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nent photographs was communicated to the Royal Society in January 1839, but no attempt was made till some years later to make it available for the stereoscope.
In a chapter on binocular pictures, and the method of executing them in order to reproduce, with perfect accuracy, the objects which they represent, we shall recur to this branch of the subject.
Upon obtaining one of these reflecting stereoscopes as made by the celebrated optician, Mr. Andrew Ross, I found it to be very ill adapted for the purpose of uniting dissimilar pictures, and to be imperfect in various respects. Its imperfections may be thus enumerated :
1. It is a clumsy and unmanageable apparatus, rather than an instrument for general use. The one constructed for me was 16 inches long, 6 inches broad, and 8 inches
2. The loss of light occasioned by reflection from the mirrors is very great. In all optical instruments where images are to be formed, and light is valuable, mirrors and specula have been discontinued. Reflecting microscopes have ceased to be used, but large telescopes, such as those of Sir W. and Sir John Herschel, Lord Rosse, and Mr. Lassel, were necessarily made on the reflecting principle, from the impossibility of obtaining plates of glass of sufficient size.
3. In using glass mirrors, of which the reflecting stereoscope is always made, we not only lose much more than half the light by the reflections from the glass and the metallic surface, and the absorbing power of the glass, but the images produced by reflection are made indistinct by the oblique incidence of the rays, which separates the image
produced by the glass surface from the more brilliant image produced by the metallic surface.
4. In all reflections, as Sir Isaac Newton states, the errors are greater than in refraction. With glass mirrors in the stereoscope, we have four refractions in each mirror, and the light transmitted through twice the thickness of the glass, which lead to two sources of error.
5. Owing to the exposure of the eye and every part of the apparatus to light, the eye itself is unfitted for distinct vision, and the binocular pictures become indistinct, especially if they are Daguerreotypes,1 by reflecting the light incident from every part of the room upon their glass or metallic surface.
6. The reflecting stereoscope is inapplicable to the beautiful binocular slides which are now being taken for the lenticular stereoscope in every part of the world, and even if we cut in two those on paper and silver plate, they would give, in the reflecting instrument, converse pictures, the right-hand part of the picture being placed on the left-hand side, and vice versa.
7. With transparent binocular slides cut in two, we could obtain pictures by reflection that are not converse; but in using them, we would require to have two lights, one opposite each of the pictures, which can seldom be obtained in daylight, and which it is inconvenient to have at night.
Owing to these and other causes, the reflecting stereoscope never came into use, even after photography was capable of supplying binocular pictures.
As a set-off against these disadvantages, it has been
1 Mr. Wheatstone himself says, "that it is somewhat difficult to render the two Daguerreotypes equally visible."-Phil. Trans., 1852, p. 6.
averred that in the reflecting stereoscope we can use larger pictures, but this, as we shall shew in a future chapter, is altogether an erroneous assertion.
Description of the Lenticular Stereoscope.
Having found that the reflecting stereoscope, when intended to produce accurate results, possessed the defects which I have described, and was ill fitted for general use, both from its size and its price, it occurred to me that the union of the dissimilar pictures could be better effected by means of lenses, and that a considerable magnifying power would be thus obtained, without any addition to the instrument.
If we suppose A, B, Fig. 11, to be two portraits,-A a portrait of a gentleman, as seen by the left eye of a person
viewing him at the proper distance and in the best position, and в his portrait as seen by the right eye, the purpose of the stereoscope is to place these two pictures, or rather their images, one above the other. The method of
doing this by lenses may be explained, to persons not acquainted with optics, in the following manner :
If we look at A with one eye through the centre of a convex glass, with which we can see it distinctly at the distance of 6 inches, which is called its focal distance, it will be seen in its place at A. If we now move the lens from right to left, the image of A will move towards B; and when it is seen through the right-hand edge of the lens, the image of A will have reached the position c, half-way between A and B. If we repeat this experiment with the portrait B, and move the lens from left to right, the image of в will move towards A; and when it is seen through the left-hand edge of the lens, the image of B will have reached the position c. Now, it is obviously by the right-hand half of the lens that we have transferred the image of a to c, and by the left-hand half that we have transferred the image of B to c. If we cut the lens in two, and place the halves-one in front of each picture at the distance of 21 inches—in the same position in which they were when A was transferred to c and B to c, they will stand as in Fig. 12, and we shall see the
portraits A and B united into one at c, and standing out in beautiful relief,-a result which will be explained in a subsequent chapter.
The same effect will be produced by quarter lenses, such as those shewn in Fig. 13. These lenses are cut into a round
or square form, and placed in tubes, as represented at R, L in Fig. 14, which is a drawing of the Lenticular Stereoscope.
This instrument consists of a pyramidal box, Fig. 14, blackened inside, and having a lid, CD, for the admission of light when required. The top of the box consists of two parts, in one of which is the right-eye tube, R, containing the lens G, Fig. 13, and in the other the left-eye tube, L, containing the lens H. The two parts which hold the lenses, and which form the top of the box, are often made to slide in grooves, so as to suit different persons whose eyes, placed at R, L, are more or less distant. This adjustment
may be made by various pieces of mechanism. The simplest of these is a jointed parallelogram, moved by a screw forming its longer diagonal, and working in nuts fixed on the top of the box, so as to separate the semi-lenses, which follow the movements of the obtuse angles of the parallelogram. The tubes R, L move up and down, in order to suit eyes of different focal lengths, but they are prevented from turning round by a brass pin, which runs in a groove cut through the movable tube. Immediately below the eyetubes R, L, there should be a groove, G, for the introduction of convex or concave lenses, when required for very long