In the preceding illustration we have supposed the solid to consist only of a base and a summit, or of parts at _two_ different distances from the eye; but what is true of two distances is true of any number, and the instant that the two pictures are combined by the lenses they will exhibit in relief the body which they represent. If the pictures are refracted too little, or if they are refracted too much, so as not to be united, their tendency to unite is so great, that they are soon brought together by the increased or diminished convergency of the optic axes, and the stereoscopic effect is produced. Whenever two pictures are seen, no relief is visible; when only one picture is distinctly seen, the relief must be complete. In the preceding diagram we have not shewn the refraction at the second surface of the lenses, nor the parallelism of the rays when they enter the eye,—facts well known in elementary optics. CHAPTER V. ON THE THEORY OF STEREOSCOPIC VISION. Having, in the preceding chapter, described the ocular, the reflecting, and the lenticular stereoscopes, and explained the manner in which the two binocular pictures are combined or laid upon one another in the last of these instruments, we shall now proceed to consider the theory of stereoscopic vision. [Illustration: FIG. 19.] In order to understand how the two pictures, when placed the one above the other, rise into relief, we must first explain the manner in which a solid object itself is, in ordinary vision, seen in relief, and we shall then shew how this process takes place in the two forms of the ocular stereoscope, and in the lenticular stereoscope. For this purpose, let ABCD, Fig. 19, be a section of the frustum of a cone, that is, a cone with its top cut off by a plane C_e_D_g_, and having AEBG for its base. In order that the figure may not be complicated, it will be sufficient to consider how we see, with two eyes, L and R, the cone as projected upon a plane passing through its summit C_e_D_g_. The points L, R being the points of sight, draw the lines RA, RB, which will cut the plane on which the projection is to be made in the points _a_, _b_, so that _ab_ will represent the line AB, and a circle, whose diameter is _ab_, will represent the base of the cone, as seen by the right eye R. In like manner, by drawing LA, LB, we shall find that A′B′ will represent the line AB, and a circle, whose diameter is A′B′, the base AEBG, as seen by the left eye. The summit, C_e_D_g_, of the frustum being in the plane of projection, will be represented by the circle C_e_D_g_. The representation of the frustum ABCD, therefore, upon a plane surface, as seen by the left eye L, consists of two circles, whose diameters are AB, CD; and, as seen by the right eye, of other two circles, whose diameters are _ab_, CD, which, in Fig. 20, are represented by AB, CD, and _ab_, _cd_. These plane figures being also the representation of the solid on the retina of the two eyes,