If we look at A with one eye through the _centre_ of a convex glass, with which we can see it distinctly at the distance of 6 inches, which is called its _focal distance_, it will be seen in its place at A. If we now move the lens from _right to left_, the image of A will move towards B; and when it is seen through the _right_-hand edge of the lens, the image of A will have reached the position C, half-way between A and B. If we repeat this experiment with the portrait B, and move the lens from _left to right_, the image of B will move towards A; and when it is seen through the _left_-hand edge of the lens, the image of B will have reached the position C. Now, it is obviously by the _right_-hand half of the lens that we have transferred the image of A to C, and by the _left_-hand half that we have transferred the image of B to C. If we cut the lens in two, and place the halves—one in front of each picture at the distance of 2½ inches—in the same position in which they were when A was transferred to C and B to C, they will stand as in Fig. 12, and we shall see the portraits A and B united into one at C, and standing out in beautiful relief,—a result which will be explained in a subsequent chapter. [Illustration: FIG. 12.] The same effect will be produced by quarter lenses, such as those shewn in Fig. 13. These lenses are cut into a round or square form, and placed in tubes, as represented at R, L, in Fig. 14, which is a drawing of the _Lenticular Stereoscope_. [Illustration: FIG. 13.] This instrument consists of a pyramidal box, Fig. 14, blackened inside, and having a lid, CD, for the admission of light when required. The top of the box consists of two parts, in one of which is the right-eye tube, R, containing the lens G, Fig. 13, and in the other the left-eye tube, L, containing the lens H. The two parts which hold the lenses, and which form the top of the box, are often made to slide in grooves, so as to suit different persons whose eyes, placed at R, L, are more or less distant. This adjustment may be made by various pieces of mechanism. The simplest of these is a jointed parallelogram, moved by a screw forming its longer diagonal, and working in nuts fixed on the top of the box, so as to separate the semi-lenses, which follow the movements of the obtuse angles of the parallelogram. The tubes R, L move up and down, in order to suit eyes of different focal lengths, but they are prevented from turning round by a brass pin, which runs in a groove cut through the movable tube. Immediately below the eye-tubes R, L, there should be a groove, G, for the introduction of convex or concave lenses, when required for very long-sighted or short-sighted persons, or for coloured glasses and other purposes. [Illustration: FIG. 14.]