RESPONSE OF RETINA TO STIMULUS OF LIGHT 419 



noticed that the true excitatory effect on the retina is in the same 

 direction as the existing injury-current, and thus constitutes 

 a positive variation of it. 



Thus, as regards forms of stimulus other than light, such 

 as, for example, the electrical, the responsive reaction of the 

 retina is by induced galvanometric negativity. This at once 

 disposes of the doubt that the general reaction of the retina 

 might be of a different sign from that of any other tissue. 

 But we have still to determine whether or not the stimulus 

 of light, in particular, induces the same normal negative 

 change. The next question to be taken up, then, is that of 

 the true nature of the responsive variation induced in the 



FIG. 251. Series of Photographic Records of Excitatory Responses in Frog's 

 Retina to Equi-alternating Electric Shocks at Intervals of One Minute 



Responsive current from retina to nerve. First series show response to 

 stimulus of moderate intensity ; the second series to stimulus of intensity 

 nearly twice as great. 



retina by light. We have seen that the effect recorded as 

 normal by numerous observers, whether in the eyeball or in 

 the isolated retina (Kiihne and Steiner) was a positive varia- 

 tion. But since we now know that retinal response to stimu- 

 lation in general is not unlike that of other tissues, and since, 

 with regard to the stimulus of light in particular, we have 

 seen it induce true excitation in vegetable tissues, we might 

 expect the responsive reaction of the retina to light to take 

 place by galvanometric negativity. We must then accept 

 one of two conclusions. Either the positive change is a 

 misnomer for the phenomenon observed in the eye, or the 

 inference which we have drawn from the analogy of vegetable 

 tissues is not justified. 



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