It is obvious, however, from the slightest consideration, that reflectors of such a size are wholly unnecessary, and that one square inch of reflecting surface, in place of thirty-two, is quite sufficient for uniting the binocular pictures. We can, therefore, at a price as low as that of the 4-inch glass reflectors, use mirrors of speculum metal, steel, or even silver, or rectangular glass prisms, in which the images are obtained by total reflexion. In this way the stereoscope becomes a real optical instrument, in which the reflexion is made from surfaces single and perfectly flat, as in the second reflexion of the Newtonian telescope and the microscope of Amici, in which pieces of looking-glass were never used. By thus diminishing the reflectors, we obtain a portable tubular instrument occupying nearly as little room as the lenticular stereoscope, as will be seen from [Fig. 28], where ABCD is a tube whose diameter is equal to the largest size of one of the binocular pictures which we propose to use, the left-eye picture being placed at CD, and the right-eye one at AB. If they are transparent, they will be illuminated through paper or ground-glass, and if opaque, through openings in the tube. The image of AB, reflected to the left eye L from the small mirror mn, and that of CD to the right eye R from the mirror op, will be united exactly as in Mr. Wheatstone’s instrument already described. The distance of the two ends, n, p, of the mirrors should be a little greater than the smallest distance between the two eyes. If we wish to magnify the picture, we may use two lenses, or substitute for the reflectors a totally reflecting glass prism, in which one or two of its surfaces are made convex.[40]
2. The Single Reflecting Stereoscope.
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.
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.
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.