RETINAL CHANGES INVOLVED IN VISION 633 



difference. The substance B reacts less rapidly than A. On lighting, 

 it develops a positive, on darkening a negative potential difference. 

 Its action is especially marked in a dark eye which is illuminated for a 

 short time with a weak light. Substance c reacts in the same sense as 

 B, but much more slowly. Its action is wanting in a completely light 

 eye. The actions of these three substances are represented diagram- 

 matically in Fig. 292, I. and II. 



It is interesting to note that the latent period of the photo-electric 

 reaction agrees with the latent period of the light perception of the 

 human eye, and may vary from '01 sec. with strong stimuli to as much 

 as 2 sec. with very weak stimuli. 



