6Color vision

return to this renewed interest in colors after a short review of the history of some color vision theories.

Although it was already pointed out by Newton (1979) that ‘the rays are not colored’, scientists have not always taken care to distinguish between the description of light rays and the description of qualitative experiences of colors. It also took a long time before it became clear which aspects of color phenomena needed a scientific explanation, and which were outside the scope of traditional scientific methods and language. The physical concepts of electromagnetic radiation, for instance, had to be developed before a scientific theory of color and color vision could be established. Then the whole complex needed to be sorted out and proper answers be sought to questions like ‘Which parts of the color phenomena belong to the external, physical world?’, ‘Which parts are due to sensory processes?’ and ‘What roles do neural networks and cognitive processes play?’ Even today this is a thorny issue (Chalmers, 1995a, b). Isaac Newton believed that every natural body reflects ‘its own color’ more conspicuously than the others, and many of us have been taught that color is caused by the wavelengths of light reflected off an object, i.e. that red is caused by long-wavelength light and blue by short. This is a simplistic view that is no longer maintained, except for within the darkness of the laboratory, as clearly demonstrated by Land (see Chapter 5). Since the time of Newton, many aspects of color phenomena have been omitted from a natural scientist’s description of light, and Goethe’s polemics against the Newtonian view can, at least in part, be interpreted as a fundamental disagreement on which aspects of experience should be dealt with by natural science.

Newton was more concerned with the properties of light rays than with colors as such, and in this field of optics his achievements were outstanding. The correlates he found between colors and the refrangibility of light rays were derived in a dark room illuminated only by a sunbeam. These correlates have a rather limited value for predicting the perception of colors in an everyday environment, but they have been intractably linked to color theory ever since. Goethe, on the other hand, was more interested in the phenomenology of color. He carried out many beautiful experiments and developed ideas and concepts that have had a great impact in art and culture. Particularly impressive are the demonstrations of boundary colors that appear when the slit used in Newton’s experimentum crucis is wide. The colors are yellow and red on one side of the border, and blue and violet on the other. Goethe also devoted a lot of attention to the spectrum of colors that appears when the narrow slit in Newton’s experimentum crucis is replaced by a narrow opaque strip. This Goethian inverted spectrum is complementary to the Newtonian spectrum and consists of a transition from blue via non-spectral magenta (purple) to a broad band of yellow. Goethe also experimented with colored shadows, an impressive manifestation of contrast colors that had no place in Newton’s theory, and emphasized the important role of the experiment in serving as a mediator between object and subject.

One may call Newton’s investigations analytic or theory-oriented. The predictive power of this approach has proved very successful in the natural sciences. It has given

us a tool with which to manipulate nature to such an extent that we today experience an increasing conflict between technology and science and culture. Goethe’s experimentation was more intuitive and exploratory. This latter method has had prominent representatives in natural science, and Michael Faraday’s investigation of electromagnetism is probably the most striking example (Ribe and Steinle, 2002). Exploratory experimentation typically comes to the fore in situations where a useful conceptual framework is not yet available and where the creative investigator must find ways and concepts along with experimentation (Fynn, 1979).

Thomas Young or George Palmer?

Some time before Goethe published his Theory of Colors (Goethe, 1810/1963), the physiological basis of color vision had caught the interest of the English physicist, physician and philologist Thomas Young (1773–1829). In his famous 1801 Bakerian Lecture at the Royal Society of London, he presented an idea of how color vision worked. There were three types of receptors in the retina, he said, each excited by light from a different wavelength band in the Newtonian spectrum, and each associated with one of the three primary colors. Differential excitation of these receptor types would lead to mixtures of the primary colors.

Light mixtures of red, green and blue we can see every day on color TV. If you look more closely at the screen, for instance through a strong magnifying glass, you will find that the picture is constructed of many small red, green and blue dots or squares of varying intensity. All the colors you see on the screen some distance away are a result of the melting together (called an additive mixture) of these three ‘primary’ colors, modified by spatial contrast effects (simultaneous contrast).

It seems likely that this trichromatic hypothesis was developed from ideas that had become familiar to Young when, in 1795–1796, he studied for a doctor’s degree in medicine at the University of Go¨ttingen in Germany. The original idea probably came from an English glass manufacturer and carpet merchant, George Palmer, alias Giros von Gentilly (Walls, 1956; Mollon, 1993, 1995). This man was colorful in more than one sense of the word; he is supposed to have had some dubious business in France that made it necessary for him to use a pseudonym. Already in 1776 Palmer had published a pamphlet with the title Theory of Colors and Vision, in which he explained color vision by means of three receptors in the retina. This pamphlet was translated into French and published under the name of Giros von Gentilly.

Palmer’s ideas must have been well known in the academic circles of Go¨ttingen. His hypothesis for color vision, and the related thought that color blindness was caused by a missing or a defect receptor type, was discussed in 1781 in Lichtenberg’s

Magazin, and color blindness was treated again in a new pamphlet in 1785 (Lee, 1991a). Lichtenberg’s Magazin was devoted to popular science and it was edited by the brother of a famous professor in physics in Go¨ttingen, Georg Christoph Lichtenberg (1742–1799). Lichtenberg was a well-known personality in German science in the second half of the eighteenth century and well acquainted with Thomas Young in Go¨ttingen. The hypothesis about color vision that was later credited Young does not seem to have been entirely his own, but rather a development of ideas that he must have heard about in Go¨ttingen.

Young–Helmholtz’s three-receptor theory

The three-receptor theory lived a quiet life until, 60 years after Young’s lecture in 1801, it was taken up by von Helmholtz and developed further into what today is called the Young–Helmholtz three-receptor theory of color vision. At about the same time, another well-known physicists, James Clerk Maxwell, developed an interest in color and color vision. He made experiments that prepared the ground for color photography. The following citation is from his writings (Maxwell, 1872): ‘if the sensation which we call color has any laws, it must be something in our own nature which determines the form of these laws’.

In the years after von Helmholtz’s (1866/1911) revival of Young’s hypothesis, color perception was explained by light absorption in three types of cone photoreceptors in the human retina. The spectral sensitivities of these cone types are shown in Figure 4.12(A). The belief (that we now know to be wrong) was that three different classes of cone activated three basic color processes or sensations: red (L-cones), green (M-cones) and violet or blue (S-cones). Consequently, these receptor types were called the R-, G- and B-cones, respectively. All colors were said to be due to the excitation of these primary sensations in different proportions. The perception of yellow would, for instance, result from equal stimulation of the ‘green’ and the ‘red process’, whereas white resulted from equal stimulation of all three.

Referring to Young, von Helmholtz (1911, p. 119) writes:

‘Es gibt im Auge drei Arten von Nervenfasern. Reizung der ersten erregt die Empfindung des Rot, Reizung der zweiten die des Gru¨n, Reizung der dritten die Empfindung des Violett’ (‘In the eye there are three types of nerve fibers. Stimulation of the first one excites the sensation of red, stimulation of the second the sensation of green, stimulation of the third the sensation of violet’),

and on p. 120 he continues

‘(das Wesentliche in der Hypothese von Young ist), dass die Farbempfindungen vorgestellt werden als zusammengesetzt aus drei voneinander vollsta¨ndig unabha¨ngigen Vorga¨ngen in der Nervensubstanz’. (The essence of Young’s hypothesis is) that the sensations of color are imagined as composed of three mutually completely independent processes in the neural substrate’.