M. Claudet, whose name has long been celebrated in connexion with the art of photography, has described an instrument by which a single picture is made to simulate the appearance of solidity, and he states that by means of this arrangement a number of persons may observe the effect at the same time. The apparatus required is very simple, consisting of a large double convex lens, and a screen of ground glass. The [Page 324] object A, Fig. 311, is highly illuminated, and placed in the focus of a double convex lens B, when an image of the object is projected, and will be found suspended in the air in the conjugate focus of the lens at C, and from this point the rays of light will diverge as from a real object, which will be seen by separate spectators at D D and E E; and if the screen of ground glass is placed at G G, the image will appear with all the effect of length, breadth, and depth, which belong to solid bodies. (Fig. 311.) [Illustration: Fig. 311. The stereomonoscope.] An image formed on ground glass in this manner can be seen only in the direction of the incident rays, and the stereoscopic effect is not apparent when the image is received on a calico or transparent screen, on account of the rays being scattered in all directions. XVI. _The Stereomoscope._ [Illustration: Fig. 312. 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 [Page 325] 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.) XVII. _The Pseudoscope._ [Illustration: Fig. 313. Horizontal section of the pseudoscope, showing at A B two prisms placed against a block of wood about two inches long and one inch and a half wide, and cut out in the centre to admit the nose at D. The eyes are supposed to be looking at the globe, C, in the direction of the arrows. E E. Brass plates blackened, which shut out the side light, and assist in keeping the prisms in position.]