COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 39 



and that the peripheral image receives less light than does the central 

 image. It was but natural, then, that he should attempt to correlate the 

 observed changes of color sensation, with the observed changes in 

 physical and physiological conditions, and to explain the former wholly 

 in terms of the latter.* But it has since been established that Aubert's 

 analogy between the two series of phenomena is far from being com- 

 plete. Nor is his second principle of explanation adequate to account 

 for all of the facts which his first principle leaves unexplained. It is 

 true that the adaptation phenomena of the peripheral retina have never 

 been submitted to a thoroughgoing investigation; but unless it be 

 granted that this domain has genuine surprises in store for the investi- 

 gator, it must be admitted that Aubert's explanation is unsatisfactory. 



Maxwell conceived that the chromatic absorption of the yellow 

 spot is sufficient to account for all of the phenomena which he observed. 

 But inasmuch as color changes of unmistakable character occur beyond 

 the limits of the macula, Maxwell's view must also be held to be in- 

 adequate. 



Helmholtz believed that he was justified in concluding that the 

 peripheral region of the normal retina is analogous in structure and 

 function with the retina of the red-blind. He conceives both to be 

 devoid of red-sensing substance, and finds in this defect an explanation 

 of the phenomena which occur in the two cases. This conception was 

 extended by certain of his disciples, who assumed that the violet- 

 sensing fibers also are lacking in the normal periphery. This view is 

 untenable alike in its original and in its modified formulation. For if, 

 as Helmholtz teaches, the sensation of white is the product of the simul- 

 taneous functioning of all three sorts of color-sensing substance, and 

 if two, or even one, of these be absent from the periphery, then the 

 periphery is obviously incapable of furnishing the sensation of white. 

 Moreover, if the periphery is supplied with but a single sort of visual 

 substance, every sort of stimulus, chromatic and achromatic, mechanical 

 and electrical, must there arouse the sensation of blue (or green, p. 15). 

 The later advocates of the Young-Helmholtz theory have not failed 

 to see the force of this objection; and Kick has advocated a modifica- 

 tion of the original conception which, it was hoped, would obviate the 

 difficulty. Kick posits a uniformity of structure for the whole retinal 

 surface, but conceives that the function of the eccentric regions has 

 undergone a peculiar modification. He still holds to the essential fea- 



*Aubert's position with regard to this question is not altogether clear. The 

 writer is convinced, however, that he has done no violence to the views of this 

 noted investigator in the above summary statement of his position. 



