632 



PHYSIOLOGY 



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

 whether fatigued or fresh, light- or dark-adapted. A typical response 

 to a momentary flash is shown in Fig. 291, 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 then, after a considerable latent 



II. 



B 



FIG. 292. 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. I, light ; d, darkness. 



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 varia- 

 tion. This is shown in Fig. 291, 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 experiment is shown in Fig. 291, 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 



