The following notes on the stereoscope were communicated by Professor Wheatstone to the Photographic Society :—
" The most perfect and generally useful form of the stereoscope is that with reflecting mirrors described in my earliest memoir ' On Binocular Vision,' published in the Philosophical Transactions for 1838. Pictures of any size may be placed in it, at the proper point of sight, with the proper convergence of the optic axes, and it admits of every requisite adjustment to make the pair of binocular pictures coincide correctly.
" I have described in my second memoir a portable stereoscope which folds into a small compass, and which is well suited for pictures not exceeding six inches by four. I have since constructed an instrument, very convenient for carrying about, which is adapted to exhibit pictures of the largest dimensions usually taken, as well as smaller ones, and which may be made use of either for mounted or unmounted pictures. When closed it occupies a space of 9 inches in length, 5 in breadth, and 4 1/2 in height; when expanded the instrument is 2 feet in length, 1 foot in height, and 9 inches in depth. The base and sides consist of jointed bars on the principle of the lazy-tongs; the two mirrors fold together back to back, and, by means of a hinge on their support, fall into a groove on the base fitted to receive them. On the top of each of the expanding sides a clip 9 inches in length receives the picture (which there is no need to mount on cardboard), and holds it by the pressure of a suitably disposed spring; and a similar but detached spring clip is applied to the lower end of the picture, in order to keep it flat and in a vertical position.
"The pictures being fixed in the clips, so that their reflected images shall appear single and coincide in all their parts, the accurate adjustment to the sight of different persons is effected by sliding to and fro the pillar which supports the mirrors; the optic axes being caused to converge more as the mirrors are moved towards the eyes, and vice versa. As the height of the sides is variable through every degree, the pictures are easily adjusted to the same level by pressing on the side which is highest. The length of the base being also variable, the pictures, if it be required, may be placed at different equal distances from the mirrors. If the pictures are not straight with respect to the sheets of paper on which they are placed, one end may be brought lower than the other merely by drawing down that end so that it shall not enter the clip so far as the other.
" The instrument is furnished with a pair of ordinary spectacle lenses, No. 24. If the pictures were so placed that their reflected images coincided when the optic axes made an angle of 15°, corresponding to the distance of 12 inches, no lenses would be requisite, as the distance of the binocular image, the convergence of the optic axes, and the adaptation of the eyes to distinct vision, would have their customary correspondence. But, for reasons I have elsewhere stated, a much better effect is produced, and the objects appear larger and more distant, when the pictures are so placed that, to cause their most distant corresponding points to coincide, the optic axes are parallel, or nearly so; in this case, however, in order to see the objects distinctly, the rays proceeding from them must be rendered less convergent, and for this purpose lenses are necessary.
"The lenses are movable in a vertical direction, in order that they may be fixed at the proper point of sight; the effect of a stereoscopic picture greatly depends on its being thus viewed, though it is a circumstance which is very generally disregarded".
With the single camera, taking the precaution named, with two lenses of the same focal length, or with the semi-lenses, stereoscopic pictures may he obtained without difficulty.
The magic result of the resolution of two plain pictures into one, possessing to the eye the most positive solidity, is so striking when witnessed for the first time, that it appears to be a deception of the senses. Even when fully accustomed to the phenomena of the stereoscope, there is an indescribable charm in the beautiful pictures, that they are gazed at again and again with increasing admiration. Living forms appear to stand out in all the roundness of life; and where colours have been judiciously applied to the daguerreotype or calotype portrait, it is not possible to conceive a more perfect realization of the human form than that which stands forth, prominently, from the back ground of the stereoscopic picture. Statues, in like manner, are almost realized again in their miniature representations. Architectural piles are seen in all that exactness of proportion and gradation of distance, which is, in their minute reproduction, singularly interesting; and in landscapes, the stereoscope gives us a reformation of every image in apparently the most perfect solidity and truth of distance. In the stereoscope we have at once an instrument which enables us to study many of the phenomena of vision, and to reproduce loved and beautiful objects, or interesting scenes, through the agency of those rays by which they were illuminated, in that strange perfection which, in its mimicry of visible external nature, almost baffles the examination of human sense.