The Stereoscope by David Brewster — Read

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.

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_.