux’s book. Fux, therefore, laid the foundation of the classical Viennese style. It replaced the older, strictly sequential harmonies of the Palestrina style by laying down rules for simultaneous shifts in harmonic relationships in a polyphonic setting.This very brief and incomplete summary is provided only to give the reader 223

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inexperienced in music theory some idea of why a painter, who is essentially interested in depicting three-dimensional situations on two-dimensional canvas or paper, should, like Klee, develop a fascination for music theory. Several authors of publications on Klee’s relationship with music emphasize similarities between his character and Mozart’s. Both were charming and humorous persons, both had a deep-rooted understanding for children and their reactions, both were able to work at varying degrees of seriousness, influencing their creative activity. Klee and Kandinsky became well-acquainted when Klee joined the Blaue Reiter (blue rider) group in Munich in 1911 (or possibly before then). It is an open question whether Kandinsky was thinking of Klee’s affection for Mozart when he (Kandinsky) wrote, in Über das Geistige in der Kunst 32 (p. 108–109), that harmonization of colors was inappropriate for the times and that ‘…we can listen to the works of Mozart perhaps with envy, with a sad feeling of sympathy. They are a welcome pause in the roaring storms of our inner life, a consolation and a hope … but from a time unfamiliar to us; … contrasts and contradictions – that is our harmony’. He made real the desire of his generation for rhythm in painting, for geometrical constructions, and for setting color in motion. It is interesting to note that Kandinsky and Klee painted pictures with almost identical titles taken from Baroque music: ‘Fugue’ (1914, Beyeler Foundation, Riehen-Basel), and ‘Fugue in Red’ (1921, Livia Klee, Berne), respectively. Phillips recently published a book of short essays on music in art, written for laypersons interested in the subject77. How, at the end of the twentieth century, do we understand the developments since Kandinsky and Klee made their pronouncements at its beginning? During and immediately after the First and Second World Wars, two styles developed: Dadaism and Tachism, respectively. Both were clearly initiated in reaction to these disastrous wars. The destructive war activities were compensated by a pacifistic working ideal which, however, disappeared again after about a decade (the picture of Morris Louis,1961,mentioned earlier in this chapter, may be classified as pacifistic). Even if we incline to neglect these styles because of their origins, I see little development in this century’s visual art which might be said to stem from the synesthetic hypotheses of Kandinsky and Klee. Doesn’t Mozart still come over to us today as little more than a welcome pause,just as Kan224

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dinsky said then? When he mentions hope, it is Beethoven whose music is full of hope, particularly if the dark circumstances of his fate are borne in mind. The quintessential tormented genius, Beethoven was afflicted by illness of hearing,which started when he was thirty years old and led to complete deafness in his early forties.Almost equally oppressive, however, was the long-running custody battle the bachelor Beethoven was forced to fight for the guardianship and education of his nephew Carl. In spite of it all he was still able to compose such momentous works as his Ninth (Choral) Symphony and the Missa Solemnis.Beethoven’s composition represents for me a typical philosophy of life characterized by hope for the future. Don Hoffner, the former Director of the Bezalel Academy of Art in Jerusalem, recently brought to my attention American conductor and composer Leonard Bernstein’s remark that ‘Music has no content’. ‘And that’s its content’,Hoffner added.He and I came to the conclusion that the same also applies to color, because it too has such an inherent basis of immense opportunities. Yet this boundlessness is a danger in an era like ours, characterized by the atomized society in which so many people exist. Art, however, is also a means of communication between the artist and those who hear, see, or read his or her works. This communication must exist in art and must be understandable.As rationalism and empiricism hold considerable sway in the world, it is therefore not surprising that some artists try to combine sensuousness with rationality. Richard Paul Lohse’s painting (Fig. 7.14) is an excellent example of such an approach. In his book Farbe als Sprache, Albrecht27 wrote (p. 112) that a number of naive optical effects presented in modern art result in the complete passivity of reaction in the viewer. Abhorring such a result, a good painter should devote all his or her energy towards the goal of making the viewer see more than the painter can offer. Albrecht calls this an afteroptic psychological effect. Several Lohse paintings are based on this principle: they require not only to be looked at, but actively deconstructed. Initiation of this kind of activity in the viewer is a characteristic of the value of an artist’s works. It is next to impossible to predict the future of any branch of art. Painter Bridget Riley’s quote (see Lamb and Bourriau, Ref. 8 in Chapt. 1, p. 63) that ‘there is certainly a pause, but … the spirit of artistic enquiry … does not die’, may well be correct.

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Let me close this section with an example of a triple synesthesia of three senses: vision, sound, and smell. It is a quotation from the Polish writer Czeslaw Milosz (born 1911 in Lithuania, later lived in Poland, France (after 1951) and the United States). The text is taken from his 1953 essay Zniewolony umysl (The Captive Mind 78). His treatment of sensory impressions demonstrates the richness – but also the complex synesthetic interplay – of our senses, way beyond what science is capable of analyzing.‘Never has there been a close study of how necessary to a man are the experiences which we clumsily call aesthetic. Such experiences are associated with works of art for only an insignificant number of individuals. The majority find pleasure of an aesthetic nature in the mere fact of their existence within the stream of life. In the cities, the eyes meet colorful store displays, the diversity of human types. Looking at passers-by, one can guess from their faces the story of their lives. This movement of the imagination when a man is walking through a crowd has an erotic tinge; his emotions are very close to physiological sensations. He rejoices in dresses, in the flash of lights; while, for instance, Parisian markets with their heaps of vegetables and flowers, fish of every shape and hue, fruits, sides of meat dripping with every shade of red offer delights, he need not go seeking them in Dutch or Impressionist paintings. He hears snatches of arias, the throbbing of motors mixed with the warble of birds, called greetings, laughter. His nose is assailed by changing odors: coffee, gasoline, oranges, ozone, roasting nuts, perfume. It would seem that the exciting and invigorating power of this participation in mass life springs from the feeling of potentiality, of constant unexpectedness, of a mystery one ever pursues’.

References and Notes 1. 2. 3. 4. 5. 6.

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See some examples discussed in Gage’s book (see Ref. 4 in Chapt. 1), p. 15. In modern terms, this might be called a technique of kinetic art (see Sect. 7.3). See Gage’s book (Ref. 4 in Chapt. 1), p. 52. Vermilion is a brilliant red pigment, related to cinnabar. They are named after the dyehouse of the Gobelin family in Paris, where manufacture started in 1662. B. Stutzer, L. Windhöfel, Augusto Giacometti. Leben und Werk, Verlag Bündner Monatsblatt,Chur,1991.This book contains discussions of the work of art reproduced here. Augusto Giacometti was a cousin of the painter Giovanni Giacometti and uncle of Alberto Giacometti ,who became well-known for his metal sculptures.

Color in Art and in Other Cultural Activities 17. ‘As the spirit and the world of the Bible occupy a large part of my inner life, I tried to express that in this work. It is essential to represent those elements of the world that are not visible, but not to reproduce nature in all its aspects. In spite of the difficulties of our world, I kept my love of inner life in which I grew up as well as my human hope for love. In our life, just as on an artist’s palette, there is only one color which gives sense to life and to art. It is the color of love’ (My translation from French, published in I. Vogelsanger-de Roche, Die Chagall-Fenster in Zürich, Orell Füssli Verlag, Zurich 1971). 18. See the translation in Gage’s book (see Ref. 4 in Chapt. 1; p. 118) of a long paragraph from the more extended Latin edition of Alberti’s treatise. 19. ‘Red color is formed from fire, blue from air, green from water, lead-gray and ashgray from the earth … white and black are not really colors … one finds a certain affinity of colors; putting them side by side lends them dignity and grace’. 10. ‘Painting is a combination of light and shadow in a close mixture with the diverse properties of all the simple and the complex colors.’ 11. Another blue mineral is azurite, a copper compound. It was used by the Egyptians. 12. D. Bomford in the book edited by Lamb and Bourriau, (see Ref. 8 in Chapt. 1), p. 20. 13. See the large reproduction of a detail (Eleasar) in Gage’s book (see Ref. 4 in Chapt. 1), p. 126. 14. See Poussin’s painting Holy Family on the Steps (1648), reproduced in Gage’s book (see Ref. 4 in Chapt. 1), p. 158. 15. Quoted from Gage’s book, p. 173. 16. The arrangement of colors on the palette is considered to be very important to painters.There are known statements to the effect that this arrangement is more decisive for the result than the subject of the picture. Gage (see Ref. 4 in Chapt. 1) wrote an interesting chapter on this problem; see also Kandinsky’s book discussed in Sect. 7.3. 17. Why does a British warship have a French name? It was actually ‘Téméraire II’. The first warship with this name belonged to the French navy and was captured by the British.( In French,‘téméraire’means rash,daring,bold,like ‘temerarious’ in English.) 18. A typical example of Monet’s early work is the depiction of the sky in Le port de Zaandam (1871), see reproduction in Heinrich’s book19, p. 30. 19. Ch. Heinrich, Claude Monet, Benedict, Cologne (in English), 1994. 20. See, for example, van Gogh’s nighttime painting of a café in Arles (1888) with its very sharp red/green contrasts. Reproduced in Gage (see Ref. 4 in Chapt. 1), p. 196. 21. Some painters distinguish between Pointillism as an ‘optical mixture’ of small touches which cannot be seen as separate and distinct identities, and Divisionism with an ‘interaction’ of color, i.e., larger touches with small spaces inbetween. 22. See the reproduction of the whole picture in Kemp’s book23 (color plate XV) and that of a significant portion of it in Gage’s book (see Ref. 4 in Chapt. 1), p. 220. 23. M. Kemp, The Science of Art. Optical Themes in Western Art from Bruneleschi to Seurat, Yale University Press, New Haven, 1990.

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Color in Art and in Other Cultural Activities 24. F. Ratliff, Paul Signac and Color in Neo-Impressionism, Rockefeller University Press, New York, 1992. 25. J. D. Mollon,‘Signac’s Secret’, Nature 1992, 358, 379–380. 26. This painting is, therefore, twelve years older than Disque simultané (1911) by Robert Delaunay (1885–1941), often considered to be the first abstract painting in France (see, for example, Albrecht27, p.25). Delaunay’s influence is discussed briefly later in this book (p. 196). 27. H. J. Albrecht, Farbe als Sprache. Robert Delaunay – Josef Albers – Richard Paul Lohse, DuMont Schauberg, Köln, 1974. 28. Translated from the German editions of the books by Malevitch29 (p. 89) and Vallier30 (p. 15–25). 29. K. Malevitch, Suprematismus – Die gegenstandslose Welt, DuMont Schauberg, Köln, 1962. 30. D. Vallier, Braque. La peinture et nous. Cahiers d’Art 1954, 29, 13–24; German: Kunst und Zeugnis, Arche-Verlag, Zürich, 1961, p. 11–28. 31. See reproduction by Riley in Lamb and Bourriau’s book (see Ref. 8 in Chapt. 1, p. 62). 32. W. Kandinsky, Über das Geistige in der Kunst, R. Piper, München, 1912. English edition: On the Spiritual in Art, S. R. Guggenheim Foundation, New York, 1946. Quotations in this book from the tenth German edition, Benteli, Bern, 1973. 33. ‘Picasso sticks at nothing, and if color distracts him from the problem of drawing form properly,he throws it overboard and paints a picture in brown and white. Such problems are actually his main strength. Matisse – color. Picasso – form. Two great directions to one grand goal.’ 34. ‘All these assertions are the results of empirical and spiritual impressions, and they are not based on any positive science’, p. 88 of Ref. 32. 35. My statement is somewhat broader than that of Kandinsky. A more extensive discussion of Kandinsky’s book by Gage (see Ref. 4 in Chapt. 1), p. 207–209 and 212, is worth reading in this context. 36. J. Itten, Kunst der Farbe, Otto Maier, Ravensburg, 4th edn., 1974. 37. The development of Malevitch’s style was, however, different. 38. Mondrian considered that color in abstract painting was a ‘precise mathematical way of expression’, and that it could be brought to ‘precision, first by reduction of natural color to primary color, second, by reduction of color to plane and, third, by demarcation…[i.e.,] as units of rectangular planes.’ 39. Perspective viewing is strongly culturally influenced, as shown by Segall et al.40 He showed Fig. 7.23, the so-called Müller-Lyer optical illusion, to 1800 subjects in the USA and to groups of twelve tribes in Africa. The result showed that subjects living in Western cultures were more often deceived by this illusion than

Fig. 7.23. Müller-Lyer optical illusion: Which of the horizontal lines is the longer?

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40. 41. 42. 43. 44.

45. 46. 47. 48. 49. 50.

51. 52. 53.

the Africans (correct answer: the lengths are the same). The cause is the fact that straight lines and right angles are ubiquitous in Western environments, but not in Africa (circular buildings etc.). M. H. Segall, D. T. Campbell, and M. J. Herskovits, The Influence of Culture on Visual Perception, Bobbs-Merrill Co., Indianapolis, 1966. J. Albers, Interaction of Color, Yale University Press, New Haven, 1963 ; German edn., DuMont Schauberg, Köln, 1970. An example is his painting ‘Theta’ (1961) in the Museum of Fine Arts in Boston. I see a similarity between minimal art and Feyerabend’s laisser-faire philosophy of science (anything goes) which was developed at the same time (1975). ‘Painting is wedded to the present and is hence timeless and static. Surrealism sought to depict reality by pushing it too far, Cubism by geometrizing it, and, in doing so, straight away found itself trapped by its own contradictions (Cubism is planimetric, its stereometry simulated), abstract art resorted to pure logic and so to security.’ (translated from Versuche, Diogenes, Zurich, 1985, p. 124.) In the interests of brevity, I shall mention as few names of people and places as possible and omit the traditional names of periods of Japanese history and general information on Japanese geography. A peculiarity of the Japanese religious sense is the fact that the majority are adherents of both Shintoism and Buddhism. This aspect of Japanese art and its influence on European art (van Gogh) was briefly discussed by Gage in Lamb and Bourriau (Eds.; see Ref. 8 in Chapt. 1, p. 189) in a section entitled Disdain of Color. The family names of these artists are Katsushika and Andô, respectively. Family names precede first names in Japanese. In Western languages, however, they are generally called Hokusai and Hiroshige. The number of pictures is higher than the number of places in both series, because there are some places which the artists depicted from two vantage points. For example,M.Narazaki,Masterworks of Ukiyo – E. Hiroshige,Kodansha International, Tokyo. 1996. This book contains all of the 53 Stations of the Tôkaidô pictures. Selected examples can be found in many other books, such as: Y. Awakawa, Zen Painting, Kodansha International, Tokyo, 1970; M. Ishizawa (and six co-authors), Japanese Art, Kodansha International, Tokyo, 1981; German edition: Japanische Kunst, Krüger, Frankfurt a. M., 1982. For an excellent selection of Japanese art from archaic times to the nineteenth century, see I. Tanaka, K. Koike, Japanese Coloring, Libro Port, Tokyo, 1982; L. Smith, Ukiyo – E. Images of Unknown Japan, British Museum exhibition catalogue, London, 1988. H. Zollinger, ‘Zusammenhänge zwischen Neurobiologie des Farbensehens der Farbwortlinguistik und Jungs Arbeiten über die psychologische Bedeutung der Farben’, Analytische Psychologie 1985, 16, 88–103. C. G. Jung, Collected Works, Princeton University Press, Princeton, NY, 1953 and 1968.Most work relevant to color psychology can be found in Vols.9,I (2nd edn., 1968) and 12 (1953). Spontaneous use of colors plays an important role in difficult psychological situations,as the following incident in my own family illustrates. My wife was babysitting our youngest son’s three children for an afternoon. They live in a mainly agricultural area, and a neighbor suddenly brought the news that our

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54. 55. 56. 57. 58. 59.

60. 61.

62. 63.

64. 65. 66.

67.

68.

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grandchildren’s cat had been killed in a field by a mowing machine. The two boys shed bitter tears, but seven-year old Nina at first did not seem to react. After about a minute, however, she asked my wife to draw the mowing machine and the cat in bold outline, as she wanted to color them in using her color pencils. She did so in several colors while saying very little, finding comfort that way. C. G. Jung, Mandala. Bilder aus dem Unbewussten, Walter Verlag, Olten (now Zurich), 5th edn., 1983. I. C. McManus,A. L. Jones, J. Cottrell,‘The Aesthetics of Colour’, Perception 1981, 10, 651–666. Saito57 provides a good example of a reliable comparative study of color preferences in Japan and other Asian regions, with special emphasis on the preference for white. M. Saito, ‘Color Preferences in Japan and Other Asian Regions’, Color Research and Application 1996, 21, 35–49. J. Liberman, Light – Medicine of the Future, Bear & Co., Santa Fe, NM, 1991. It is known, for example, that sales of a certain brand of cigarette almost doubled within a few months of changing the color of the package from a dull green to a bright royal blue. It was assumed that many smokers of this brand were looking for a world of illusion. J. W. von Goethe, Zur Farbenlehre, Cotta, Tübingen, 1810; English translation: Theory of Colours, M. I. T. Press, Cambridge, MA, 1970. In its English editions, the title of Zur Farbenlehre is translated as Theory of Colo(u)r(s). I agree with Sepper’s criticism62 that this translation overlooks the important didactic goal Goethe had in mind (Lehre). Stylistically, moreover, the preposition Zur here means ‘contributions to the’ or ‘on the’, a meaning which, however, Goethe clearly did not intend. D. L. Sepper, Goethe contra Newton, Cambridge University Press, Cambridge, 1988. J. W.von Goethe,Die Schriften zur Naturwissenschaft,(1790–1810),edited by the Deutsche Akademie der Naturforscher Leopoldina in Weimar, German Democratic Republic, 1975. Zur Farbenlehre and related subjects are the subjects of the First Section, volumes 3 to 7. This is the only post-1945 edition to contain the Polemic Part and all the figures. A. Schöne, Goethes Farbentheologie, C. H. Beck, München, 1987. ‘It took little deliberation for me to perceive that a boundary is necessary to produce these colors, and I immediately spoke it out aloud, as if by instinct, that the Newtonian teaching is false’. In 1796, Goethe and Schiller jointly published 414 short poems, satirical distichs (couplets), called Xenien (from the Greek expression for hospitality gifts). Between 1821 and 1827, Goethe published six series of, as he called them, Zahme Xenien (tame poems). ‘… the colors diametrically opposite one another in the color circle are those which reciprocally call for each other’s presence. Thus yellow demands violet; orange blue; purple green, and vice versa’. Goethe calls complementary colors ‘geforderte Farben’ (called-for colors). I can recommend two relatively recent books for additional and deeper information. Sepper’s Goethe contra Newton62 emphasizes philosophical and scien-

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69. 70. 71.

72.

73. 74. 75.

76.

77. 78.

tific aspects, while Schöne’s book64 is written by a well-known scholar of German literature who is also well acquainted with physics. The latter’s provocative title Goethes Farbentheologie relates to Schöne’s finding that the style of Goethe’s writing in Farbenlehre reflects that of Martin Luther. Goethe never quoted these statements by Newton, another sign of his prejudice against him. Steiner attributed a very key role in anthroposophy to Goethe, but made no critical assessment of works by him, the Polemic Part of Farbenlehre included. ‘In living nature, nothing happens that does not exist in some relationship to the whole, and if experiences appear to us only in isolation, if we regard experiments solely as facts in isolation, that is not to say that they truly are isolated. The question is, how are we to find the relationships between these phenomena, these givens?’ ‘The things I have achieved as a poet I do not pride myself on at all. There have been excellent poets during my lifetime; still more excellent ones lived before me, and after me there will be others. Yet, in the difficult science of color, I am proud that I am the only one in my century to know the truth, and there I have an awareness that is superior to others’. Gage gives a thorough description from the Greek era to the mid-twentieth century in Chapt. 13 of his book (see Ref. 4, Chapt. 1). ‘Color and sound do not allow direct comparison at all, but both refer to a higher formula, both are derived each but for itself from this higher law.’ The thorough-bass was developed in Italy (basso generale or basso continuo) in the sixteenth century as a notational system for accompanying instruments (organ, harpsichord, lute). Its function is to establish the harmonic structure, i.e., the chordal structures, relationships, and progression. ‘This prophetic remark of Goethe’s is a premonition of the situation in which painting finds itself today. This situation is the starting point of the path by which, through the means of its own techniques and materials, painting will develop into an art in the abstract sense and where it will ultimately attain the condition of pure composition in paint’. T. Phillips, Music in Art: Through the Ages, Prestel, Munich, 1997. C. Milosz, The Captive Mind. Alfred A. Knopf, New York, 1953.

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Epilogue

8. Epilogue This book’s seven chapters have described various aspects of the phenomenon that is color. I hope that they have conveyed just how multidimensional and multidisciplinary color indeed is. This short epilogue contains some thoughts on a very central question, that of the relationship of color to human consciousness. It is short because, as Ludwig Wittgenstein put it (in his Tractatus logico-philosophicus), ‘Wovon man nicht sprechen kann, darüber muss man schweigen’(‘whereof one cannot speak, thereof one must be silent’). That particular statement comes at the very end of the Tractatus, but the same idea can also be found in its preface – in a slightly longer but (in my view) more informative version: ‘Was sich überhaupt sagen lässt, lässt sich klar sagen; und wovon man nicht reden kann, darüber muss man schweigen’ (‘what can be said at all can be said clearly; and whereof one cannot speak, thereof must one be silent’). Wittgenstein’s statement is easily applicable to Chapts. 2 and 3: Our present knowledge in color physics and color chemistry is well established, although by no means complete. Yet, even in physics and chemistry, we have to stay mindful that the relatively clear status of color in these branches of science was arrived at only through assiduous research over several centuries. The best example is the explanation of the physical cause of the rainbow. As discussed in Sect. 2.3, it took more than 700 years from the pioneering work of Robert Grosseteste to understand this impressive phenomenon of nature, which we can see, but neither reach nor touch. For many of our contemporaries, the rainbow is still as much a mystery as it was for Noah after the Flood, when God told him that the rainbow ‘is the token of the covenant’ which he made between Noah and himself (Genesis 9, 12). Colorimetry (Chapt. 4),as the very term suggests, also belongs to the exact natural sciences. We measure colors with quantitative physical methods, and so can define them within well defined numerical parameters. Yet, such colorimetric results do not seem to be as clear as expected – as is shown in the comic strip in Fig. 8.1 by Lucy’s reaction to ‘grass green’. This little story demonstrates better than many words that quantitative measurements and emotions are close but not intimate neighbors in the world of colors.

233 Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

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Fig. 8.1. What is ‘grass green’ ?

Emotions are also related to the use of color terms and of the word color itself in situations which are colorless to a scientist. A ‘poisonous green’ does not contain any poison in the sense a chemist or medical doctor would recognize, and two books with the same title – Primary Colors – can have entirely different contents, as was shown in Sect. 1.1. These emotions bring us to human vision, color vision in particular. Our knowledge of vision began with the development of anatomy in the sixteenth century and of physiology some two hundred years later. Thomas 234

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Young’s research at the very beginning of the nineteenth century opened up the field of color-vision physiology. It concentrated first on studies of the eye,then from the mid-twentieth century on the brain – by far the most complex organ and the center of all functions of man and animals.‘We see with our brain, not with our eyes’ became a slogan. Today, neuroscientists measure chemical potentials in nerve cells using micro-electrodes, patients suffering from partial vision deficiencies undergo psychophysical vision tests, and PET(positron emission tomography)-based physicochemical localization of brain activities makes it possible to identify non-invasively those areas of the cortex (and other parts of the brain) active in various types of vision processes. One major general finding was that the visual system of the brain includes individual nerve cells – and even whole regions – that are specifically responsive to particular features of objects, such as form, color, brightness, movement, lines, and angles. Differential processing of the signals from the two eyes makes it possible to estimate the distance to an object, and so to optimize the accommodation of the lenses to that distance. How do scientists observe these processes in the brain? They record membrane potentials, extremely small changes in blood pressure, positron emissions and so on: in a nutshell, they measure physical and chemical processes. The achievements of this analytical approach have been spectacular, but how are these unitary processes organized into the complex functional system of cognition and comprehension? We experience a unified perception of a single object – and yet it remains wholly imperceptible to us that the incoming information has been processed in what have been found to be widely separated cells and regions of the brain. In the same decades that brain research was making these findings, enormous progress was taking place in information research and technology. Computers of the 1990s are unbelievably more effective than those of the 1950s in speed and capacity, but also in many applications beyond numerical calculation. Is not our brain, therefore, simply an excellent computer? Why do many people use the term ‘artificial intelligence’ for some nontrivial tasks which can be solved by computers today?1 Numerous experts in computer technology, mathematics, philosophy, and other disciplines have pronounced on the matter: their answers to the question cover the whole spectrum from yes to no. I cannot discuss them here, not only for reasons of space, but because I am not able to evaluate them. Therefore, I think it best to follow the second part of Wittgenstein’s comment. That does not mean, however, that I recommend the decision I made to all readers of my book – and I made it only for this book2. 235

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Yet, I hope that Wittgenstein would not object to the following remarks by a spectator of brain research. Recently, I attended the Inaugural Lecture of a young professor of architecture at my home institution, ETH in Zurich. She spoke on computeraided architectural design (CAAD),and mentioned as a comparison to her work the chess competition between the world champion Kasparov and the chess supercomputer Deep Blue 7. She said that a chess champion is able to evaluate about three moves per second,but Deep Blue one million moves per second.If so,why doesn’t Deep Blue win all the competitions? The much slower reaction of the brain is easy to explain neurobiologically; transmissions of signals between neurons are diffusion reactions and chemical processes. Both are many powers of ten slower than the transfer of electrical charges in computers. The difference between Kasparov and Deep Blue, however, is not only one of powers of ten in speed: ‘powers of ten’ is only a one-dimensional feature. Therefore, there must be other ‘features’ which make the brain an effective competitor to a computer in chess games.Unlike that of the brain, the ‘world’ of any computer is limited to given areas with specified and classified values, or – in other words – with an abstraction of reality (see also Searle’s comments6, p. 57 ff. and 208 ff.). My own conclusion is that we are really still far from understanding how the brain works in solving the logical problems of chess and still further regarding the (less logical, but more emotional) redness of red, let alone how it works when seeing a beautiful painting or when experiencing the feeling of hope. I agree, therefore, with the essence of one of Searle’s statements3 (p. 228): ‘There are brute, blind neurophysiological processes and there is consciousness, but there is nothing else’8. Today, mental processes, subject of repeated study for over two thousand years, once more take center stage in the mind-body problem. Diderot’s explanation – almost 250 years ago – of vision and perception by eye and brain (see Fig. 5.1) illustrates a little of the inherent difficulty as regards the subject of this book. Diderot’s application of optics to the eye is correct: the ‘picture’of the arrow on the retina is upside-down.Yet,he then extrapolates an analogous second process in the brain, because we do not see the world upside-down. Diderot’s explanation may be called a homunculus process – a transfer of a reaction by a person in the real (outside) world to a reaction by an analogous mannequin (homunculus) in the brain10. Hypotheses based on homunculi were not only a seventeenth century development; they still crop up today, in computational theories of cogni236

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tion and consciousness, for instance. It is possible to describe the vision process,in all its above-mentioned aspects,starting off from a two-dimensional visual array on the retina and progressing to a ‘description’ of the three-dimensional external world as the output of the vision process. But isn’t it still just a homunculus who then has to ‘read’that description? Aren’t we, therefore, just replacing one big black box, the mind, by a large number of small black boxes? As an analytical method, this has been – and still is – stupendously successful in classical physics and chemistry, but it is inadequate for integrated investigation of living organisms. These questions remain unanswered; maybe they cannot be answered at all. A major difficulty in solving them is highlighted in Max Planck’s observation that ‘Wissenschaftliches Denken erfordert immer einen weiten Abstand und eine scharfe Trennung des denkenden Subjekts von dem gedachten Objekt’11. This caveat of Planck’s is closely related to the distinction philosophers make between those features of the world that are intrinsic, in the sense that they exist independent of any observer, and those features that they consider observer-relative, in the sense that they only exist relative to some outside observer or user (quoted after Searle3, p. XIII). Planck’s remark is exemplified by Heisenberg’s uncertainty principle – itself a consequence of Planck’s original discovery of quantum processes (see Sect. 2.1). The uncertainty principle is what restricts the validity of determinative laws in nature to macrophysics, while elementary-particle processes react acausally.This made it clear early on (1927) that our understanding of the world of classical physics, such as the nature of light (see Sect. 2.1), is inherently confined within certain limits. Extensions of quantum mechanics into life sciences have been striven after for several decades (for example, by Pascual Jordan, 1947) but have not as yet led to any breakthrough of an importance comparable to that of Heisenberg12. Planck’s opinion of the subject/object relationship can also be expressed by saying that,in cognition and consciousness,we are simultaneously producers and products of our own personal, interpersonal (i.e., social), and environmental histories, and, therefore, we are unable to recognize our products ‘objectively’. This paradox of self-reflection is also nicely expressed in a 1993 book by Peter D. Kramer13, based on his experiences of using Prozac (then a new antidepressant drug) to treat patients in his psychiatric practice. On page 133, he mentions an aside by one of the pharmacologists who developed that drug:‘If the human brain were simple enough for us to understand, we would be too simple to understand it’. 237

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This empirical statement fits the elaborate discussion of cognitive science given by Pinker in his most recent book How the Mind Works14. It demonstrates that he is at the forefront of brain science after being a pioneer also in the philosophy of language (see Sect. 6.2). His goal is to reconstruct all mental activities with the help of analytical techniques, and by combination of computer technology, neurobiology, and theory of evolution. His work indeed leads him to very remarkable degree of comprehension of how the mind works. Yet, he realized also that his methodology does not allow understanding of problems like free will and ethics. He contradicts philosophers like Daniel Dennett who postulated that phenomena that are analyzable scientifically do not exist at all. Pinker says ‘we are organisms, not angels, and our brains and organs are no secret channels to truth’(translated from the German edition). Therefore, Pinker also comes to the conclusion that certain states of facts stay impervious to scientific approach. In the last pages of this epilogue, I shall devote a few paragraphs to the relationship between brain and visual art and to the fundamentals of beauty in general.I closed Chapt. 1 with a quotation from Einstein,that the most beautiful experience is the mysterious and that emotion ‘stands at the cradle of true art and true science.’ Artists and scientists are both seekers for truth: artists in our inner (human) world, scientists in the world around us.For both activities,emotion is important from the very outset,and both are mysterious. Is beauty intrinsic to the object or dependent upon the observer, the subject? With these questions in mind,an international group – consisting of physicists, a chemist, neuroscientists, psychologists, anthropologists, a philosopher, a visual artist, a musician, and a poet – met for a few days on seven occasions between 1979 and 1983 on invitation of the Werner Reimers Foundation in Bad Homburg, Germany. Their aim was to discuss biological ‘aspects’ of aesthetics15, and a book edited by Rentschler, Herzberger, and Epstein16 contains most of the contributions considered in general discussions within the whole group. How does the brain decide that information received by one or more of the senses is beautiful? If a number of subjects with normal color vision see a piece of green paper whose hue is close to unique green, it is very likely that all of them will say it is green. If we ask them, however, if that color is beautiful we will get different answers.This result is,of course,psychological in nature. And, if we then present to these observers a paper with a uniform green background, but sporting various patches of color in the same or different forms,the answer will be even more complex.Such 238

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constellations of optical stimuli on our retinae influence our psyches in very complex ways, but can hardly lead us to an understanding of beauty. Certain features are likely to be important as to whether an individual person considers something to be beautiful. From what we have already seen in this chapter, it is clear that we do not know at all how the following features are weighted and combined or coordinated in our brain: 1) What we consider as visually beautiful is likely to be a type of input that corresponds optimally to perception in the eye and further processing in the various parts of the visual cortex. 2) Plasticity is an important characteristic of the central nervous system. Plasticity of the brain, particularly in substructures where signals from different centers (for form, color, movement etc.) come together, is the basis of learning. Therefore, we can learn to appreciate works of art which we did not like as much when younger, and vice versa. 3) All ‘good’ works of visual art, after the immediate physical and neurobiological processes of seeing them, exert a continuing mental effect on the viewer, beginning with a tendency to more attentive observation followed by deeper insight into correlations, sometimes combined with thoughts and speculations about the inner world of the artist, or about the above-mentioned ‘seeking for truth’. There might be a discrepancy between the immediate process of seeing and this mental effect. (Many modern artists unfortunately emphasize such discrepancies with naive or intellectual intent.) Synesthetic phenomena are even more complex17. 4) The search for essentials is a feature related to this mental effect, and also to the search for essentials in scientific research.This search for essentials in art – like in science – should not, however, be confused with the search for the shocking and unusual, as is sometimes the case (in some works of kinetic art, for example). ‘Essential’ is a term applicable to various areas of reference. In brain-related studies of visual art objects, it may refer to the dominance of one brain center in the evaluation of visual input; the movement center when viewing a work of kinetic art,for example.Personally,I find that most, but not all,works of visual art which I like because of their beauty involve several brain centers and form a pleasing wholeness. As an example, I mention Titian’s Ariadne and Bacchus, which I discussed earlier in this book (Fig. 1.1, see also Sect. 7.2). As well as form and color, this work has a great feel of movement about it, but it is movement 239

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at a higher cognitive level than just seeing objects which move in the literal sense of the word. You fairly often hear opinions expressed that listening to works by Mozart gives you the feeling that God must have dictated the particular work to Mozart18, or that archaic levels of consciousness or emotion are involved in that listening. These two feelings are actually not too far removed from one another: theologians have arrived at the conclusion that faith in the divine is a manifestation of a very ancient form of consciousness. In this context, a remark by Cézanne is interesting: ‘Je me sens coloré par toutes les nuances de l’Infini’ (‘I feel I am colored by all hues of the Infinite’, quoted by Guerry19, p. 180). Cézanne indicates that he felt liberated from the three-dimensionality of the ‘real-world’ color space. This idea is clearly beyond the understanding of our consciousness. These remarks bring me back to Wittgenstein’s sayings at the beginning of this epilogue.All that I said afterwards had the purpose of showing that there are three types of phenomena in color vision – and in all other processes involving mental and perceptory human activity: namely those which we are able to understand, others which we understand in part, and which we or following generations may understand better20, and finally those which we do not understand, and which are not likely to be understandable at all. These last are the subject of religious revelation. The difference between human and divine knowledge was expressed in a very concise form by the Apostle Paul who said ‘we know in part’ (First Cor. 13, 9. The German version is more informative: ‘Unser Erkennen ist Stückwerk’; ‘Stückwerk’ has the meaning of ‘in irregular patches’). At a time when the manuscript of this book was already at the publishers, the theologian Werner Meyer brought to my attention C. G. Jung’s psychological concept of the self 21. Jung calls it ‘a construct that serves to express an unknowable essence which we cannot grasp as such … It might equally well be called the ‘God within us’ … [It] lies beyond the bounds of our understanding … By affixing the attribute ‘divine’ to the workings of autonomous contents [i.e., of the self], we are admitting their relatively superior forces.’ Doesn’t Jung’s concept of the self fit remarkably well with the third type of phenomena in color vision mentioned above? (My putting this question should not be taken though to imply any doubt at all on my part that there exist specific biological mechanisms – involving neurons in the brain – which are necessary for our sentience.)

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The French mathematician, physicist and philosopher of religion Blaise Pascal (1623–1662) said ‘science sans conscience n’est que ruine de l’âme’ (‘science without conscience is no more than the ruination of the soul’). Pascal’s conviction has lost none of its veracity and value today. Most of these personal thoughts have developed slowly over some decades in tandem with my scientific work in research and teaching. I cannot trace them back specifically. While on the topic, I would, however, like to add one humorous small event. When I was elected to a professorship at ETH Zurich in 1959, a regional Swiss newspaper published a short note which is part of Fig. 8.2. This notice was taken over by the Swiss satiric weekly newspaper Nebelspalter (literally ‘Cleaver of mist’) for its column Unfreiwilliger Humor kommt auch in Gazetten vor (unconscious humor can also be found in newspapers, Swiss newspapers being notoriously serious-minded),and the editor made the comment about the notice shown in italics (Even theology is in the process of being modernized). The misprint-creating gremlin was, I think now, not totally incorrect!

Fig. 8.2. Is the misprint-creating gremlin correct?

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References and Notes 11. In this context, it is interesting to note that neurons work in an analogue mode (see Sect. 5.2) whereas (modern) computers are digital processors.Yet, relatively few scientists who investigate models for the brain use analogue computers (e.g., Carver Mead). 12. I might, however, mention that during my work for this book I read two books cover-to-cover, namely those of John Searle3 and of Gerhard Roth4, and that of Francis Crick5 in part. Also most informative to me was Searle’s extended review of books by Francis Crick, Daniel C. Dennett, Gerald M. Edelman, Roger Penrose (a lecture by whom I also attended in 1997), and Israel Rosenfeld. The review was first published in The New York Review of Books (1995) and later (1997) in extended form as a book6. To me, as a reader trained in experimental science, it was astonishing how the mystery of consciousness was discussed by these five authors in such a diversified, non-interrelated way (that statement is clearly not applicable to Searle’s book). They are not easy reading, but worth recommending. 13. J. R. Searle, The Rediscovery of the Mind, MIT Press, Cambridge, MA, 1992. 14. G.Roth, Das Gehirn und seine Wirklickeit. Kognitive Neurobiologie und ihre philosophischen Konsequenzen, 5th edition, Suhrkamp, Frankfurt a. M., 1997. 15. F. Crick, The Astonishing Hypothesis, The Scientific Search for the Soul. Simon and Schuster, New York, 1997. 16. J. R. Searle, The Mystery of Consciousness, New York Review of Books, New York, 1997. 17. See Sect. 1.1., where Deep Blue is mentioned in another context. 18. So that no reader of this book should misunderstand Searle’s statement,I should add that his work is clearly in accordance with the hypothesis that mental processes are caused by neurophysiological processes. A discussion of emotional intelligence relative to rational intelligence is given in Goleman’s book9. Both types of intelligence can be subsumed under the ability to handle new,unknown situations. 19. D. Goleman, Emotional Intelligence. Why it can matter more than IQ, Bantam, New York,1995.German edition: Emotionale Intelligenz. Carl Hanser,München, 1996. 10. As a symbolic pictorial idea, the homunculus was developed in late Antiquity. 11. ‘Scientific thinking always requires a large distance and a sharp separation between the thinking subject and the object thought of ’. 12. Penrose (mentioned in Ref. 2 on recent books on consciousness) thinks that further developed physics of quantum mechanics may aid better understanding of the brain. 13. P. D. Kramer, Listening to Prozac. A Psychiatrist Explores Antidepressant Drugs and the Remaking of the Self, Viking Penguin, New York, 1993. 14. S. Pinker, How the Mind Works, Norton, New York, 1997, German edition: Wie das Denken im Kopf entsteht, Kindler, München, 1998. 15. It was originally intended to use the word ‘basis’in the name of that study group. I suggested ‘aspects’ because that avoids the implication that biology is the only basis of aesthetics.

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Epilogue 16. I. Rentschler, B. Herzberger, and D. Epstein (Eds.), Beauty and the Brain. Biological Aspects of Aesthetics, Birkhäuser, Basel, 1988. 17. I find it doubtful that the neuropsychological localization of synesthetic effects in the brain, as performed by Richard Cytowic in the United States, and more recently, by Hinderk Emrich in Germany, can provide any more than anatomic information about synesthesias. 18. This statement is used mainly for Mozart’s more elegant pieces, such as Eine kleine Nachtmusik. Yet, in my opinion, it is appropriate also for his dramatic and tragic work, e.g., the symphony no. 40 in G minor (K. 550) or the opera Don Giovanni. Such feelings about Mozart’s works are not far away from a statement of Stravinsky on a piece of our century: ‘I am the vessel through which ‘Le Sacre’ (du Printemps) passed.’ 19. L. Guerry, Cézanne et l’expression de l’espace, Flammarion, Paris, 1950 (quoted from J. Gebser, Der unbekannte Ursprung, Walter, Olten, Switzerland, 1970, p. 116). 20. ‘Better’ – but probably not ‘completely’ – see Popper’s statement in his autobiography Unended Quest (Ref. 5 in Chapt. 2). 21. C. G. Jung, The Collected Works of C. G. Jung, Pantheon, New York, 1953, Vol. 7, pages 236 f.

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Name Index

Name Index A Abramov, I. 106 f., 125 Addison, J. 1 Albers, J. 196, 219, 228, Alberti, L.B. 176 ff. Albrecht, H.J. 193, 225 Alexander of Aphrodisias 23 Ando, Hiroshige 206 f., 207, 208, 229 Angelico, F. 173 Antonio di Pisa 173 Apelles 162 Archimedes 13 Aristotle 3, 5, 23, 24, 77, 161 f., 163, 164 B Bach, J.S. 223 Baeyer, A. von 44 Baylor, D.A. 88 f. Beatus of Liébana 164 Beethoven, L. van 223, 225 Berlin, B. 158 Bernstein, L. 225 Biernson, G. 74 Bing, S. 206 Blanc, Ch. 180 Blonsky, M. 158 Bohr, N. 9, 15 ff., 134 Bolton, R. 150 Bornstein, M.H. 80 Bourriau, J. 225 Boyle, R. 73 Bragg, W.H. 30 Bragg, W.L. 30 Braque, G. 191, 193 Brecht, B. 13 f., 131 Bürgi, H.-B. 43 C Calder, A. 196 Canetti, E. VI f. Castel, L.-B. 222 Cennino, C. 176 ff. Cézanne, P. 188 f., 191, 206, 240 Chagall, M. 173 ff.

Chastaing, M. 137 Chevreul, M.E. 76, 188 Chittka, L. 115, 116 Chomsky, N.A. 124, 125 Conklin, H.C. 133 Corbett, G.G. 132, 148 Cranach, L. 170 Crick, F. 242 Crisp, D. 150 Crivelli, C. 176 Cytowic, R. 243 D Dalton, J. 86 Darwin, C.R. 28 f., 84, 114, 125 Davies, I.R.L. 132, 148 Daw, N.W. 100 Deinert, H. VIII, 215 Delacroix, E. 180, 186 Delauney, R. 196, 228 Democritus 7, 161 Dennett, D.C. 238, 242 Descartes, R. 24, 25 f., 26, 72 f., 180 Deutsch, N.M. 170 Diderot, D. 79 f., 82, 114, 236 Dietrich von Freiberg 24 f. Domany, E. 131 Dürer, A. 162, 170 Dürrenmatt, F. 197 Durrer, H. 32 ff. E Eckermann, J.P. 220 Edelman, G.M. 242 Ehrlich, P. 46 Einstein, A. 10, 15, 56, 238 Empedocles 161 Epstein, D. 238 F Feyerabend, P. 229 Feynman, R.P. VI, 3, 17 Forsius, A.S. 71 Franklin, B. 140

245 Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

Name Index Fraunhofer, J. von 13, 30 Fresnel, A. 13 Friedländer, P. 154 Frisch, K. von 114, 115 Fudô Myô-ô 201 Fux, J.J. 223 f. G Gage, J. VII, 77, 130, 159, 161, 188, 189, 196, 226–229, 231 Gellatly, A. 150 Giacometti, A. 173, 175, 190, 193, 226 Gibson, J. 1 Giotto di Bondone 3 Giros von Gentilly 119 Gladstone, W.E. 123, 127 Goethe, J.W. von 7, 19, 74, 78, 120, 154, 212 ff., 222, 223, 231 Gogh, V. van 184, 189, 206, 227, 229 Gordon, J. 106 f., 125 Gouras, P. 100 Graebe, C. 43, 56 Greco, El 153, 178 Gregory, J. 38 Griess, P. 44 Gropius, W. 192 Grosseteste, R. 24, 233 Guerry, L. 240 Gutenberg, J. 165 H Hadlaub, J. 166 Händel, G.F. 222 Harris, M. 74 Heider, E.R. 133, 134 Heisenberg, W. 9, 17, 57, 237 Hellwag, C. 136 Helmholtz, H. von 7 f., 84, 104, 108, 110, 119, 128, 157, 216 Heraclitus 210 Hering, E. 105, 108 Hero 20 Hershel, W. 15 Hertz, H. 15 Heumann, K. 44 Hippocrates 162 Hiroshige, see Ando Hoffner, D. VIII, 225

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Hofmann, A.W. von 42, 43 Hokusai, see Katsushika Homer 28, 123, 133, 161 Hooke, R. 11 Hubel, D.H. 93, 94, 97 f., 99, 100, 105 Hückel, E. 43 Hunt, D.M. 86 Hurvich, L.M. 105 f. Huygens, C. 11, 26, 30 I Iijima, T. 146 Ikkyû Sôjun 203 Ingres, J.A.D. 180 Ishihara, S. 86 Itten, J. 193 J Jacobs, G.H. 119 Jakobson, R. 124, 136 ff. Jameson, D. 105 Jordan, P. 237 Jung, C.G. 191, 208 ff., 240 Justinian 151, 154, 164 K Kandinsky, W. 192 f., 218, 219, 221 f., 223, 224, 227 Kant, I. 81 Katsushika, Hokusai 206, 229 Kay, P. 132, 133, 148 Kekulé, A. 43 Kirchner, E.L. 189, 218 Klee, P. 192 f., 219, 223 ff. Klein, J. 2 Köhler, W. 136 Koren, Z.C. 155 f. Kramer, P.D. 237 Kuffler, S.W. 96, 97 Kuhn, H. 13, 58 ff. Kuschel, R. 130, 147 L Lamb, T. 101, 225 Land, E.H. 67, 109 ff., 120 Laue, M. von 30 Le Blon, J.C. 73

Name Index Le Brun, Ch. 180 Leibnitz, G.W. 38 Leonardo da Vinci 18, 78, 111, 177, 178, 179 Liberman, J. 212 Lichtenberg, G.C. 214 Liebermann, C. 44, 56 Linné (Linnaeus), C. von 131, 158 Livingstone, M.S. 112 Locke, J. 221 Lohse, R.P. 194 ff., 195, 225, 228 Lorenz, K. 75 Louis, M. 196, 224 Lüscher, M. 212 Luther, M. 231 M MacLaury, R.E. 132, 134 f., 145, 151, 157, 158 MacNichol, E.F. 8, 87, 104 Maier, M. 211 Malevitch, K. 191 Mann, T. 149 Matisse, H. 189, 192 Maxwell, J.C. 7 f., 13 ff., 21, 104, 108 McDaniel, C.K. 132, 133 McManus, I.C. 211 Mead, C. 242 Menzel, R. 115, 116 Meyer, W. 240 Michelangelo Buonarroti 178 f. Milosz, C. 226 Mollon, J.D. VII, 86, 89 f., 92 f., 107 f., 109, 118, 188 Monberg, T. 130, 147 Mondrian, P. 109, 110, 193, 194, 196, 228 Monet, C. 74, 111, 183 ff., 185, 189, 206, 227 Mozart, W.A. 223 f., 240, 243 Munsell, A.H. 69 f., 120 N Nassau, K. 35, 36 ff. Nathans, J. 86, 92 f., 117 Neumeyer, C. 117 Newton, I. 4 ff., 11, 18 ff., 26 f., 64, 71 ff., 79, 104, 110, 126, 127, 128, 162, 213 ff., 219 ff., 221 Nijhawan, R. 107 f.

O Orgel, L.E. 48 Ostwald, W. 76 P Pascal, B. 241 Pauli, W. 9 Pauling, L. 61 Penrose, R. 242 Perkin, W.H. 7, 42, 43 Phillips, T. 224 Philostratus 163 Piaget, J. 80 f. Picasso, P. 191 f., 193, 228 Pinker, S. 125 f., 238 Pissarro, C. 184, 188 Planck, M. 15, 56, 237 Plato 23, 161, 163 Pliny the Elder 155, 162 Plutarch 163 Pople, J.A. 59 Popper, K.R. 17 f., 24, 158, 243 Poussin, N. 180 Pugh, E.N. 80 R Ratliff, F. 188 Reichstein, T. V Rembrandt, H. van R. Rentschler, I. 238 Riley, B. 225, 228 Ritter, J.W. 15 Roberson, D. 145 Rorschach, H. 212 Roth, G. 242 Rubens, P.P. 179 Runge, P.O. 76, 218 Ruskin, J. 173

179

S Saunders, B. 131 Schaie, K.W. 212 Scherrer, P. 17 Schiller, F. 230 Schilling, D. 167 ff., 169 Schönberg, A. 222 Schöne, A. 214, 231 Schopenhauer, A. 218

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Name Index Scriabin, A. 222 Searle, J.R. 157, 236, 237, 242 Segall, M.H. 228 Segantini, G. 186 Seurat, G. 184 ff., 187 Shôtoku, Prince 198, 207 Siegel, J.S. 43 Signac, P. 184 ff. Simpson, J. 150 Sklar, A.L. 56 Snellius, W. 20 f., 25 Snow, C.P. VI, 9 Spanier, E. 152 Sperry, R.W. 93, 96 Steiner, R. 218, 231 Stella, F. 196 Strawinsky, I. 243 Suzuki, Harunobu 205 Svaetichin, G. 97 f., 105 T Tarrant, A.W.S. 150 Teller, D.Y. 80 Theodora 154, 164 Theodoric of Freiberg, see Dietrich von Freiberg Theroux, A. 2 f., 10 Thurow, M. 151 Tinguely, J. 196 f. Tiziano Vecelli (Vecellio) (Titian) 3, 4, 178, 179, 239 Travis, A.S. 42 Trismosin, S. 29

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Turner, J.M.W. 173, 180 ff., 181, 182, 218 Tyndall, J. 30 V Vallier, D. 191 Valois, R. de 97 f., 105 Velázques, D. 179 Verguin, E. 42 Verrey, L. 102 f. Vinci, see Leonardo da Vinci Virgil (Publius Vergilius Maso)

23

W Wald. G. 8, 10, 87, 104 Wattenwyl, A. von 124, 146 Webster, M.A. 107 f. Wehner, R. 116 Werner, A. 59 Wierzbicka, A. 134, 139 Wiesel, T.N. 93, 97 f., 99, 105 Williams, J.M. 138 f., 141 Witt, O.N. 56, 58 Wittgenstein, L. 126, 233 ff., 240 Y Yang, Y. 150 Young, T. 7 f., 12, 28, 38, 87, 104, 108, 119, 235 Z Zajonc. A. 18 Zeki, S. 100 ff., 197 Zollinger, H. 108, 146 f.

Subject Index

Subject Index A Abstract art (see Art, abstract, and Non-representional pictures) Accommodation (eye) 82, 235 Achromatic colors (hues) 1, 64, 67, 75, 85, 104 f., 106 f., 128, 129, 136 ff., 142, 203, 209 Achromatopsia 101 ff. Ada manuscript 167 Adaptation experiments 107 f. Additive mixture of colors 65 ff., 73, 74, 78, 105 ff., 216 Aesthetics 238, 242, 243 After-image 74 Agnosia (see Color agnosia) Ainu, language 133 Akinetopsia 101 Alchemy 28 f., 210 f. Alexander’s dark band 24 Alizarin 42, 44 Altamira 3, 41 Amacrine cells 84 Ancient Purple 3, 42, 51 f., 78, 151 ff., 164 Angiosperm plants (see Flowers) Anomaloscope 86 Anomia (see Color anomia) Antagonism, chromatic 105 f., 107 f. Anthracene 50 f. Antocyanin dyes 53 Antroposophy 231 Aphasia 136, 138 Aphrodisian paradox 23 f. Arabic – language 130 – culture 3 Art (see also Baroque, Beauty, Gothic, Mosaics, Renaissance, Rococo) – abstract 190 ff., 223 ff. – book painting 165 ff. – color in antique art 161 ff. – color in European art 161 ff., 175 ff., 188 ff. – color in Japanese art 197 ff.

– color in twentieth-century art 188 ff. – constructivistic 194 f. – glass windows 170 ff. – in Goethe’s Farbenlehre 217 f. – kinetic art 101, 196 f., 226, 239 – minimal art 197 – textile art 76, 164, 165 Arthropods – vision 113 f. – evolution 115 f. Artificial intelligence 235 Assimilation in green plants 44, 53 Audition, physics and physiology 221 Axons 82, 84 Azo dyes 44, 52 B Bari, language 130 Baroque – painting 173, 178, 179 f. – music 223 Basic color terms – categorization 127 ff., 132 f. – definition 128 – evolution 131, 132 f., 134, 136 – general 124 – meaning 134 f. – stages 129 Bauhaus 192 f., 196, 219, 223 Beauty 238 f. Bees 54, 114 ff. Bellona Island, language 124, 131, 134, 147 Benzene 43 f. Berinmo language 145, 158 Berlin and Kay categorization 13, 124, 127 ff., 133 ff., 138, 142, 144, 148, 150, 151,162, 191, 209, 210, 218 Bible 165 – quotations related to color 22, 28, 151, 152, 164, 175, 210, 233 – quotations related to language 126, 240 Bipolar cells 84 f.

249 Color: A Multidisciplinary Approach. Heinrich Zollinger © Verlag Helvetic Chimica Acta, Postfach, CH8042 Zürich, Switzerland, 1999

Subject Index Blindsight 120 Blob cells 100 Bond, chemical – cleavage 140 – formation 57 f. – π-Bond 58 ff. – σ-Bond 58 – single/double 43, 49 ff. Book painting 165 ff. Bragg’s law 31 Brain (see also Cortex) – anatomy 93 ff. – art 238 f. – comparison with computer 235, 236 – development 99 f. – general aspects 235 – gray and white matter 95, 99 – hemispheres 94 f., 96 f. – language learning 96 – plasticity 99, 239 – speed of signal transmissions 236 Brightening agents 55 Brightness (see also Color, light/dark) – in categorization of color terms 132 f., 135 f. – in Munsell system 69 f., 128 – phonemes 136 f. Buddhism 199, 201, 203, 204, 208, 210 Butterfly colors 35 f. Byzantium (see Istanbul) C Camouflage colors 33, 37, 38 Carbon black 47 Carbonyl group 51 β-Carotene 45, 50, 53 Catalan language 132 Cave paintings 3, 41, 46 Center/surround receptive fields 97, 99 f., 105 Cephalopod molluscs 113, 114 Changeant effects 176, 177 Charge transfer 37 Chartres, cathedral 170 ff., 171, 172 Chelation 53 Chelicerata 116 Chemiluminescence 56 Chess computer (see ‘Deep Blue’)

250

Chauvet (see Grotte Chauvet) Chiaroscuro techniques 169, 177 Chiasma 94 f., 96 Chinese – color naming, influence of gender 150 – color terms 130, 133 Chlorophyll 53 Choroid 81, 82 Chromatic colors (hues) 1, 64 ff., 104 f., 107 ff., 124, 129, 136 ff., 161 f., 176, 177, 183, 186, 188, 189, 194 Chromaticity coordinates 67 Chromatism – oppositions 137 ff. – of sounds 136 ff. Chromatophoric cells 37 Chromophore of rhodopsin 89 ff. Chromosomes 92 f., 118, 126 Chronicles of Berne and Lucerne 167 ff. CIE (Commission Internationale de l’Eclairage) 8, 63, 67 ff., 70, 71, 75, 76, 78, 106, 108 Coal tar 42 Codex Manesse 166, 168 Co-extension (MacLaury) 135 Color – causes 36 ff., 56 – color effects in animals 28 ff., 33, 35 f. – of commercial products 212 – color and form 163, 175 f., 180, 196 – in folk tales 150 – historical aspects 3 ff. – inorganic compounds 59 ff., 170, 178, 187, 198, 226, 227 – Italian painting 3 f., 175 ff. – light/dark (warm/cold) 111, 129, 133, 135, 136 ff., 161, 186, 187, 209 – meaning 1 f. – mixing 7, 45, 65 ff., 73, 104, 216 – in non-European cultures 3, 197 ff. – organic compounds 58 ff. – perception 4 ff. – preferences 207 f., 211 f. – by reflection 20 ff. – by refraction 18 ff.

Subject Index – space (solid) 67 ff., 71 ff., 77, 127, 132, 134, 209, 223 – in subatomic physics 3 – as symbols 207, 210 f., 218 Color agnosia 103 Color anomia 103 Color-blindness 85 Color chemistry 7, 41 ff., 124, 233 Color constancy 67, 109 ff. Color instruments 220 Color naming – age, influence of 148, 149 – art students 149 – categorization 127 ff., 132, 134 – certainty of determination 143, 146 ff. – complementary colors 133, 138, 144 – concepts 134 – culture, influence of 134, 141 ff. – in dreams and visions 209 f. – emotions 234 ff. – etymology 123, 124 – examples of languages 130, 132 – frequency of occurrence 143 ff. – gender, influence of 150 – general 9, 123 f., 126 – history 127 f. – meaning of ‘color’ 1 f., 126 – meaning of color terms 126 f., 134 f. – metaphors 138 f. – relativism 128 f. – social groups, influence of 145, 148, 149 – techniques of testing 128, 131, 135, 141 – universalism 128 f. – vantage theory 132, 134 ff., 151 Color photography 8, 45 Color physics 1, 11 ff., 63 ff., 214, 215 f., 217, 219, 233 Color psychology 192, 207 ff., 221 Color and sound 1, 137 ff., 162, 193, 197, 220 ff. Color terms (see Color naming) Color theories – 19th and 20th century 76, 193 – classical Greek 161 f.

Color vision – of animals 108, 110, 112 ff. – of man 8, 79 ff., 216 f., 234 f. Color-vision deficiencies – cerebral 101 ff. – classical Greeks 123 – evolution 114 f. – genetics 86, 117 f. – red/green differentiation 85 f. – retinal 85 ff., 101 Colorants – episematic 44 – functional 44, 45, 59 – history 41 ff. – infrared 45 – inorganic 41, 46 ff., 59 ff., 170, 178, 187, 198, 226, 227 – organic 41, 49 ff., 58 f. – structure and color 56 ff. – types of 41 – UV 50 Colorimetry – absorption spectra 64 f. – computer-based 70 – emission spectra 64 f. – general aspects 63, 208, 211, 233 – history 71 f. – physical 63 ff., 71, 233 – sensation-based 63, 66 ff., 69 f. – stimuli-based 63, 66 ff. – three-dimensional 63 Colour Index 49 Compensatory colors 78 Complementary colors 67, 68, 73 ff., 78, 97, 138, 144, 173, 177, 181, 183, 184, 186, 187, 188, 189, 216, 230 Complementary concept (N. Bohr) 17, 134, 139 Compound eye 113 ff. Computer (see Brain) Conceptualization of colors 134 Cones – absorption maxima 87, 91, 92, 117 – evolution 92 f. – genes 92 f., 117 – sensitivity curves and maxima 88 f., 92 – structures 84 f.

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Subject Index – types 8, 88 – wavelengths 85 f. Consciousness 131, 209, 236 f., 240 f., 242 Constantinople (see Istanbul) Contrasting colors 74, 75 f., 78, 137 f., 161, 184 Copper phthalocyanine 54, 151 Cornea 81 Corpus callosum 94, 96 Cortex, visual – areas 95–101 – bypasses in the visual cortex 101 – occipital lobe 95, 102 – pathways 101, 216 – striate 93, 95 – surgery 95, 96 Cosmochlor 49 Crabs 116 Crocetin 50 Crustacea 116 Cubism 191 f., 193, 197, 223 Culture – definition 134 – influence on color naming 134 f., 141 ff., 144 ff., 150 ff., 157 – ‘The Two Cultures’ VI, 9 Cuttlefish 33 Cyalume lightstick 56 Czech language 137 D Dadaism 224 f. Daltonism 86 Dani, language 129, 130, 133, 139 Daza, language 133 ‘Deep Blue’ (chess computer) 3, 236 Deficiencies of color vision (see Colorvision deficiencies) Dendrites 82 f. Denim 52 Deuteroanomalous deficiency 86 Deuteroanopic deficiency 86 Diazo compounds 44 Diffraction – in various animals 33, 35 – in peacock’s feathers 30 ff.

252

Dioptric vision 113 f., 116 f. Disazo dyes 52 Discrete combinatorial system, language as 157 Disegno versus colore 175, 179 f. Divisionism 184, 227 Dominant wavelength 67 ff., 75 f. Double-opponent cells 100 Dyes (see also Colorants) – cationic 59, 91 – definition 41 – fluorescent 55 – general 124, 151 – infrared 45, 50 – laser 45 – natural 44, 49 – production 42, 43 f. Dyschromatopsia 103 E Earthworms 113 Echelette gratings 35 Einstein-Bohr frequency condition 56 f., 59 Electron 57 ff. Emotions 9, 149, 233 f., 236, 242 English language – color-naming tests 141 ff. – color terms 129, 133, 138, 142 Epistemology 80 Epithelium 84, 118 Ethene 58 Evangeliar of Durrow 166 ff. Evening color 109 Evolution – of angiosperm plants 115 – color vision 114 – eye types 114 – loss of visual sense 114 Excited states 56 f., 58, 140 Experimentum crucis of Newton 19, 215 Expressionism 188, 189 f., 218 Eye, human – anatomy 81 ff., 236 – comparison with compound eye 114 ff. – connection to brain 93 ff., 236 f.

Subject Index F Farbenlehre (Goethe) – color-contrast phenomena 74, 216, 217 f. – color/sound phenomena 220 – color vision 214, 217 – criticism of Newton 7, 19, 213 ff., 217 – in editions of Goethe’s work 220 – general aspects 212 ff., 218 ff., 230 – optical illusions 120 – poems on color 214, 215 – purple hue 154 Farnsworth-Munsell test 86 Field theories 13 Firefly 56 Fish 97 f., 100, 117 Flavonoid colorants 53 Flowers of angiosperm plants – color 44, 114 – evolution 115 f. – pollination 114 Fluorescence 49, 55 Folk tales 150 Fovea centralis 81, 87, 97 Free-electron model 58 ff. French language – color-naming tests 141 ff. – color terms 125, 133, 138, 142 Fuchsine 42 Fuzzy set theory 132 G Ganglion cells – in cortex 96 f. – in lateral geniculate body 99 f. – in retina 84 f., 97, 100 Gender, influence on color-naming 150 Genes of cone photopigments 92 f., 117 German language – color-naming tests 141 ff. – color terms 133, 138, 142, 143 Glass window art 170 ff. Gobelins 76, 165 Goldfish 117 Gothic 162, 166 ff., 175 Grammar – generative 124

– universal 99, 124, 126, 157 Greek language 123, 127 f., 133, 141, 161 Gray matter 95, 99 Grotte Chauvet 3 Gyrus 102, 120 H Halobacterium halobium 91 Hanunóo language 130, 133 Hearing sense (see Audition) Hebrew language – color-naming tests 141 ff. – color terms 130, 151 Hemin 54 Hemoglobin 54 Homunculus 80, 119, 236 f., 242 Honeybee (see Bees) Horizontal cells 84, 97 Hückel molecular-orbital theory (see Molecular orbitals) Hue – Munsell term 69 f. – psychologically pure hues (see Unique hues) Hue-cancellation technique 105 Humor, vitreous (eye) 81 f., 113 Hungarian language 130, 132, 133, 135 Hybridization 57 Hydrogen molecule 57 Hydroxy group 53, 54 Hymenopteran insects (see also Bees) 116 I Illuminant 109, 110 Illusion, optical 110 ff., 228 f. Impressionism 74, 183 ff., 188, 189, 218, 226 Indigo 41, 44, 51 f., 71, 72, 155, 165, 198 Indigofera tinctoria 41, 72 Indirubin 155 f. Infants – brain 98 f. – color perception 80 f. – intellectual development 80 f., 125 f., 136, 137, 138 Infrared light 15, 16, 117, 140 Insects 113 ff.

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Subject Index Intelligence, emotional 236, 242 Interblob cells 100 Interference of light 12 f., 30 f. International Commission on Illuminants (see CIE) Iridescent colors 30, 177 Iris – goddess 23 f., 30 – part of the eye 23, 81 f. Iron-oxide pigments 47 Isatis tinctoria 73 Ishihara plates 86 Islamic culture 3, 166 Istanbul – book paintings 165 f. – mosaics 154 Italian language 138, 151 J Japan (see also Japanese language) – architecture 204 – art 197, 199 ff. – history 197 ff., 229 – influence of Europe 199 – influence of Korea and China 198 f. – influence on European art 183, 206 f. – preference of colors 207 – religions (see Buddhism, Shintoism, Zen-Buddhism) – theater 204 – woodcuts 204 ff. Japanese language – color-naming tests 141 ff. – color terms 2, 130, 132, 142, 143, 148 Jeans, blue 52 Jellyfish 113 Jûni-hitoe 202 K Kekchi language – color-naming tests 124, 141 ff. – color terms 142 Kichijô-ten 199, 200 Kinetic art (see Art) Korean language 141

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L LAB System 70, 77 Language – as discrete combinatorial system 157 – general 99, 124, 125 f. – instinct 125 – learning (child) 99, 125 f., 136, 137 – in relation to psychophysics of color vision 104, 124 Laser dyes 37, 45 Lateral geniculate body 94 f., 97, 100 Latin language 133, 138, 162 Lens (eye) 81 f. Ligand-field theory 48, 61 Light – physics 11 ff., 17 f. – scattering 47 – speed 14, 21 – theories 11 ff., 15 f., 139 Lightness (see Luminosity) Linguistics (see also Basic color terms) – general 125 f. – color terms 9, 124 f. Loan words for color names 144 f., 148 Low-resolution electromagnetic tomography (LORETO) 120 Luminance 81, 100 Luminescence 56, 66 Luminosity – in art 163 – in CIE system (Y) 67 f., 107 – in LAB system (L) 70 – in Munsell system 70 Lycopene 44 f. Lysine 89 M Macaque monkeys (see Monkeys) Magnetic resonance imaging 120 Magnocellular system 100, 116 Mammals – color vision 117, 118 f. – nocturnal 118 f. Mandala 209, 210, 211 Manesse manuscript (see Codex Manesse)

Subject Index Mannerism 178 Marmoset monkey (see Monkeys) Mauve (Mauveine) 7, 42 Mbula language 2 Mechopdo language 133 Melanin 32, 33, 118 Mesoamerican languages 158 Metal-complex dyes 53 ff., 59, 61 Metameric colors (hues) 66, 109 Methane 58 Metaphors, sensory adjectives 138 f., 221 Methine group 49 f., 51 Microelectrodes 93, 96 Mimicry 33, 37 Miniature painting 165 f. Misquito language 141 ff., 146 Models – in linguistics 13, 131 – in science 13 f., 17, 57, 58 ff. Molecular orbitals 37, 57 ff., 61 Monkeys – absorption maxima of visual pigments 117 – evolution of color vision in 114 – Old World vs. New World monkeys (vision) 118 – vision experiments with 88 ff., 93, 97, 100, 120 Monoazo dyes 52, 53 Monochromatic light 30, 45, 74, 78, 80, 87 Mosaics 154 f., 163 ff. Motion, perception of 101 ff. Müller-Lyer illusion 228 Munsell color system 69 f., 120, 128, 130, 141, 143 Music (see Color and sound) Musical octave 72, 162, 221, 222 N Naphthalene 50 f., 54 Naphthalene Orange G 52 f., 54 Nautilus 113 Neo-Impressionism 164, 185 ff., 188 Neurons – activity 82, 84 f. – general structure 82 ff.

– ‘negative’ signals 105 – on/off cells 97 ff. – working mode 82 Neurotransmitter 82, 91 Non-representational pictures 223

190 ff.,

O Occipital lobe (cortex) 95, 102 Ocelli – in peacock’s feathers 32 ff. – organ for vision 113 Ommatidia 113 Opal gemstone 35 Opponent color theory (see also Complementary colors) 105 ff., 109, 116 Optical nerve 81, 83, 84, 94 Orbitals (see Molecular orbitals) OSA Uniform Color Scale 70 P Parvocellular system 100, 116 Peacock’s colors – in alchemy 28 f. – cultural history 28 ff. – electron microscopy 32 ff. – physical basis 30 f. – in psychology 28, 209 Pedicules 84 Perspective 176, 194, 228 PET (see Positron emission tomography) Phoenician culture 3, 154 f. Phonemes – and color 136 ff. – opposition of 137 f. Phonology 136 ff. Phosphors, in television 37 Photodynamic therapy of cancer 45 f. Photography – instant (Polaroid) 109 – sensitizers 8, 45, 50, 59 Photon 15 Photoreceptors (see also Cones, Rods) – in bacteria 91 – dark reactions 89 f.

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Subject Index – electrochemical potential 91 – general aspects 8, 79 – genetics 92 f. – photochemical reaction 89 ff. – structure 84 f. – types 84 Pigeon 117 Pigments (see also Colorants) – definition 41 – inorganic 46 ff. Pointillism 103, 164, 176, 185 ff., 188, 227 Polarized light 109, 115 Polaroid filters 109 Polish language 132, 133, 135 Pollination 54, 114 f. Polyene dyes 49 f., 58 Pomo language 130 Pompeii 162 Porphyrin pigments 53, 54 Positron emission tomography (PET) 93, 101, 104, 235 Primary colors – in art 193, 194 – book title 2 f., 234 – in CIE system 67 – definition 2, 73 f. – history 73, 177 Prosopagnosia 103 Protanomalous deficiency 86 Protanoptic deficiency 86 Proteins 91 f. Proto-slavic languages 133 Pseudoisochromatic devices 86 Psychologically pure hues (see Unique hues) Psychophysical investigations 88, 103, 104 ff., 235 Pupil 81 Purity of colors (hues) – pa (CIE diagram) 67 f. – unique hues 75 Purple (see also Ancient Purple) – color naming 142 – color terms in Hebrew 142, 155 – hue 73, 152 ff., 162, 173 – line (CIE diagram) 67, 73, 75, 104, 106

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Q Quantum – electrodynamics (QED) 17 – mechanics (see Quantum theory) – theory 15 f., 43, 56 ff., 61, 237 Quarks,‘colored’ 3 Quechi (see Kekchi language) Quinta essentia 211 R Rainbow – in Bible and antique history 22 ff., 233 – colors 22, 23 – Iris (goddess) 22 f. – physics 24 ff., 233 – primary, secondary, and tertiary 23, 26, 27 – supernumerary 28 – in various cultures 21 ff. Ravenna mosaics 154–156, 164 Receptors (see Photoreceptors) Reflection – in peacock’s feathers 30 f. – physics of 20 f. – in the rainbow 24 – total 20, 21 Refraction – physics of 18 ff. – in the rainbow 25 f. Refractive index 21, 26, 47, 82 Relativism 128 Renaissance 162, 173, 175, 177 ff. Retina – anatomy 79, 81 ff., 113 – photochemistry 8, 87 ff. Retinal 89 ff. Retinex theory 110 Rhodopsin – absorption maxima 91 f. – dark reactions 89 – gene of rod pigment 92 f., 117 f. – photochemical reaction 89 ff. – retinal side chain 90 – structure 90 ff. Rococo 173 Rods 8, 84 f., 86, 87, 93 Roman art 161 f.

Subject Index Royal Purple (see Ancient Purple) Ruby 48, 49, 61 Russian language 125, 130, 132, 133, 135 S Saffron 50 Salvarsan 46 Saturation – of monochromatic light 106 f. – in Munsell system 69 Sclera (eye) 81 Semiconductors 37 Semiotics and semantics 124, 127, 157 Senses 138 f. Sensitizers (see Photography) Setswana language 148 Sfumato technique 177 Shintoism 198, 199 Sistine Chapel 179 Sky 30, 37 Slavic languages (see Russian, Polish, and Proto-slavic languages) Smell – structure of sensory cells 91 – use of adjectives of 138 f. Snails – molluscan (purple) 152 f. – vision of 113 Sodium ions 89, 91 Sound (see also Color and sound) 138 f. Spanish language 144 f. Spectral colors 11, 18 f., 65 f. Spectrum – absorption 64 f. – correlation to musical scale 72, 162, 221, 222 – electromagnetic 15, 16 – emission 64 f. – infrared 15, 16 (see also Infrared light) – ultraviolet 15, 16, 50, 55 (see also Ultraviolet) – visible 4, 15, 16, 37 Spiders 116 Split-brain surgery 93, 96 Squirrel 118

Stomatopoda 116, 121 Striate cortex 93, 95 Subatomic physics 3, 17 Subtractive mixture of colors 65 ff., 74, 216 Sulfo group 53 Suprematism 191 Surrealism 197 Synapses 82, 83 Synesthesias (see also Color and sound) – of color words 136 ff. – history 222 f. – in theory of art 192, 197, 220 ff., 239, 243 T Tachism 224 f. Taste 138 f. Television 2, 8, 65 f. Tetrachromatic vision 93 f., 116, 117 Textiles 41 ff., 44, 51, 150, 151 f., 165, 176, 209 Thalamus 96 Thorough-bass 223, 231 Titanium white 46 f. Touch 138 f. Transducin 89 Transition metals 37, 61 Trichromatic (tristimulus) theory 8, 104 ff. Turkey Red 42, 44 Turquoise 142, 150 f. Turtle 117 Tyrian Purple (see Ancient Purple) Tzeltal language 130 U Ultramarine blue 178 Ultraviolet – light 15, 16, 55, 114, 117, 139, 140 – photoreceptors 115, 116 – protection agents 55 Uncertainty principle 17, 57, 237 Unique hues – in art 193 – definition 75 f., 78 – in mandalas 210

257

Subject Index – position in CIE diagram 76, 159 – in psychophysical investigations 80, 104, 105 f., 107 f., 108 f., 159 Universals – of human experience 134 – linguistic 128 ff., 133 ff. – phonological 136 ff. V Vantage theory 132, 134 f., 145, 151 Vat dyes 51 f. Vertebrate animals 113 Violanthrone 51, 52 Vision (see also Color vision) – body/mind relationship 80 – comparison with audition 221 – general aspects 8 ff., 79 ff. – pathways 101 f., 216 f. – sensation vs. perception 79 Visual pigments (see Photoreceptors) Vitamins 45, 89

258

W Wave mechanics (see Quantum theory) Wave theory 11 ff., 139 f. Wavelength sensitivity of cones 86 White matter 95, 99 Woad 41, 73 Woodcuts 169 f., 204 ff. World Color Survey 158 X Xanthophyll 53 X-Rays – diffraction 30 – spectral region 16 Y York Minster 170 Young’s modulus 38 Z Zen-Buddhism 203 f. Zuni language 130