2. If another drawing of the same object or series of objects is similarly executed on the same plane from _a second_ point of sight, sufficiently distant from the first to make the two drawings separate without overlapping, every point of its surface visible from this second point of sight will also be represented on the plane, so that we shall have two different drawings of the object placed, at a short distance from each other, on the same plane. [Illustration: FIG. 24.] 3. Calling these different points of the object 1, 2, 3, 4, &c., it will be seen from Fig. 24, in which L, R are the two points of sight, that the distances 1, 1, on the plane MN, of any pair of points in the two pictures representing the point 1 of the object, will be to the distance of any other pair 2, 2, representing the point 2, as the distances 1′P, 2′P of the points of the object from the plane MN, multiplied inversely by the distances of these points from the points of sight L, R, or the middle point O between them. 4. If the sculptor, therefore, or the architect, or the mechanist, or the surveyor, possesses two such pictures, either as drawn by a skilful artist or taken photographically, he can, by measuring the distances of every pair of points, obtain the relief or prominence of the original point, or its distance from the plane MN or AB; and without the use of the stereoscope, the sculptor may model the object from its plane picture, and the distances of every point from a given plane. In like manner, the other artists may determine distances in buildings, in machinery, and in the field. 5. If the distance of the points of sight is equal to the distance of the eyes L, R, the two plane pictures may be united and raised into relief by the stereoscope, and thus give the sculptor and other artists an accurate model, from which they will derive additional aid in the execution of their work. 6. In stereoscopic vision, therefore, when we join the points 1, 1 by converging the optic axes to 1′ in the line PQ, and the points 2, 2 by converging them to 2′ in the same line, we place these points at the distances O1, OO2, and _see the relief_, or the various differences of distance which the sculptor and others obtained by the method which we have described. 7. Hence we infer, that if the stereoscopic vision of relief had never been thought of, the principles of the instrument are involved in the geometrical relief which is embodied in the two pictures of an object taken from two points of sight, and in the prominence of every part of it obtained geometrically. CHAPTER VI. ON THE UNION OF SIMILAR PICTURES IN BINOCULAR VISION.