From what has been already stated it would seem that in order to give but a single impression, the images perceived by both eyes should be exactly alike. But experiment 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 phenomonon, 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 proportion 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 different 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 landscape 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 that we never fix the eyes except on a very limited portion of the space spread out before us; the objects thus normally seen occupy our whole attention, and turn it away from the other images of which the vagueness or duplication passes unperceived. When we endeavour to determine these facts we find that the boundary lines of the objects and the borders of the pictures appear double, but dim and confused, when outside of the point of convergence of the ocular axes.