172 THE HUMAN BODY. 



seem that in order to give but a single impression, the images 

 perceived by both eyes should be exactly alike. But experi- 

 ment demonstrates that two images differing in some respects, 

 do notwithstanding give but a single sensation to the brain. 

 When we look at a solid like the pedestal of a column or a 

 monument, a moment's attention shows us that the outlines 

 corresponding to the right of the spectator give a larger image 

 to the right eye than to the left, and that each image differs 

 from the other; the combination of the two sensations gives 

 us the idea of relief. 



If now we obtain by photography, or if we trace by a single 

 white line on a black ground, the projection of this monument 

 or pedestal, in conditions identical with those under which 

 our eyes receive a double impression, the two images placed 

 in the direction of the optical axes, as the surfaces would be 

 which they represent, will give us the impression of the solid 

 in question by a single image. We owe to Mr. Wheatstone 

 the demonstration of this phenomenon, and the invention of 

 an instrument which renders the proof very easy and simple. 

 This is the stereoscope, the application of which is so widely 

 known. 



When the eyes are first applied to the instrument, in pro- 

 portion as the optic axes converge, the two images are seen 

 one over the other, and when at last we perceive but one, 

 instead of a plane surface we have a relief under our eyes, 

 which, in certain cases, produces a complete illusion. But 

 as M. Longet observes, the unity of the image does not 

 prove that there is but a single sensation, and the two dif- 

 ferent images do not give birth to a simple sensation, but are 

 the source of one complex though indefinable one, that of 

 solidity. How this blending of two different impressions is 

 effected, is one of the mysteries of our organization ; but the 

 sensation of relief evidently arises from a combination of 

 conditions different from those which determine single vision 

 by means of two eyes. 



When vision embraces a certain extent of space, as a land- 

 scape or a gallery of pictures for instance, the objects appear 

 single to us, although for the most part they are out of the 

 direction of the optical axes, but on observing closely we find 



