THE PERCEPTION OF SIGHT. 291 



of facts which apparently made it possible to reconcile 

 the new phenomena discovered with the theory of the 

 innate identity of the sensations conveyed by the two 

 retinae. If we carefully follow the way in which we 

 look at stereoscopic pictures or at real objects, we 

 notice that the eye follows the different outlines one 

 after another, so that we see the ' fixed point ' at each 

 moment single, while the other points appear double. 

 But, usually, our attention is concentrated upon the 

 fixed point, and we observe the double images so little 

 that to many people they are a new and surprising phe- 

 nomenon when first pointed out. Now since in following 

 the outlines of these pictures, or of an actual image, we 

 move the eyes unequally this way and that, sometimes 

 they converge, and sometimes diverge, according as we 

 look at points of the outline which are apparently nearer 

 or further off; and these differences in movement may 

 give rise to the impression of different degrees of distance 

 of the several lines. 



Now it is quite true, that by this movement of the 

 eye while looking at stereoscopic outlines, we gain a 

 much more clear and exact image of the raised surface 

 they represent, than if we fix our attention upon a single 

 point. Perhaps the simple reason is that when we move 

 the eyes we look at every point of the figure in suc- 

 cession directly^ and therefore see it much more sharply 

 defined than when we see only one point directly and the 

 others indirectly. But Briicke's hypothesis, that the 

 perception of solidity is only produced by this movement 

 of the eyes, was disproved by experiments made by Dove, 

 which showed that the peculiar illusion of stereoscopic 

 pictures is also produced when they are illuminated 

 with an electric spark. The light then lasts for less 

 than the four thousandth part of a second. In this 

 time heavy bodies move so little, even at great velocities. 



