If we unite two dissimilar pictures by the simple convergency of the optical axes, we shall observe a certain degree of relief, at a certain distance of the eyes from the pictures. If we diminish the distance, the relief diminishes, and if we increase it, it increases. In like manner, if we view the dissimilar pictures in the lenticular stereoscope, they have a certain degree of relief; but if we use lenses of a higher magnifying power, so as to bring the eyes nearer the pictures, the relief will diminish, and if we use lenses of a less magnifying power, the relief will increase. By bringing the eyes nearer the pictures, which we do by magnifying them as well as by approaching them, we increase the distance between similar points of the two pictures, and therefore the distance of these points, when united, from any plane in the picture, that is, its relief will be diminished. For the same reason, the diminution of the distance between similar points by the removal of the eyes from the picture, will produce an increase of relief. This will be readily understood if we suppose the eyes R, L, in Fig. 24, to be brought nearer the plane MN, to R′L′, the points 1, 1 and 2, 2 will be united at points nearer MN than when the eyes were at R, L, and consequently their relief diminished. Now we have seen, that in taking portraits, as explained in Fig. 46, we view the two pictures, _ab, a′b′_, with the eyes at E and E′, exactly, and with the same relief in the air, as when we saw the original AB, from L, L′, and therefore E_c_ is the distance at which the dissimilar pictures should be viewed in the stereoscope, in order that we may see the different parts of the solid figure under their proper relief. But the distance E_c_ = L_c_ is the conjugate focal length of the lens L, if one lens is used, or the conjugate equivalent focal length, if two achromatic lenses are used; and consequently every picture taken for the stereoscope should be taken by a camera, the conjugate focal length of whose lens corresponding to the distance of the sitter, is equal to _five_ inches, when it is to be used in the common stereoscope, which has generally that depth.