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CAMERA APPLIANCES
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actually see it as a flat surface is proved by the deception produced by theatrical decorations, where the remote background, if properly painted, often produces an extremely natural effect ; but we perceive this background to be flat directly we move the head on one side. When this is done, a solid object presents a different appearance, but a flat surface remains unchanged. . FiS67'
The Stereoscope. — Wheatstone was impressed by the fact that the solid impression made by an object is caused by the combination of the different views of it by the right and left eye. Accordingly, he tried to substitute for a single picture a view of the right side of an object for the right eye, and of the left side for the left eye. He obtained in this manner a perfectly solid impression, though the double picture occasioning it is no solid at all. Some people, indeed, are able to see stereoscopic pictures as solids without using an instrument. But most persons require an apparatus which renders it possible for both eyes to see in the same place the two separate pictures. This apparatus is the stereoscope (fig. 67). Its most essential features are, the two pictures on the slide and the partition in the interior of the box which prevents the right eye seeing the picture on the left, and vice versa ; and, further, the lid, which is generally provided with a mirror which can be either shut or opened, so as to exclude or let the light into the box ; and, lastly, the two lenses at the top. These lenses are represented in the diagram (fig. 68) ; they are two halves of one lens, and work in the same manner. We are indebted to Brewster for the construction of this instrument.
Fig. 68.