PROPERTIES OF THE RETINA. 



335 



w 



V 



,'" 



r 



B 



color blind. It assumes that the colorless sensations white, 

 gra}r, black are occasioned by the reactions of a photochemical 

 material which for convenience may be designated as the gray sub- 

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

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

 pable of giving color sensations. When the molecules of this sub- 

 stance are completely disso- 

 ciated by the action of light, 

 gray sensations result, and 

 as this is the only reaction 

 possible in the rods these 

 elements can furnish us only 

 sensations of this quality. 



The molecules of gray 

 substance in the cones, on 

 the other hand, have under- 

 gone a development such 

 that certain portions only of 

 the molecule may become 

 dissociated by the action of 

 light of certain periods of 

 vibration. This development 

 may be supposed to have 

 taken place in two stages: 

 first, the formation of two 

 groupings within the mole- 

 cule one of which is dissoci- 

 ated by the slower waves 

 and gives a sensation of yel- 

 low, and one of which is dis- 

 sociated by the more rapid 

 waves and gives the sensa- 

 tion of blue. This stage re- 

 mains still on portions of the 

 periphery of the retina, and 

 is the condition present in 

 the fovea also in the eyes 

 of the red-green blind. The 

 second stage consists in the 

 division of the yellow component into two additional groupings 

 in one of which the atomic movements are of such a period as 

 to be affected by the longest visible waves, the red of the spec- 

 trum, while the other is dissociated by rays corresponding to the 

 green of the spectrum and gives rise to the sensation of green. 

 If the red and green groupings are dissociated together the resulting 



Fig. 146. 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. 



