This very simple instrument, which, however, answers only for symmetrical figures, such as those shewn at A and B, which must be either two right-eye or two left-eye pictures, is shewn in Fig. 29. A single reflector, MN, which may be either a piece of glass, or a piece of mirror-glass, or a small metallic speculum, or a rectangular prism, is placed at MN. If we look into it with the left eye L, we see, by reflexion from its surface at C, a reverted image, or a right-eye picture of the left-eye picture B, which, when seen in the direction LCA, and combined with the figure A, seen directly with the right eye R, produces a _raised_ cone; but if we turn the reflector L round, so that the right eye may look into it, and combine a reverted image of A, with the figure B seen directly with the left eye L, we shall see a _hollow_ cone. As BC + CL is greater than RA, the reflected image will be slightly less in size than the image seen directly, but the difference is not such as to produce any perceptible effect upon the appearance of the hollow or the raised cone. By bringing the picture viewed by reflexion a little nearer the reflector MN, the two pictures may be made to have the same apparent magnitude. [Illustration: FIG. 29.] If we substitute for the single reflector MN, two reflectors such as are shewn at M, N, Fig. 30, or a prism P, which gives two internal reflexions, we shall have a general stereoscope, which answers for landscapes and portraits. [Illustration: FIG. 30.] The reflectors M, N or P may be fitted up in a conical tube, which has an elliptical section to accommodate two figures at its farther end, the major axis of the ellipse being parallel to the line joining the two eyes. 3. _The Double Reflecting Stereoscope._ This instrument differs from the preceding in having a single reflector, MN, M′N′, for each eye, as shewn in Fig. 31, and the effect of this is to exhibit, _at the same time, the raised and the hollow cone_. The image of B, seen by reflexion from MN at the point C, is combined with the picture of A, seen directly by the _right_ eye R, and forms a _hollow_ cone; while the image of A, seen by reflexion from M′N′ at the point C′, is combined with the picture of B, seen directly by the left eye L, and forms a _raised_ cone. [Illustration: FIG. 31.] [Illustration: FIG. 32.] Another form of the double reflecting stereoscope is shewn in Fig. 32, which differs from that shewn in Fig. 31 in the position of the two reflectors and of the figures to be united. The reflecting faces of the mirrors are turned outwards, their distance being less than the distance between the eyes, and the effect of this is to exhibit at the same time the _raised_ and the _hollow_ cone, the hollow cone being now on the right-hand side.