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 16½ inches long, 6 inches broad, and 8½ inches high.
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,[34] 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.