KETINAL CHANGES INVOLVED IN VISION 567 



strength and duration of the stimulus, and to the condition of the eye, whether fatigued 

 Of Ano gh T~ L d f k ' ada P ted - A ^cal response to a momentary flash is shown 

 m Fig. 293, I. Within a very short latent period after the incidence of the flash i e 

 after a latent period of not more than -01 sec., there is a small short negative variation 

 of the resting current, which is immediately followed by a large positive variation i e 

 the resting current is increased. This is followed immediately by a diminution and 



FIG. 294. Diagrammatic representation of the reactions to light of the three 



hypothetical substances A, B, and c. (EINTHOVEN and JOLLY.) 

 I, the three effects shown separately ; II, the three effects combined to form a 

 single curve. A is the lighting and A' the darkening effect of the first substance. 

 /, light ; d, darkness. 



then, after a considerable latent period,' by a second slow prolonged increase of the 

 current. When the duration of the stimulus is longer the moment of shutting off 

 the light is seen to be followed immediately by a second positive variation. This 

 is shown in Fig. 293, II. It is possible to obtain this response to darkness by shutting 

 off for a short period of time the light falling into the eye. The result of such an ex- 

 periment is shown in Fjg. 293, III. Einthoven and Jolly explain these results by the 

 assumption that three separate processes are concerned when the retina is stimulated. 

 For convenience they speak of the changes in three distinct substances, A, B, and c. 

 Of these A reacts more rapidly than the other two, and its action is specially marked 

 in a ' light ' eye, appearing almost isolated on sudden darkening of short duration 

 (a flash of darkness). On lighting, it develops a negative, on darkening a positive 

 potential 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. Sub- 

 stance 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 

 diagrammatically in Fig. 294, 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. 



