in.] 



THREE TYPES 



51 



(i) Both reactions positive; (2) Both reactions homodrome ; 

 and (3) Both reactions negative ; and although I do not find any 



Direction of exciting current 



Negative 



Positive 



TVI-E I. 



The response to excitation in both direc- 

 tions is positive, 

 response. 



This is the normal 



TYPE II. 



The response to excitation in the negative 

 direction is negative, and in the positive 

 direction positive. 



TYPE III. 



The response to excitation in both direc- 

 tions is negative. 



DEAD EYEBALL. 



The response to negative excitation is 

 positive and to positive excitation 

 negative. 



FIG. 23. Frog's eyeball. The three types of response to excitation by single 

 induction shocks in positive and negative directions. 



strict parallelism between these three types of electrical response 

 to electrical stimuli, and the three types described above of 

 electrical response to luminous stimuli, I do find correspondence 

 to this extent that a perfectly normal eye gives positive response 

 of the first type in both series, while a considerably compressed 

 or massaged eye gives negative response of the third type in 

 both series. The extremes in the two series correspond, but I 

 fail to trace correspondence in the intermediate and transitional 

 forms of the two series. It is hardly necessary to point out that 

 the very existence of such a series of responses graded from 

 positive to negative, is confirmatory evidence of the coexistence 

 of two contrary electro-motive changes, a conclusion which we 

 saw reason to infer from the effects of light already described. 



