PROPERTIES OF THE RETINA. 



361 



O 



\.y 



w 



this theory, that only the white-black substance is present, while 

 red and green blindness — both of them, it will be recalled, really 

 forms of red-green blindness — are explained on the view that in such 

 persons the red-green substance is deficient or lacking. On this 

 theory, complementar}^ colors — red and blue-green, yellow and 

 blue — ^are, in reality, antagonistic colors. When thrown on the 

 retina simultaneously their 

 effects neutralize each other, 

 and there remains over only 

 the disassimilatory effect on 

 the white substance which is 

 exerted by all the visible 

 rays. The effect of the vari- 

 ous visible rays of the spec- 

 trum on the three photo- 

 chemical substances is illus- 

 trated by the chart given in 

 Fig. 154. Ordinates above 

 the abscissa representing dis- 

 assimilatory effects; those 

 below, assimilatory. 



///. The Franklin Theory 

 of Color Vision (Molecular 

 Dissociation Theory) . — This 

 theory, proposed by Mrs. C. 

 L. Franklin,* takes into ac- 

 count the fact of a gradual 

 evolution of the color sense 

 of the retina from a primitive 

 condition of colorless vision 

 such as still exists in the 

 periphery of the retina and 

 in the eyes of the totally 

 color blind. It assumes that 

 the colorless sensations — 

 white, gray, black — are occa- 

 sioned by the reactions of a 

 photochemical material 

 which for convenience may 

 be designated as the gray 



substance. This substance in the normal eye exists in both rods 

 and cones; in the latter, however, in a differentiated condition 

 capable of giving color sensations. When the molecules of this 

 substance are completely dissociated by the action of light, gray 



* Franklin, " Zeitschrift f. Psychologic and Phys. d. Sinnesorgane," 1892, 

 iv; also "Mind," 2, 473, 1893, ancl " Psychological Review," 1894, 1896, 1899. 



Fig. 155. — Schema to illustrate the Frank- 

 lin theory of color vision (Franklin) : W, The 

 molecule of the primitive visual (gray-perceiv- 

 ing) substance; Y and B, the first step in the 

 differentiation into a yellow- and a blue-per- 

 ceiving substance, whose combined dissociation 

 gives the same effect as that of the original sub- 

 stance, W; G and R, the second step in the 

 differentiation of the yellow-perceiving sub- 

 stance, the combined dissociation of the two 

 giving the same effect as that of the yellow-per- 

 ceiving substance alone. The complete devel- 

 opment of color vision as it exists in the central 

 part of the retina consists in the existence of 

 three substances, which, taken separately, give 

 red, green, and blue color sensations. 



