RESPONSE TO STIMULUS OF LIGHT 



409 



of stimulation by light, I have obtained similar multiple 

 after-effects. And in this connection I have discovered certain 

 characteristic peculiarities of the electrical after-effect which 

 would appear to throw much light on the obscure subject 

 of after-effects in the retina. The electrical after-effect of 

 stimulus of light varies greatly in different conditions of the 

 tissue, but is capable of classifica- 

 tion under three different types. 



The first of these types refers 

 to those multiply responding tis- 

 sues which give the usual single 

 response of galvanometric nega- 

 tivity under short exposure to 

 light. When such specimens are 

 subjected to the continuous action 

 of this stimulus they give multiple 

 responses whose phases alternate 

 in the sequence of minus-plus- 

 minus-plus ( 1 h). Confining 



our attention to the first two pairs 



of such phasic alternations under 



continuous stimulation of light 



(fig. 243), we observe that, in the 



first period, excitatory negativity 



attains its maximum at b y after 



which the phase becomes reversed 



to positive, in which process the 



curve may arrive at the original 



base line, or stop short of this or 



go beyond it. The maximum of 



this phase, a', we shall designate as maximum positivity. 



Under the still-continued action of the stimulus the phase 



now changes once more to negative, and so on. 



It has already been stated (p. 100) that these periodic 

 alternations in phase were brought about by antagonistic 

 reactions becoming effectively predominant by turns. Thus, 

 as was there pointed out, in those cases which normally give 



FIG. 243. Diagrammatic Repre- 

 sentation of Phasic Alter- 

 nations, and After-effect in 

 Type I. 



During action of light, the 

 phasic alternations are a b, 

 b a!, a' 6',6' a" (- + - + ); 

 b is here the maximum 

 negative, and a' the maxi- 

 mum positive. Withdrawa 

 of stimulus at point of re- 

 versal a' causes unmasking 

 of positive component, which 

 is exhibited by overshooting 

 of the curve in the positive 

 direction, a' c. 



