STEREOSCOPE (Gr. stereos, solid, and skopein, to see), an optical instrument of modern invention, by means of which pictures of objects possessing three dimensioes are seen not as plane representations, but with an appearance of solidity or relief, as in ordinary vision of the objects themselves. The more recond,te principles of the stereoscope, which are of high interest and importance in their bearing on the philosophy of perception, will be fully considered under VISION, BINOCULAR. The present article will be limited to an historical sketch of its invention and subsequent developments, coupled with an exposition of the optical and mechanical details of its construction.
The essential principle of the stereoscope, the first conception of which by prof. Wheatstone justly ranks as one of the most brilliant optical discoveries of the age, may be thus explained. It is an obvious fact that the eyes being separated by a certain interval of space, all solid objects so near to the observer as to be seen with a sensible convergence of the optic axis, necessarily form retinal pictures, differing as to their perspective projections for each eye. Singular to say, the true import of this plain fact was wholly unsuspected prior to the investigations of prof. Wheatstone, who, in his first paper on this subject, pablisbed in the Philosophicat Transactions for 1838, clearly established the important conclusion that this dissimilarity of the retinal images is made to subserve an important end in the use of our visual organs—that it is, in fact, the principal originating cause of our immediate perception of the solidity (or relief) of objects adjacent to the sight. The problem he set himself to investigate was: "What would be the visual effect of presenting to each eve, instead of the object itself, its projection on a plane surface as 11, appears to that eye?" and in order to bring this question to the test of experiment, he devised au instrument which he named the stereoscope.
The pictures being attached to the slides, the observer places himself with his nose close to and immediately in front of the vertical angle made by the reflectors, so that the view by each eye is limited to the rays reflected by its appropriate mirror; the pic tures are then seen, as it were, behind the mirrors, and. the eyes being made slightly to converge, either by an effort of the will or by drawing the slides a little forward, the effect of either of which is to refer the reflected images to the same part of space, the observer sees no longer mere pictorial resemblances, but, to all appearance, the objects themselves, exquisitely modeled, occupying a certain extent of space, and standing forth with a substantiality of aspect truly wonderful. At the outset the only stereoscopic pictures obtainable were the outlines of geometrical solid figures, which it was possible fora skillful artist to depict with perspective projections adapted for the right and left eye respectively; and the pictures so prepared excited the greatest interest and admira tion. They, moreover, ahundantly exemplified the truth and importance of the binoc ular principle, though the universality of its application to purposes of pictorial illustra tion only became apparent on the introduction and gradual improvement of the photo graphic arts. In 1819 sir David Brewster originated that convenient, portable, and in
all respects admirable form of the stereoscope which is now in general use over the whole civilized world. For this—the lenticular stereoscope—the pictures (taken, be it remembered, from two different points of view), are mounted side by side, on a piece of cardboard, and, being nlaced in the instrument, arc viewed through semi-lenses, fixed at a distance apart of the two eyes. To effect the displacement of the pictures, so that they shall be referred to the same part of space, which we have above defined to be an essential condition, sic David Brewster most ingeniously availed himself of an optical principle, which enabled him at the same time to fulfill several collateral ends of con siderable importance. This principle may be described as follows• If an object be viewed through the center, or, more properly. along, the axis, of a convex lens, it will be seen exactly in front of the eye; i.e., in it line with the eye, the center of the lens, and the actual place of the object. if now the lens be moved slightly to the left, the object will appear to advance toward the right; and, conversely, as the lens is moved toward the right, the object is displaced in the opposite direction. Let the lens he in half, transversely, and the two semi-circular pieces reversed as to their former posi tion, i.e., placed side by side, and so that their thin edges shall be adjacent, while the two plane edges, formed by the section of the lens, are kept in mutual parallelism, and have their faces turned outward, toward the left and right respectively: the right eye will now look through the left half of the lens, and vice versa; and the two pictures, each placed opposite its appropriate eye, and in the principal focus of the eye-piece, will be seen, not in their actual places, but in a position midway between the two. The subsidiary purposes served by this arrangement are that the pictures are magnified as well as caused to coalesce; and that the equality of the magnifying power of the eye. 'pieces (a result by no other means certainly attainable) is secured by the fact of their being cut from the same lens, the whole of which is thus advantageously and econom ically utilized. In too many of the instruments offered for sale the conditions stated above are very imperfectly fulfilled; the parallelism of the two sectional planes of the semi-lenses, and their rectangurarity with two imaginary planes joining their opposite ends respectively, are not maintained, and, as a consequence, the coalescence of the plc tures is effected, if at all, by a forced and more or less painful displacement of the eye balls, entirely destructive of all pleasure in the use of the instrument, And it is impor tant to recollect that this parallelism of the sides of the semi-lenses may be either actual or virtual; for to whatever shape they may be cut (and the circular form is the one most often adopted), the foregoing conditions are in nowise altered. The best lenticular form of the instrumeut with which we are acquainted is the achromatic stereoscope devised by Messrs. Smith, Beck, and Beck, the well known London opticians, which combines excellencies of a very varied character.