CHAPTER XXVIII 



RESPONSE OF RETINA TO STIMULUS OF LIGHT 



Response of retina Determination of true current of rest Determination 01 

 differential excitabilities of optic nerve and cornea, and optic nerve and 

 retina The so-called positive variation of previous observers indicates the 

 true excitatory negative Retino-motor effects Motile responses in nerve 

 Varying responsive effects under different conditions Reversal of the normal ' 

 response of light due to (i) depression of excitability below par ; (2) fatigue- 

 The sequence of responsive phases during and after application of light 

 Demonstration of multiple responses in retina under light, as analogous to 

 those in vegetable tissues Three types of after-effect Multiple after- 

 excitations in human retina Binocular Alternation of Vision Demonstra- 

 tion of pulsatory response in human retina during exposure to light. 



THE nature of the electrical reaction of the retina under 

 stimulus of light constitutes a problem which has attracted 

 many investigators, chief and earliest among these being 

 Holmgren, Kiihne and Steiner, Dewar and McKendric. The 

 results obtained by these workers have been confirmed by 

 the later researches of others ; but while their general observa- 

 tions are fairly concordant, the way in which the phenomena 

 they have described are related to the excitatory reaction is a 

 question which has hitherto remained undetermined. 



Observed results, in fact, would seem to show that the 

 electrical reactions of the retina are of a nature quite different 

 from those exhibited by other animal tissues. For whereas 

 nerve or muscle, for example, responds on excitation by 

 negative variation, the retina, under the same normal* con- 

 ditions, is said to exhibit a positive variation. The subject 

 is very .much complicated, moreover, by the confusion which 

 has unfortunately arisen as to the meaning of the terms 

 positive and negative. These positive and negative varia- 

 tions are so named in reference to the existing current ot 



