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CHAPTER VII.

DESCRIPTION OF DIFFERENT STEREOSCOPES.

ALTHOUGH the lenticular stereoscope has every advantage that such an instrument can possess, whether it is wanted for experiments on binocular vision-for assisting the artist by the reproduction of objects in relief, or for the purposes of amusement and instruction, yet there are other forms of it which have particular properties, and which may be constructed without the aid of the optician, and of materials within the reach of the humblest inquirers. The first of these is

1. The Tubular Reflecting Stereoscope.

In this form of the instrument, shewn in Fig. 28, the pictures are seen by reflexion from two specula or prisms placed at an angle of 90°, as in Mr. Wheatstone's instrument. In other respects the two instruments are essentially different. In Mr. Wheatstone's stereoscope he employs two mirrors, each four inches square-that is, he employs thirty-two square inches of reflecting surface, and is therefore under the necessity of employing glass mirrors, and making a clumsy, unmanageable, and unscientific instrument, with all the imperfections which we have pointed out in a preceding chapter. It is not easy to understand why mirrors of such

a size should have been adopted. The reason of their being made of common looking-glass is, that metallic or prismatic reflectors of such a size would have been extremely expensive.

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 thirtytwo, 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

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a tube whose diameter is equal to the largest size of one of the binocular pictures which we propose to use, the lefteye 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.1

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

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may be either a piece of glass, or a piece of mirror-glass, or a small metallic speculum, or a rectangular prism, is placed

We may use also the lens prism, which I proposed many years ago in the Edinburgh Philosophical Journal.

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 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

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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,

B

M

C'

N'

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.

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