To this assumption we may easily add the further condition, that for certain medium intensifies of the stimuli the chromatic components are the strongest, while for greater and smaller intensifies the achromatic components come more and more to the front. The second fact, is that any two opposite colors are complementary; that is, when mixed in suitable proportions, they produce an achromatic sensation. This phenomenon is most easily understood when we assume that opposite colors, which are subjectively the greatest possible differences, represent objective photochemical processes that neutralize each other. The fact that as a result of this neutralization an achromatic stimulation arises, is very readily explained by the presupposition that such a stimulation accompanies every chromatic stimulation from the first, and is therefore all that is left when antagonistic chromatic stimulations counteract each other. This assumption of a relative independence between the chromatic and achromatic photochemical processes, is supported in a very striking way by the existence of an abnormity of vision, sometimes congenital, sometimes acquired through pathological changes in the retina, namely total color-blindness. In such cases all stimulations are, either on the whole [p. 71] retina or on certain parts of it, seen as pure brightness, without any admixture of color. This is an incontrovertible proof that the chromatic and achromatic stimulations are separable physiological processes.
If we apply the principle of parallelism to the chromatic stimulation, two facts present themselves. The first is that two colors separated by limited, short distance, when mixed give a color that is like the intermediate simple color. This indicates that color-stimulation is a process that varies with the physical stimulus, not continuously, as the tonal stimulation, but in short stages, and in such a way that the stages in red and violet are longer than in green, where the mixture of colors fairly near each other, shows the effects of complementary action. Such a non-continuous variation of the process corresponds entirely with its chemical nature, for chemical disintegration and synthesis must always have to do with qroups of atoms or molecules. The second fact is that certain definite colors, which correspond to rather large differences in the stimuli, are subjectively opposite colors, that is, are maximal differences, and the same colors are objectively complementary, that is, mutually neutralizing, processes.
Chemical processes, however, can neutralize each other only when they are in some way opposite in character. Any two complementary color-stimulations must, therefore, stand in a relation to each other similar to that which exists between the neutralizing processes operative in the case of antagonistic achromatic stimulations. Still, there are two very essential differences here. First, this opposition in the character of color-stimulations is not limited to one case, but appears for every color distinguishable in sensation, so that we must conclude, according to our presupposition, that for every stage of the photochemical process of. chromatic stimulation which is to be assumed on the ground of the results obtained by mixing neighboring colors, there is a certain complementary process. Secondly, the difference between two opposite colors, which is subjectively the greatest possible difference, is mediated by transitional forms, not merely in one direction from each color, as in the case of black and white, but in two opposite directions. In a similar way, the objective complementary action of two colors gradually diminishes as, starting from opposite colors, they approach each other in either of [p. 72] these two directions. We may, then, infer from this twofold elimination of complementary action that the return of the color-line to its starting point corresponds to a repetition of related photochemical processes, on the same grounds that led us to infer the opposite character of the processes corresponding to opposite colors, from the fact that they are complementary. The whole process of chromatic stimulation, beginning with red and passing beyond violet through purple mixtures to its starting point, running parallel, as it does., with continuous changes in the wavelength of objective light, is to be regarded as an indefinitely long succession of photochemical processes. All these processes together, form a closed circle in which, for every stage there is a neutralizing opposite and a possible transition to this opposite in two different directions.
We know nothing about the total number of photochemical stages in this circle of processes. The numerous attempts made to reduce all color-sensations to the smallest possible number of such stages, lack adequate foundation.