Stereoscope

Stereoscope. Since our two eyes are separated by a definite distance, it follows that near objects will produce slightly different images on the two retina. The two pictures are mentally blended, and the combination gives us the perception of solidity or relief. Wheatstone was the first to point out the importance of this double vision, and illustrated its effect by means of an instrument he called a stereoscope, which was afterwards improved by Brewster. Two pictures are drawn of an object from two near but different points of view. They represent the pictures formed by the two eyes. The one picture will show a little more of the right side of the object, just as would be seen by the right eye; the other picture will show more of the left side. The two pictures are then placed in two halves of a box, over each of which a half lens is placed. The right eye views the one picture, while the left eye sees the other, a partition down the centre of the box preventing either eye seeing what was intended only for the other. The two half-lenses slightly magnify the picture, and, what is more important, cause the two virtual images to coincide as nearly as possible. Thus, rays from the object a b to the lens enter the eye R as though they came from A B. In a similar way the other lens forms an image of e d which appears to the eye L to be superposed upon A B. The result is that the observer sees a solid object instead of a plane picture. The whole thing stands out in relief and looks exactly like any actual still dimensional object. The inability of a person to detect which eye sees an object is exhibited by the stereoscope. A piece of white paper may be put in one side, and a paper with a clot on it in the other; the observer cannot tell in which side the dot occurs - i.e. he does not know which eye sees it.