RETINAL CHANGES INVOLVED IN VISION 631 



eyeball of the frog led off from its anterior and posterior surfaces shows 

 a current directed in the eyeball from behind forwards (the resting or 

 demarcation current). It was first shown by Holmgren that this 



II. 



III. 



FIG. 291. Electrical variation in frog's eye as recorded by the string 



galvanometer. (EINTHOVEN and JOLLY.) 



I, on exposure to a single flash ; II, on exposure to light of moderate 

 duration ; III, effect on a light eye of momentary darkening. 



resting -current undergoes modification when light is allowed to fall 

 on the eyeball. Of late years the nature of this modification has been 

 studied especially by Waller with the galvanometer, by Gotch with 

 the capillary electrometer, and by Einthoven and Jolly with the string 

 galvanometer. The nature of the response varies according t 





