STEREOSCOPE, an optical apparatus which enables us to look at one and the same time upon two photographic pictures nearly the same, but taken under a small difference of an gular view, each eye looking upon one picture only; and thus, as in ordinary vision, 'two images are conveyed to the brain which unite into one, the objects being thus represented under a high degree of relief. The stereo scope is constructed in accordance with the visual phenomena which convey to the mind impressions of the relative forms and positions of an object. When a near object having three dimensions is looked at, a different perspective representation of it is seen by each eye; • in other words, there is distinct binocular parallax. Certain parts are seen by the right eye, the left being closed, that are invisible to the left eye, the right being closed and vice versa, and the relative positions of the portions visibly to each eye in succession differ. These two visual im pressions are simultaneously perceived by both eyes, and are combined into one image, pro ducing the impression of perspective and relief. If, then, truthful right-and-left monocular pic tures of any object be so presented to the two eyes that the optic axes, when directed to them, shall converge at the same angle as when directed to the object itself, a solid image will be seen. This is effected with the stereoscope, a reflecting form of which was invented by Professor Wheatstone in 1838. It con structed so that the two dissimilar pictures are reflected to the eyes from two small plane mirrors placed at right angles, the faces being toward the observer. Subsequently Sir David
Brewster invented the refracting or lenticular stereoscope, based on the refractive properties of semi-double convex lenses; and this instru ment, of which there are numerous forms, is now in general use. Convex lenses magnify the pictures besides producing a stereoscopic effect. Photography greatly assists the stereo scope in providing perfectly accurate right and-left monocular views, which are taken si multaneously on a plate in a twin camera. Sometimes, in the case of objects which from their distance have no sensible binocular paral lax, photographs are taken from two points con siderably apart; and when such views are com bined in the stereoscope the effect of relief is greatly exaggerated. A most interesting ap plication has been made of this principle to the photography of comets, the position of the observer between the two exposures being greatly changed owing to the fact that the earth on which he stands is continually mov ing about the sun with a speed of more than 18 miles a second. Though the results are some what marred by the fact that the comet itself is moving, and also changing its shape, yet when the two pictures are combined, the stereo scopic effect is in many cases beautifully shown.
Several modifications of the reflecting stereo scope are distinguished by the names pseudo scope, iconoscope, telesterdoscope and poh& tereoscope, the last being an apparatus which serves the purposes of all the others. See CAMERA; LIGHT.