STEREOSCOPE, from (meat (solid), and eaconafs (a view, or eacesiai, to view), au instrument by which two pictures of any object, taken from different points of view, are seen as a single picture of that object, having the natural appearance of relief or solidity.
The theory of the stereoscope is sufficiently discussed under Scour. The reflecting stereoscope was first described in a paper by Professor Wheatstone, entitled ' Contributiorri to the Physiology of Vision: Part 1. On some Remarkable and hitherto Unobserved Phenomena of Binocular Vision,' read before the Royal Society, June 21st, 1838, and printed in the' Philosophical Transactions' a few months later. The refracting stereoscope is described by Sir David Brewster in a paper ' On the Law of Visible Position in Single and Binocular Vision, and on the Representation of Solid Figures by the Union of Dissimilar Plano Figures on the Retinte,' which he communicated to the Royal Society of Edinburgh in January, 1843. Ile further explained and defer led his views in subsequent papers, which, like the former, appeared in the 'Edinburgh Transactions' of that and following years. The opinions of Sir David Brewster are further set forth in his work Stereoscope' (8vo, 1856); those of Mr. Wheatstone must be sought in the paper already referred to, and in another which formed the liakerian Lecture of the Royal Society for 1852, being 'Part II. of Contributions to the Physiology of Vision, and on Binocular Vision.' The dissimilarity of the pictures, as seen by each eye separately, does not appear to be a modern discovery. It was recognised by Euclid 2000 years ago, and minutely described by Galen. The idea was revived by Baptista Porta, in 1593; also by Leonardo da Vinci ; by Aguilonius, in a work on the vision of solids (1613); by Harris, in 1775; by Dr. Smith, Dr. Porterfield, and others. Thus, as Brewster remarks, writers in every age knew the two facts that the pictures on the retinae of the two eyes were dissimilar, and that by the union of these two flat distinct pictures we obtain the vision of solids. But in order to obtain accurate pictures of objects as seen by each eye, and the method, of uniting them, photography and the binocular camera were required. The first was already sufficiently advanced, and the latter was introduced by Brewster ; while, in order to view the pictures with effect, the lenticular stereoscope was contrived. ' The first instru
ment of this kind was constructed by the late Andrew Ross for the inventor, and was exhibited to the British Association in Birmingham in 1849. It did not, however, attract attention until the French optician Du Boscq showed his remarkable collection of stereoscopic views in the Great Exhibition of 1851, after which the demand for the stereoscope warmed into a passion which has scarcely since cooled down. The lenticular stereoscope, as described by the inventor, " consists of a pyramidal box of wood or metal, or any other opaque material, blackened on the inside, and having a lid for the admission of light when the pictures are opaque. The box is open below, in order to let the light pass through the pictures when they arc transparent. Another lid is sometimes added, so as to open externally on the bottom of the box, for the purpose of exhibiting dissolving views in the stereo scope. The bottom of the box is generally covered with ground-glass, the surface of which ought to be very fine, or very fine-grained paper may be used. The top of the box consists of two portions, in one of which is the right eye-tube, containing a semi-lens, or quarter-lens, and in the other the left eye-tube, also containing a semi-lens or quarter lens. These two portions may be advantageously made to approach or recede, in order to suit eyes at different distances from one another ; and the tubes containing the lenses should draw out, in order to suit long- and short-sighted eyes." The two dissimilar pictures (which, for convenience, are mounted on a thick card, forming the universally known "slide ") are placed in a groove in the bottom of the box, when, on looking through the eye-tubes, they are seen united into a single picture, and the object or objects, if a proper amount of light is obtained, stand out with an almost magical appearance of relief and solidity. For opaque slides, a mirror may be used, and made to move on a hinge, so as to throw light into the instrument. The employment of photography for the stereographs has wonderfully extended the range of the instrument.