how comes it that we see the solid and not the plane pictures? When we look at the point B, Fig. 19, with both eyes, we converge upon it the optic axes LB, RB, and we therefore see the point single, and at the distances LB, RB from each eye. When we look at the point D, we withdraw the optic axes from B, and converge them upon D. We therefore see the point D single, and at the distances LD, RD from each eye; and in like manner the eyes run over the whole solid, seeing every point single and distinct upon which they converge their axes, and at the distance of the point of convergence from the observer. During this rapid survey of the object, the whole of it is seen distinctly as a solid, although every point of it is seen double and indistinct, excepting the point upon which the axes are for the instant converged. [Illustration: FIG. 20.] From these observations it is obvious, that when we look with both eyes at any solid or body in relief, _we see more of the right side of it by the right eye, and more of the left side of it by the left eye_. The right side of the frustum ABCD, Fig. 19, is represented by the line D_b_, as seen by the right eye, and by the shorter line DB′, as seen by the left eye. In like manner, the left side AC is represented by CA′, as seen by the left eye, and by the shorter line C_a′_, as seen by the right eye. When the body is hollow, like a wine glass, _we see more of the right side with the left eye, and more of the left side with the right eye_. If we now separate, as in Fig. 20, the two projections shewn together on Fig. 19, we shall see that the two summits, CD, _cd_, of the frustum are farther from one another than the more distant bases, AB, _ab_, and it is true generally that _in the two pictures of any solid in relief, the similar parts that are near the observer are more distant in the two pictures than the remoter parts, when the plane of perspective is beyond the object_. In the binocular picture of the human face the distance between the two noses is greater than the distance between the two right or left eyes, and the distance between the two right or left eyes greater than the distance between the two remoter ears. We are now in a condition to explain the process by which, with the eyes alone, we can see a solid in relief by uniting the right and left eye pictures of it,—or the theory ocular stereoscope. In order to obtain the proper relief we must place the right eye picture on the left side, and the left eye picture on the right side, as shewn in Fig. 21, by the pictures ABCD, _abcd_, of the frustum of a cone, as obtained from Fig. 19. [Illustration: FIG. 21.]