206 LIFE AND WORK OF SIR JAGADIS C. BOSE 



crucible ; but on quickly closing the eyes the visual after- 

 effect of the smoke, being of less luminescence, cleared 

 away first, leaving the after-image of the molten and boiling 

 metal growing clearer on the retina. 



Under continuous action of light the artificial retina 

 exhibited periodic fluctuations in response. In trying 

 to determine the corresponding phenomenon in human 

 vision, he discovered ' the curious fact that in normal eyes 

 the two do not see equally well at a given instant, but 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, must be of obvious advantage. 



For demonstration he uses a stereoscope carrying, 

 instead of stereo-photographs, an incised plate, through 

 which we look at the light. The design consists of two 

 slanting cuts, one eye looking at one and the second at 

 the other. In this way not only is the different binocular 

 alternation of vision demonstrated, but also the after- 

 effects. When the design is looked at through the stereo- 

 scope, the right eye will see the right slanting cut R, and 

 the left the other incised cut L ; the two images will appear 

 superimposed, and we see an inclined cross. When the 

 stereoscope is turned towards the sky, and the cross looked 

 at steadily for some time, it will be found, owing to the 

 alternation already referred to, that while one arm of 

 the cross begins to be dim, the other becomes bright, and 

 vice versa. The alternate fluctuations become far more 

 conspicuous when the eyes are closed ; the pure oscillatory 

 after-effects are then obtained in a most vivid manner. 

 After looking through the stereoscope for ten seconds or 

 more, the eyes are closed. The first effect observed is 

 one of darkness, due to the rebound. Then one luminous 

 arm of the cross first projects aslant the dark field, and 



