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 [Page 323] 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.) [Illustration: 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._