It remains to speak of the pictures in their relation the one to the other as a stereo scopic pair. Evidently, exactly to reproduce the conditions of normal vision, they should be taken from points of view separated laterally by a space equal to the distance between the eyes, viz., about 21 in.; and for all objects within narrow limits of distance this rule is observed. But taking a wider range, such as would include, for iustauce, an extensive architectural pile, photographers usually take their pictures front spots separated by a considerable interval; and the stereoscopic slides thus obtained, when viewed in the stereoscope, exhibit effects of solidity or relief of a very striking charac ter. Inasmuch, however, as these effects are due to a gross exaggeration of the ordinary difference of perspective relatively to the two eyes, they to a like extent misrepresent the actual appearance of the scene; and it were to be wished that for all stereoscopic pairs alike, whether representative of near or of remote objects, photographers would be content to adopt that exact relation of the two retinal pictures which subsists in ordinary binocular vision. As to the mounting of the pictures, it is of course highly important that they be placed exactly in the same line; it has further been pointed out by Mr. Claudet that, as the apparent solidity-of the objects viewed in the stereoscope conflicts with the evident flatness of the cardboard mount, it is advantageous to adopt the follow ing expedient. The pictures must be of the same size, but instead of having them identically the same as regards the objects represented on each, let the left-hand picture include on its left-hand margin somewhat less than is found on the same margin of the right-hand picture; similarly, let its right-hand margin contain somewhat more than is found on the same margin of the right-hand picture: then will the view appear to extend well out of and beyond the cardboard, which forms, as it were, a framework around it. A moment's consideration will show that this ingenious arrangement does but reproduce the conditions which obtain whenever we look out upon a scene through a easement distant from us by a few feet. Availing himself of the libration of the mom, Mr. War ren De La Rue has obtained lunar stereoscopic photographs, which exhibit that body with a general appearance of rotundity, while the other objects on her surface arc seen in conspicuous relief. These effects are, however, evidently due to an exaggeration of the "binocular parallax ;" for by no human eyes, how near soever,they might be placed to the lunar surface, could such a view be obtained. It is, as sir John Herschel has remarked, as though the moon were seen with the eyes of a giant, placed thousands of miles apitrt.
Among the minor applications of the stereoscope may be mentioned the STEREO MONOSCOPE and the STEREOTROPE, the former devised by Mr. the latter by
Mr. William Thomas Shaw; and severally described by them in the Proceeding.4 of the Royal Society of June, 1S57, April, 1858, and Jan. 1861. In the stereomonoscope,the two pictures of a stereoscopic pair are projected, by means of lenses, on to the posterior surface of a piece of ground glass, one upon the other, or so that they occupy the, same place; when the observer, looking from the opposite side of time glass, sees them not as a confused mixture of two pictures, but as a single stereoscopic representation, possessing the usual attributes of solidity or relief. The stereotrope consists in an application of the principle of time stereoscope to that class of instruments variously termed thauma tropes, phenakistoscopes, etc., which depend for their results on "persistence of vision." In these instruments, .as is well known, an object represented on a revolving disk in the successive positions it assumes in performing a given evolution, is seen to execute the movement so delineated; iu the stereotrope, the effect of solidity is superadded, so that the object is seen as if in motion, and with an appearance of relief as in nature. A highly ingenious application of the principle of the stereoscope to portraiture has been described by 3Ir. henry Swann in time Report of the British Assoczatioa for 1863. In this arrangement, the portrait is seen as a solid bust imbedded in a cube of crystal. A foram of the reflecting stereoscope, in which the/planes of reflection are vertical, has been pro posed by \l r. Walter I birdie.
But by far the most important application of the stereoscopic principle, is its realiza tion in the binocular microscope of Mr. Wenham, the advantages of which over tho monocular form of that instrument are increasingly appreciated by microscopists. In this, the right and left eye pictures, respectively, are thus obtained. Immediately behind the object-glass, a small and peculiarly shaped prism is placed in such a position, that it shall receive the whole of the rays coming through the right half of the lens. These rays, after being twice reflected Nvithin the body of the prism, finally emerge at such an angle to their original direction, that they cross the undiverted pencil of rays transmitted by the other half of the lens, and are then received into a second tube, which, being inclined to the first or main tube at an appropriate angle, conveys them to the left eye; while the other complement of rays pursues an undeviating coarse to the right eye. Each of the two tubes is fitted with the usual eye-pieces; and object-glasses of all but the highest powers may be used with pleasure and advantage. For a fuller explanation, see the original paper by Mr. Wenham in the Transactions of the .ificroscopic Society, new series, vol. ix., page 15.