quence of the bending of the rays of light, they are projected upon similar parts of the two retinæ, and seem to be produced by a single object. Whether the two impressions are made by the double picture or by a single solid, the same sensation is produced, as in either case the mind combines the two impressions into the idea of solidity. The stereoscope, therefore, enables us to give a true notion of the form and position of objects from two flat representations on paper or glass; in fact, we may see the objects quite as well as if they stood before us.
Although the stereoscope was discovered some twenty years ago, it has only lately become popular. So long as mere drawings by hand were used as stereoscopic slides, only regular bodies, such as crystals and geometric solids, could be represented; but now, by the aid of photography, we may obtain pictures of any natural or artistic objects.
When we look at a double photograph in the stereoscope, the picture to the right is seen by the right eye only, and that to the left, by the left eye. The two pictures are taken from different points of view, and are exactly similar to the views we obtain of solid objects, by alternately closing the right and left eyes. There is, therefore, no longer any doubt as to the use of two eyes, since by the aid of photography we may obtain pictures similar to those which the eyes receive, and these pictures combine to produce the effect of solidity.
We are indebted to Professor Wheatstone for the
