VISUAL ANALOGUES ey es 
tion ismeant 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 retine 
there is then a relative retardation of half a period. 
This may be seen by means of a stereoscope, carrying, 
mstead of stereo-photographs, incised plates through 
which we look at light. The design consists of two 
