The principle of the lenticular stereoscope is perhaps better seen by reference to the next diagram, in which the centres of the semi-lenses (i.e., a lens cut in half) are placed at 2½ inches apart, with their thin edges towards each other, and marked, a b, Fig. 310. The centres of the two stereoscopic pictures c d correspond with the centres of the lenses, and the rays of light diverging from c d fall upon the semi-lenses, and being refracted nearly parallel are, by the prismatic form of the semi-lenses, deflected from their course, and leave the surfaces of the lenses in the same direction as if they actually emanated from e; and as all images of bodies appear to come in a straight line from the point whence they are seen, the two pictures are superimposed on each other, and together produce the appearance of solidity, so that a stereoscopic result is obtained when the spectral images of the two stereoscopic pictures are made to overlap each other. By taking one of the semi-lenses in each hand, and looking at the two pictures, the over-lapping of the spectral images becomes very apparent, so that the combined spectral images, and not the pictures themselves, are seen when we look into a stereoscope. (Fig. 310.)
Fig. 310.
Sir David Brewster says, "In order that the two images may coalesce without any effort or strain on the part of the eye, it is necessary that the distance of the similar parts of the two drawings be equal to twice the separation produced by the prism. For this purpose measure the distance at which the semi-lenses give the most distinct view of the stereoscopic pictures, and having ascertained by using one eye the amount of the refraction produced at that distance, or the quantity by which the image of one of the pictures is displaced, place the stereoscopic pictures at a distance equal to twice that quantity—that is, place the pictures so that the average distance of similar parts in each is equal to twice that quantity. If this is not correctly done, the eye of the observer will correct the error by making the images coalesce, without being sensible that it is making any such effort. When the dissimilar stereoscopic pictures are thus united, the solid will appear standing as it were in relief between the two plane representations."
XV. The Stereomonoscope.
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 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.)
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.