Herrick, Color Vision. 279 



To the above Miss Calkins remarks : It may be assumed from 

 psychological analysis that there are four and not three fundamental 

 colors and that white is not a mixed but a fundamental sensation. 

 This disposes of the Youxg-Helmholtz theory. 



The fact above noted, that a mixture of red and green light does 

 not produce white light is not reconcilable with Hering's theory. 



The anatomical structure and distribution of the rods indicates 

 that these structures can produce only colorless light and this confirms 

 the view shared by v. Kries, Konig and Ladd-Franklin. 



The fact that rods and cones originally were similar and that the 

 cones differentiated in the course of evolution, makes it probable that 

 a chemical process which goes on in the same way in the rods and 

 cones produces white light and, furthermore, that various phases or 

 stages of this chemical process in the cones are the causes of colored 

 light. These considerations recommend the Ladd-Franklin theory 

 of molecular dissociation. 



According to the Ladd-Franklin theory, the basis for color dis- 

 crimination is a four-fold chemical process in the cones but white light 

 is simply produced by the decomposition of the elementary form of the 

 pigment, which decomposition may be supposed to produce a stimulus 

 communicable along the fibers of the optic nerve. Certain other 

 rates of vibration are capable of producing a change in the more 

 complicated cone-pigment corresponding to the sensation of "red," 

 "green" "yellow" or "blue" respectively. It may be ventured as a 

 suggestion in line with this theory that, if the complicated pigment of 

 the cones is genetically related to that in the rods, it is also probable that 

 in its process of formation it will pass through a stage like that in the 

 rods. In this event, there will always be material in the cone capable 

 of reacting to white light independently of a decomposition of the 

 proper complicated pigment in its mature state. 



Up to this point no psychological question has been raised except 

 in so far as in the use of language there has been an incautious impli- 

 cation that there has been a mixing of sensations. But suppose wave 

 lengths corresponding to red and blue impinge at the same time on the 

 cones, then either the double stimulant causes a new kind or degree of 

 chemical decomposition, or both the red and blue phases of decompo- 

 sition are going on concurrently in different ingredients — at any rate 

 the chemical resultant of this mixing is a nerve stimulus different from 

 that for red or blue alone and must be conveyed along the fibers or 

 fibrils of the optic nerve as such, or else the retinal ganglia, as a portion 



