VISUAL ANALOGUES 175 



tion is meant either the presence of fatigue- stuffs, or the 

 breaking-down of the sensitive element of the tissue, or 

 both of these. In such a case we should expect that 

 this fatigue, with its consequent negative image, would 

 gradually and finally disappear on the restoration of the 

 retina to its normal condition. 



We find, however, that this is not the case, for the 

 negative image recurs with alternate positive. The 

 accepted theory of fatigue is incapable of explaining 

 this phenomenon. 



In the sensitive silver cell, we found that the mole- 

 cular strain produced by light gave rise to a current of 

 response, and that on the cessation of light an oscillatory 

 after-effect was produced. The alternating after-effect 

 in the retina points to an exactly similar process. 



Binocular alternation of vision. It was while ex- 

 perimenting on the phenomena of recurrent vision that 

 I discovered the curious fact that in normal eyes the 

 two do not see equally well at a given instant, but that 

 the visual effect in each eye undergoes fluctuation from 

 moment to moment, in such a way that the sensation in 

 the one is complementary to that in the other, the sum 

 of the two sensations remaining approximately constant. 

 Thus they take up the work of seeing, and then, relatively 

 speaking, resting, alternately. This division of labour, 

 in binocular vision, is of obvious advantage. 



As regards maximum sensation in the two retinae 

 there is then a relative retardation of half a period. 

 This may be seen by means of a stereoscope, carrying, 

 instead of stereo-photographs, incised plates through 

 which we look at light. The design consists of two 



