THE PERCEPTION OF SIGHT. 255 



in our sensation depended upon any anatomical arrangement of 

 the conducting nerves. 



Again, after the invention of the stereoscope, a fresh difficulty 

 arose in explaining our perceptions of solidity by the differences 

 between the two retinal images. First, Briicke 1 called attention 

 to a series 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 retinse. If we 

 carefully follow the way in which we look at stereoscopic pic- 

 tures or at real objects, we notice that the eye follows the dif- 

 ferent 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 phenomenon 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 succession 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 per- 

 ception 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, 

 1 Professor of Physiology in the University of Vienna. 



