RESPONSE OF RETINA TO STIMULUS OF LIGHT 431 



look towards a well-lighted wall. The dark phases will now 

 become the more noticeable. If, however, the wall be dimly 

 lighted, both the dark and bright phases will be noticed alter- 

 nately. It has been suggested that these phenomena might 

 be due to some obscure form of fatigue, but the regular alter- 

 nations observed clearly demonstrate them to be a case of 

 oscillatory changes and multiple response. 



The demonstration of multiple responses in the human 

 retina, as an after-effect, is thus seen to be easy. But their 

 occurrence during continuous exposure of the eye to light is 

 more difficult to prove. This arises from the fact that the 

 waxing and waning of the effect are so gradual as not to be 

 noticeable, unless against some definite standard of com- 

 parison. I shall now prove that, during the continuance of 

 constant light, pulsatory visual effects are produced. These 

 pulsations go unnoticed in visual sensation, not only for want 

 of a standard of comparison, but also because of the remark- 

 able fact which I have discovered, that while the impressions 

 of each individual retina undergo variation, the sum total of 

 the two remains always approximately constant. I have 

 been able to provide the necessary comparison-standards by 

 having two distinguishable images produced in the two eyes, 

 the fluctuation of the visual excitation in one eye being thus 

 capable of detection, by comparison with that in the other. 

 It would have been impossible to detect this fluctuation, had 

 the excitatory variation taken place in the two eyes simulta- 

 neously, i.e. if the maximum excitation in the one had 

 occurred at the same moment as the maximum excitation in 

 the other. But I have found that, as regards excitation 

 there is a relative difference of phase, of half a period, 

 between the two retinae, so that the maximum effect at a 

 given moment in one eye corresponds to the minimum in 

 the other. This constitutes the phenomenon which I have 

 designated the Binocular Alternation of Vision. It is owing 

 to this fact that the periodic excitations in each retina are 

 unmistakably brought out by the following experiment, 

 which consists in looking through a stereoscope that holds, 



