THE EYE AS A SENSORY APPARATUS 249 



apart from its failure to accord with the doctrine of specific nerve 

 energies, is that its assumption of similar nervous activities result- 

 ing from opposing chemical processes is unwarranted by any 

 knowledge that we have of the relation between chemical processes 

 and nervous activities in other parts of the body. 



The Franklin Theory is based on the idea that the peculiar dis- 

 tribution of color vision over the retina is significant as suggesting 

 that the more complex color perceptions are evolved from simpler 

 ones. According to this theory the primary photochemical sub- 

 stance is a gray-perceiving substance; white and black represent- 

 ing the ends of the gray color series. This substance is in all the 

 rods, and in the cones of the retinal margin where only gray per- 

 ception occurs. In the cones of the yellow-blue field, the funda- 

 mental gray-perceiving photochemical substance is supposed to 

 be dissociated into two different photochemical substances, one 

 yellow-perceiving, the other blue-perceiving. Since these are 

 products of the gray-perceiving substance when both are stimu- 

 lated together the effect is the same as when the gray-perceiving 

 substance itself is stimulated, namely, a shade of gray. 



In the central cones of the retina a still further decomposition 

 is assumed to have occurred, of the yellow-perceiving substance 

 into red and green-perceiving substances. The central cones, then, 

 contain three photochemical substances, a red-perceiving one, a 

 green-perceiving one, and a blue-perceiving one. Since all are 

 ultimately derived from the gray-perceiving substance their com- 

 bined stimulation produces gray sensations; simultaneous stimu- 

 lation of the red and green substances gives the same result as 

 stimulation ,of their parent substance, that for perceiving yellow. 



This theory puts the distribution of color vision in the retina and 

 the phenomenon of color blindness, which it explains as due to 

 failure of dissociation of the yellow-perceiving substance, upon a 

 more rational basis than do either of the other theories described. 

 In most other respects it offers little advantage over them. 



While we must admit that at present a full understanding of 

 color vision is beyond us we may properly look forward to its ulti- 

 mate mastery, as physiology is able to penetrate more deeply the 

 processes which underly it. 



Visual Perceptions. The sensations which light excites in us we 

 interpret as indications of the existence, form, and position of ex- 



