XVI. The Stereomoscope.
This arrangement is an important modification of the other, and consists of a screen of ground glass (a b, Fig. 312), and two convex lenses (c d, and e f) arranged in such a manner that they will project images of the stereoscopic pictures, g h, at the same point on the screen, a b.
It might be thought that a confusion of images would result from projecting two pictures on one point, p—viz., the focus of the two lenses; but as each photograph can be seen only in the direction of its own rays, it follows that if the eyes are so placed that each receives the impression of one stereoscopic picture, the two images must coalesce, and a stereoscopic effect will be the result, as is apparent at k k and l l; so that several persons may look at the stereoscope at one time. (Fig. 312.)
Fig. 312.
The stereomoscope.
XVII. The Pseudoscope.
This curious optical instrument, as its name implies, produces a false image by the refracting power of prisms, and is the invention of Professor Wheatstone. When used with both eyes, the same as the stereoscope, it inverts the relief of a solid body, and makes it appear exactly as if it were an intaglio, or sunk beneath the line surrounding it. For instance, a terrestrial globe when looked at through the pseudoscope appears to be concave, like Wyld's Globe in Leicester-square, instead of convex. A vase with raised ornaments upon it looks as if it had been turned (to reverse the usual expression) outside in, and the whole of its convexity is turned to concavity; and of course a face seen under these circumstances looks very curious. (Fig. 313.) The cause is perhaps somewhat difficult to understand; but by taking other and more simple examples of the same effect, the principle may be gradually comprehended.
Fig. 313.