68 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



composed of Raehlmann, Landolt, Lamansky, and possibly Aubert. 

 There can be no doubt that the peripheral image is relatively faintly 

 illuminated, and it may be granted that it is also characterized by ill 

 definition. But one finds it difficult to see how these differences in objec- 

 tive conditions of stimulation can account for all of the phenomena of 

 indirect vision. For if the phenomena in question are a product of the 

 lesser luminosity of the peripheral image, one would expect to find a 

 thoroughgoing parallelism between the changes of tone which occur 

 when an image moves across the retina and those which occur when an 

 image occupying a constant retinal position is varied in brightness. 

 That the analogy between these two series of changes is anything but 

 perfect has frequently been demonstrated. Nor can the problematic 

 influence of defective refraction save the theory. For Albini himself 

 has shown that the progressive correction of refraction in indirect vision 

 does not run parallel with the different refrangibility of the colored 

 lights employed. Indeed it seems probable that increased brightness of 

 image was the determining factor in extending Albini's color-zones, and 

 that refractive correction had nothing to do with the matter. Moreover, 

 the fact that the keenness of the peripheral " light sense " is undimin- 

 ished, or even increased, would seem to indicate that the imperfect 

 optical properties of the visual organ have been over-emphasized. 



(2) The second view conceives the histological structure or chemi- 

 cal constitution of the peripheral retina to be different from that which 

 obtains at the center, and brings this structural or chemical differen- 

 tiation into relation with the difference between the functions which 

 have been established in the two cases. The elaboration of this concep- 

 tion has taken different forms in the hands of different theorists. 



(a) Maxwell believed that the absorption which results from the 

 pigmentation of the yellow-spot is sufficient to account for the transi- 

 tons of color-tone which he observed. Maxwell's principle is, of course, 

 powerless to account for changes in tone which occur beyond the limits 

 of the macula. , 



(b) Helmholtz conceived that of the three visual substances but 

 two are distributed over the whole retinal surface. The red-sensing 

 substance is absent from the periphery, which is therefore red-blind. 

 This conception was supported and extended in different directions by 

 certain of his disciples. But the later Helmholtzians and subsequently 

 Helmholtz himself concede its inadequacy. 



(c) Closely allied with the earlier 'Helmholtzian view is the theory 

 which posits an integrity of structure of the whole retinal surface, but 

 assumes that an impairment of function has taken place in the sensitive 



