There are two principal theories of visual sensation, two hypothetical statements of the physiological conditions which underlie the vari ous phenomena of color-vision. They are known as the Young-Helmholtz and the Hering theories.
The Young-Helmholtz theory sets out from the laws of color mixture as derived by Newton. It posits the existence, in the visual apparatus. of three kinds of nerve fibres—a red fibre, a green fibre, and a blue or violet fibre—each of which if it. were excited alone, would give its particu lar sensation. These three colors, red, green. and blue, are chosen as the primary colors because they give, by mixture, all the various colors and brightnesses at a relatively high degree of satu ration (although approximately the same result may be obtained with other combinations of three). Any light stimulus is said to excite all three kinds of nervous elements, but in varying proportions. Red. e.g. excites the red fibre maxi mally and the green and blue fibres minimally. The color seen under any kind of stimulation de pends upon the proportion in which the three kinds of nervous apparatus are affected. Yellow, e.g. results from a strong excitation of the red and green fibres and a weak excitation of the blue fibre. White is due to the equal excitation of all three fibres.
The Hering theory says that there are three `visual substances' which undergo metabolic changes under the influence of light rays. Each substance is the seat of antagonistic processes: processes of dissimilation (decomposition) and assimilation (recomposition). Dissimilation in one substance gives white, assimilation black; dissimilation in a second gives yellow, assimila tion blue; dissimilation in it third, purplish red, assimilation bluish green. From these six prin cipal sensations all others are derived by com bind excitation of the three substances. A bal ance of assimilation and dissimilation, in any substance. means absence of function; i.e. corn plementaries cancel each other and the gray that is left over is said to be due to a cortical excitation (O. E. Milller).
The two theories must he tested by an appeal to all the facts of color mixture, contrast, after images, indirect vision, adaptation. Purkinje's phenomena, and color-blindness. The Hering theory meets the various classes of facts more easily and with less recourse to subsidiary hy potheses than does the Young-Helmholtz theory. The latter. indeed, would appear to have sur vived rather in virtue of the authority of Helm holtz than by its intrinsic merits. At the same time, it cannot he said that the Hering theory is wholly adequate to all the minute of visual phenomena.
Numerous other theories have been devised with a view to meeting the outstanding diffi culties of visual sensation. Thus the Franklin theory supposes that at an early period of de velopment the eye is sensitive only to luminosity and not to color: at this period it possesses only a gray-perceiving substance, present in all parts of the retina. which is affected by all lumi
nous vibrations. but mast markedly by those eor responding to the middle of the spectrum: stimu lation of this substance produces the sensation of white of all Andes. In the course of the eye, this gray-perceiving material becomes differentiated in the cones first into two differ ent substances, which are especially affected by the two halves, respectively, of the spectrum; this is the stage of development which corre sponds to the yellow-blue color system of the partially color-blind, and also to that of the middle zone of the normal retina. In the final stage of development, the yellow constituent be comes again differentiated into two substances which respond respectively to red and to green light. The mixed colors are produced by simulta neous stimulation of two or more color substances in varying proportions. The Hering theory is for mulated in static, not in genetic. terms; but if we put a genetic interpretation upon it. we must say that for it. too. the black-white substance is the earliest, the blue-yellow the next, and the red green the last to appear in the course of evolu tion. Such an interpretation is borne out espe cially by the facts of eolor-blindness and of in direct vision, as understood by Hering and his school.
Another important theory is that of \Vundt, who posits only two excitatory processes, a chro matic and an achromatic, and regards the former as a multiform, the latter as a uniform, photo chemical process. thus gives up alto gether the idea of primary 'components' of visual sensation—of the three ultimate colors of Helm holtz or the four of Hering—and substitutes for these 'component theories' a theory of gradations within a single process. h may be remarked that Wundt's recognition of a separate achro matic process antedates the formulation of Her ing's theory. The independence of the achromatic process is accepted also by Von Kries, the author of another theory, which, while based on that of Helmholtz as regards the function of the retinal cones. sees in the rods special organs of ac•hro matic vision.
BIBLIOGRAPHY. Ebbinghaus, Grunliziifie der Bibliography. Ebbinghaus, Grunliziifie der Psychologic (Leipzig. 1897) ; Wundt, Human and Animal Psychology (New York. 1S94) ; Helmholtz, Physiologischc Ovid,. (Leipzig, 1896) ; Hering, Lchrc coin Lichtsinne (Vienna. I878); Titchener. Experimental Psychology (New York, 1'301) : Kfilpe. Outlines of Psychology (New York, 1895) Von Kries. Gesichtsemplindungen (1882) : Gcsam melte Abhandlungen PhYsioloaischen Optik (Leipzig, 19031. See AFTER-1 MACE; CONTRAST; COL0R-BLINDNESS; WEBEICS LAW; QUALITY; INTENSITY OF SEN SATION; PSYCIIOLOGICAL APPARATUS.