VISUAL SENSATION. 183 VISUAL SENSATION. component." Some of the complementary pairs <Trc rod and verdigris, orange and groenish-blue, vcllow and blue, green and purple. (2) The law of neigldxiring colors: "'i'lie mixture of any two colors which ai'e not eoniplenientaries gives an intermediate color, varying in color-tone with the relative amounts of the two original colors, and varying generally in saturation with their nearness or remoteness in the color series." The mixtures of yellow and green, red and orange, lilue and purple, arc instances. (3) "The mix- ture of any two combinations which match will itself match either of the original combinations, jpriividcd that the illumination of the colors re- main approximatel.y the same" (TItchcnor). Mix, e.g. a gray made from red and verdigris with a like gray made from yellow and blue. The re- sult is a gray which matches either of the com- ponent grays. The fact that this law does not hold when the illumination is diminished (fail- ure of Newton's law of color mixtures) — that the mixture which contains green will then be far too bright — constitutes the phenomenon known as the 'extended' Purkinje phenomenon ( i.e. for colorless light). 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. Fio. 2. The Young-Hclnilicltz Theory. The letters R ■O. Y. etc., (l08ipnate the spectral colors. The three curves Hliow roughty the relative values of the excitatory process, 1, In the red-sensitive fibres. 2, iu the green-sensitive, and y, In the violet-sensitive. 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 filire, and a blue or violet filire — 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 tliree kinds of nervous elements, but in varying pro|)ortions. ReiU e.g. excites the red fil>re maxi- niallj' and the green and Idue 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 sub.stances' which undergo metabolic changes under the influence of light ravs. Each Bubstance is the .seat of antagonistic processes: processes of dissimilation (decomposition) and assimilation (recomposition) . Dissimilation in one suli^tancc gives white, assimilation black; di-similation in a second gives yellow, assimila- tion blue; dissimilation in a tliird. ]iurplish red, assimilation bluish green. ]<"rom these six prin- cipal sensations all others are derived by com- Fid. 3. The Hering Theory, The line r«- shows the effect of 'rays of illfferent wave-lenRths on the red-Kreen visual substance: the effect is disslnillative to the (loint Y, ae- simllHtive from Y to B. and then againdisslniilatlve to the end of the spectrum (broken line). The line ylj shows the effect ujion the yellow-hlue substance. The line iv Indicates the dissimilation of the black-white substance throughout the whole length of the spectrum. bined excitation of the three substances. A bal- ance of assimilation and dissimilation, in any sulistance, means absence of function; i.e. com- plementaries cancel each other and the gray that is left over is said to be due to a cortical excitation (G. E. Miiller). The two theories must be tested by an appeal to all the facts of color mixture, contrast, after- images, indirect vision, adajitation, I'uikinje'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 be said that the Hering theory is wholly a<lequate to all the minute variations of visu:il jihenoniena. Xumerous other theories have been devised with a view to meeting the outstanding diffi- culties of visual sensation. Thus the Franklin tlu-ory 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 jiarts of the retina, which is affected liy all lumi- nous vibrations, but most markedly by those cor- responding to the middle of the spectrum: stimu- lation of this substance produces the sensation of white of all shades. In the course of development of the eye. this gray-perceiving material becomes diflerentiated in the cones first into two differ- ent substances, which are especially affected by the two halves, respectively, of the spectrum"; this is the sta.ge of development which corre- S]ionds 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 sta.L'e of development, the yellow constituent be- comes again differentiated into two sub.stances which res])ond respectively to red and to .creen light. The mixed colors are produced by siraulta-
