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STEREOSCOPE (Fr., SterEoscope; Ger., Stereo shop) An optical instrument for uniting into one image two plane representations as seen by each eye separately, and giving to them the appearance of relief and solidity. The subject of binocular vision was studied by various optical writers who have flourished since the time of Galen. Baptista Porta, one of the most eminent of them, repeats, in his work " On Refraction," the propositions of Euclid on the vision of a sphere with one and both eyes.

Leonardo da Vinci referred to the dis similarity of the pictures seen by each eye as the reason why " a painting can never show a relievo equal to that of the natural objects, unless these be viewed at a distance and with a single eye," which he thus demonstrates. If an object, C (in diagram A), be viewed by a single eye at A, all objects in the space behind it—included, as it were, in a shadow E C rt, cast by a candle at A—are invisible to that eye ; but when the other eye at 33 is opened, part of these objects become visible to it, those only being hid from both eyes that are included, as it were, in the double shadow c n cast by two lights at A and B, and terminated in D ; the angular space E D G, beyond D, is always visible to both eyes. The hidden space C D is so much shorter as the object c is smaller and nearer to the eyes.

Elliot was the first to construct a stereoscope ; but as it was devoid of lenses, the coalescing of the two pictures depended upon a somewhat scope," the fundamental principle of which forms the basis of all modem instruments. He conceived the idea that if prisms or sections of complete lenses were used to divert the rays coming from two pictures placed side by side, so that their images were thrown to a common centre, the evils of reflection obtaining in all reflecting instruments would be altogether dis missed. Diagram D shows his instrument. A unnatural accommodation of the eyes. In a box, P (see diagram B), he made apertures, D and E, and at the opposite end a single aperture c.

The pictures to be combined were placed at a distance behind the box, with the picture belonging to the left eye at B, and the picture belonging to the right eye at A. On placing the eyes at R their axes crossed at c, and then, by a slight effort in focal accommodation to A B, the two pictures A and B were seen combined and presented the desired illusion. Only persons possessing the power to accommodate the focus of the eyes to a remote plane whilst their axes were converged to a nearer plane, could obtain the desired result. Wheatstone's " Reflecting Stereoscope " (C) proved to be the first practical stereoscope. Two plane mirrors MM, set at an angle of 45°, reflect the stereo scopic pictures A and B into the eyes at 1, and R, so that the combined result is seen somewhere in the direction of o. The mirrors are fixed, but the distance between A and B is made adjustable by a screw, threaded (left and right) at each end.

Sir David Brewster, who appears to have been investigating stereoscopic phenomena at the same time as Wheatstone, devised what is known as the " Refracting or Lenticular Stereo and B represent the pictures ; D and E are a pair of prisms fitted to a suitable framework and with their thinner edges turned towards each other. On placing the eyes at 1, R, the pictures A B are seen at a common point c, and stereoscopic solidity and relief results. Brewster soon found that the use of sections of convex lenses effected the necessary refraction, and at the same time provided magnifiers. The best known form of Brewster's Lenticular Stereoscope is that of a tapering box, with the lenses at one end, and at the other a groove for the reception of the pictures. The box contains further a ground-glass panel at the back, so that when transparencies are under observation they are viewed by transmitted light ; while for opaque pictures, the box is fitted with a reflector hinged at the top.

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