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If we look into it with the left eye L, we see, by reflexion from its surface at c, a reverted image, or a righteye 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 BCCL 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.
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.
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.
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.
If in place of two right or two left eye pictures, as shewn in Figs. 29, 31, and 32, we use one right eye and one left eye picture, and combine the reflected image of the one with the reflected image of the other, we shall have a raised cone with the stereoscope, shewn in Fig. 31, and a hollow cone with the one in Fig. 32.
The double reflecting stereoscope, in both its forms, is a general instrument for portraits and landscapes, and thus possesses properties peculiar to itself.
The reflectors may be glass or metallic specula, or total reflexion prisms.
4. The Total Reflexion Stereoscope.
This form of the stereoscope is a very interesting one, and possesses valuable properties. It requires only a small prism and one diagram, or picture of the solid, as seen by one eye; the other diagram, or picture which is to be combined with it, being created by total reflexion from the base of the prism. This instrument is shewn in Fig. 33,
where D is the picture of a cone as seen by the left eye L, and ABC a prism, whose base BC is so large, that when the eye is placed close to it, it may see, by reflexion, the whole of the diagram D. The angles ABC, ACB must be equal, but may be of any magnitude. Great accuracy in the equality of the angles is not necessary; and a prism constructed, by a lapidary, out of a fragment of thick plate
glass, the face BC being one of the surfaces of the plate, will answer the purpose. When the prism is placed at a, Fig. 34, at one end of a conical tube LD, and the diagram
D at the other end, in a cap, which can be turned round so as to have the line mn, Fig. 33, which passes through the centre of the base and summit of the cone parallel to the line joining the two eyes, the instrument is ready for use. The observer places his left eye at L, and views with it the picture D, as seen by total reflexion from the base BC of the prism, Figs. 33 and 35, while with his right eye R, Fig. 33, he views the real picture directly. The first of these pictures being the reverse of the second D, like all pictures formed by one reflexion, we thus combine