The stereoscope

by David Brewster

Fig. 36.

If we take the drawing of a six-sided pyramid as seen by the right eye, as shewn in [Fig. 36], and place it in the total-reflexion stereoscope at D, [Fig. 33], so that the line MN coincides with mn, and is parallel to the line joining the eyes of the observer, we shall perceive a perfect raised pyramid of a given height, the reflected image of CD, [Fig. 36], being combined with AF, seen directly. If we now turn the figure round 30°, CD will come into the position AB, and unite with AB, and we shall still perceive a raised pyramid, with less height and less symmetry. If we turn it round 30° more, CD will be combined with BC, and we shall still perceive a raised pyramid with still less height and still less symmetry. When the figure is turned round other 30°, or 90° degrees from its first position, CD will coincide with CD seen directly, and the combined figures will be perfectly flat. If we continue the rotation through other 30°, CD will coincide with DE, and a slightly hollow, but not very symmetrical figure, will be seen. A rotation of other 30° will bring CD into coalesence with EF, and we shall see a still more hollow and more symmetrical pyramid. A further rotation of other 30°, making 180° from the commencement, will bring CD into union with AF; and we shall have a perfectly symmetrical hollow pyramid of still greater depth, and the exact counterpart of the raised pyramid which was seen before the rotation of the figure commenced. If the pyramid had been square, the raised would have passed into the hollow pyramid by rotations of 45° each. If it had been rectangular, the change would have been effected by rotations of 90°. If the space between the two circular sections of the cone in [Fig. 31] had been uniformly shaded, or if lines had been drawn from every degree of the one circle to every corresponding degree in the other, in place of from every 90th degree, as in the Figure, the raised cone would have gradually diminished in height, by the rotation of the figure, till it became flat, after a rotation of 90°; and by continuing the rotation it would have become hollow, and gradually reached its maximum depth after a revolution of 180°.

5. The Single-Prism Stereoscope.

Although the idea of uniting the binocular pictures by a single prism applied to one eye, and refracting one of the pictures so as to place it upon the other seen directly by the other eye, or by a prism applied to each eye, could hardly have escaped the notice of any person studying the subject, yet the experiment was, so far as I know, first made and published by myself. I found two prisms quite unnecessary, and therefore abandoned the use of them, for reasons which will be readily appreciated. This simple instrument is shewn in [Fig. 37], where A, B are the dissimilar pictures, and P a prism with such a refracting angle as is sufficient to lay the image of A upon B, as seen by the right eye. If we place a second prism before the eye R, we require it only to have half the refracting angle of the prism P, because each prism now refracts the picture opposite to it only half way between A and B, where they are united. This, at first sight, appears to be an advantage, for as there must always be a certain degree of colour produced by a single prism, the use of two prisms, with half the refracting angle, might be supposed to reduce the colour one-half. But while the colour produced by each prism is thus reduced, the colour over the whole picture is the same. Each luminous edge with two prisms has both red and blue tints, whereas with one prism each luminous edge has only one colour, either red or blue. If the picture is very luminous these colours will be seen, but in many of the finest opaque pictures it is hardly visible. In order, however, to diminish it, the prism should be made of glass with the lowest dispersive power, or with rock crystal. A single plane surface, ground and polished by a lapidary, upon the edge of a piece of plate-glass, a little larger than the pupil of the eye, will give a prism sufficient for every ordinary purpose. Any person may make one in a few minutes for himself, by placing a little bit of good window glass upon another piece inclined to it at the proper angle, and inserting in the angle a drop of fluid. Such a prism will scarcely produce any perceptible colour.

Fig. 37.

If a single-prism reflector is to be made perfect, we have only to make it achromatic, which could be done extempore, by correcting the colour of the fluid prism by another fluid prism of different refractive and dispersive power.

With a good achromatic prism the single-prism stereoscope is a very fine instrument; and no advantage of any value could be gained by using two achromatic prisms. In the article on New Stereoscopes, published in the Transactions of the Royal Society of Arts for 1849, and in the Philosophical Magazine for 1852, I have stated in a note that I believed that Mr. Wheatstone had used two achromatic prisms. This, however, was a mistake, as already explained,[41] for such an instrument was never made, and has never been named in any work previous to 1849, when it was mentioned by myself in the note above referred to.

If we make a double prism, or join two, as shewn at P, P′ in [Fig. 38], and apply it to two dissimilar figures A, B, one of which is the reflected image of the other, so that with the left eye L and the prism P we place the refracted image of A upon B, as seen by the right eye R, we shall see a raised cone, and if with the prism P′ we place the image of B upon A we shall see a hollow cone. If we place the left eye L at O, behind the common base of the prism, we shall see with one-half of the pupil the hollow cone and with the other half the raised cone.