PROBLEMS IN THE THEORY OD PERCEPTION OF COLOUR: 1800 - 1860 Thesis presented for the Degree of Doctor of Philosophy In the Field of History of Science by Paul D. Sherman Department of History of Science and Technology Imperial College of Science and Technology University of London October, 1971 -2- Abstract Colour and colour perception have long been subjects for scientific enquiry. It was only in the 19th century, however, that some understanding of colour vision and its relationship to colour mixing emerged. Much of this knowledge evolved through the work of Young, Brewster, Helmholtz and Maxwell. It is not too much to say that 19th century investigations of colour perception were initiated by Young's cele- brated trichromatic theory of vision. While the concept of primary colours is as old as art and science itself, and the concept of three primaries can be traced to the 17th century, it was Young who first saw that this idea might be made the basis of a theory of colour vision. Young's idea of red, green and violet as primary colours was contrary to the received opinion of red, yellow and blue. There was no argument about choosing between violet or blue but there was considerable confusion over whether yellow or green should be chosen as primary. Since .Young's choice had been made on the basis of doubtful spectral observations, other scientists were led to repeat his experiments. This, in part, led Brewster to a series of experiments on the spectrum from which he evolved in 183/ an ingenious but erroneous spectral theory. All spectral colours were to be compounded from three primary colours, red, yellow and blue which occupied equal lengths in the spectrum. Brewster's theory raised considerable controversy among physicists which was not settled until 1852 when Helmholtz showed that Brewster had been the victim of several subjective illustions concerning colour. He showed also that the confusion over colour mixing, and hence over -3- Brewster's theory, was due to two different processes being considered equivalent, additive and subtractive colour mixing. The distinction between these processes marked a turning point in understanding colour mixtures and colour vision. Helmholtz's work served as the basis of a mathematical theory of colour by Grassmann and that theory together with certain ideas from photometry served as the basis of Maxwell's experiments in colour vision. Maxwell showed that colour could be measured, and hence transformed a qualitative science into a quantitative one. He confirmed Young's trichromatic hypothesis as well as his choice of primaries and showed that colourblindness, a much discussed parallel study, could be understood in terms of Young's hypothesis. Equally interesting is the fact that Maxwell's experiments reveal him as a first rate experimental physicist, a fact which is apparently little appreciated; for this reason his experiments are described in detail. -4- Preface The nature of vision, and especially colour vision, is a fascinating subject to scientist and layman alike. Undoubtedly it is simply because colour plays such an important role in our daily lives. But there is certainly another reason as well. The subject cuts across all boundaries, embracing the widest possible spectrum of knowledge. Philosophers, physiologists, psychologists, physicists and others have all had something to say about vision. It is in this, perhaps, that both its fascination and challenge lie. Because of its interdisciplinary nature, a thesis on the history of colour vision cannot fall neatly into any of the usual pigeon-holes such as history of physics, history of physiology, etc. Of necessity it is an eclectic study along with its subject. However, to bring the subject within manageable boundaries, I have concentrated on one aspect of the subject which to me is one of the most exciting. That is simply the fact that to a remarkable extent the subject does fall within the scope of physics in the sense that optical experiments can be performed which yield considerable knowledge about colour vision. Therefore, I have concentrated on those aspects of the science which historically could be treated by the methods of physics. Restricting the field even more I have chosen the period of 1800 to 1860 because this was one in which three immortal scientists, Young, Helmholtz and Maxwell, brought the infant science of colour vision to a new maturity. The story opens with a great speculation by Young which itself had roots in the 18th and 17th centuries and closes with its confirmation by Maxwell some 55 years later. But the close of this story is also the opening of another, one which continues today as a field of active research. It is interesting to see in this respect that the science of colour perception participated along with the other sciences in the 19th century in evolving toward its modern form. It is hoped that this thesis will constitute a small contribution to the growing body of scholarship in 19th century science. In preparing this study I am indebted to my thesis supervisor, Professor A. R. Hall who has given most graciously and generously of his time and advice. Dr. M. B. Hall has also been a cheerful and willing source of help. Special thanks are due to Mr. Leslie Jones who drew many of the figures. Grateful acknowledgement must be made to Central School District No. 1, Harrison, N.Y. for their generous support during 1969-70. Last, but not least, I must acknowledge my wife whose moral support contributed considerably in completing this work. -6- TABLE OF CONTENTS Page Introduction 7 I. Thomas Young: His Speculation About Three-Colour Vision 9 II. Spectral Analysis by Coloured Filters and Brewster's Theory of the Spectrum 27 III. Hermann von Helmholtz and the Demise of Brewster's Theory 80 IV. The Primary Colours and Colour Classifications; Part I: to thei'Mid-19th Century 115 V. The Primary Colours and Colour Classifications; Part II: Helmholtz and the Theory of Compound and Primary Colours 158 VI. Hermann Gunther Grassmann: Toward a Quantitative Theory of Compound Colours 196 VII. Interlude: On Colourblindness 224 VIII. James Clerk Jewell:T The Beginning of Modern Methods 273 IX. Further Contributions of Maxwell 331 X. Conclusions 385 Appendix; On Photometry 398 Bibliography 439 -7- Introduction Studies of colour combinations, colour classification schemes and the mechanism of colour perception were continuing themes throughout 19th century science. The latter problem might seem to be a subject suitable for physiologists, while colour classification schemes might seem suitable problems for artists; but both were in fact attacked largely by physicists in the first half of the 19th century. The greatest figures in 19th century physics were deeply interested in various aspects'of colour. Young, Brewster, John Herschel, Biot, Airy, Helmholtz, Maxwell and others all made important contributions to the science of colour vision. Helmholtz, physiologist as well as physicist, attacked the problem more as a physicist than a physiologist, while that universal man, Thomas Young, broached a theory of colour vision as part of a physical theory of light. Thus many 19th century physicists saw among their problems that of finding a theory for colour perception. which they considered as a problem in optics. As in so many areas they were eminently successful, laying the foundations for much 20th research into the subject. A peculiar difficulty in all colour problems lay in the fact that no experimental basis existed either for understanding colour combinations or for determining the sensitivity of the eye to individual colours. Without an experimental basis hardly any theoretical understanding was possible. Both problems lay largely outside the realm of the great physical theories emerging in the early 19th century. Hence the study of colour combinations and colour perception was perforce a descriptive -8- one concerned more with qualitative observations and speculative hypotheses than with exact measurements and mathematical expression. This being the case it seems remarkable that it should have attracted the mathematical physicists. And yet it did so with the result that by about 1860 a good understanding of the colours perceived by the eye was available together with a mathematical understanding of colour combinations, the latter supplied essentially by the eminent mathematician Grassmann. Moreover, from being a science with practically no experimental techniques available in 1800, by about 1860 a definite corpus of devices and techniques had evolved for studying colour combinations and colour perception. It is the purpose of this thesis to review the work in colour combinations, colour classification, colourblindness and colour perception in the period of 1800 — 1860 which led to such understandings. In passing, it is worth noting that while the investigators of this period succeeded in determining the sensitivity of the eye to certain colours, the physiological mechanism of colour perception is still an active research problem. The events described form the prelude to modern research in colour vision. -9- CHAPTER I Thomas Young: His Speculation About Three-Colour Vision The first steps towards a theoretical understanding of colour perception came early in the 19th century. Thomas Young in his Bakerian 1 Lecture of 1801 proposed to the world his new theory of light, the undUlatory theory, which is so much associated with his name. It was, 2 however, as Young himself said, not an entirely new theory but more intended to "refer some theories...to their original inventors, support them by additional evidence, and to apply them to a great number of diversified facts, which have hitherto been buried in obscurity."3 It was in the context of this last phrase that Young introduced his famous theory of three colour vision. Following a few introductory remarks, Young initiated the exposition of his undulatory theory by first hypothesizing a universally pervading luminiferous ether and next hypothesizing that "undulations are excited in this Ether whenever a Body becomes luminous."4 These statements are the substance, respectively of his Hypotheses I and II.