stereoscope (stĕrˈēəskōpˌ) [key], optical instrument that presents to a viewer two slightly differing pictures, one to each eye, to give the effect of depth. In normal vision the two eyes, being a certain distance apart, see slightly different aspects of a scene. The impression of depth is obtained when the brain combines the images. A single photograph shows no more than what one eye would see. In a stereoscope two photographs, taken from positions related approximately as the positions of a person's two eyes, are placed side by side. When a person observes these photographs, his brain combines the separate images from each eye into a single three-dimensional one. Scientists, among them the English physicist Sir Charles Wheatstone in 1838, constructed stereoscopes for use with drawings, but suitable views were not generally available until the development of photography. In 1849, Sir David Brewster, a Scottish physicist, improved the stereoscope and invented the double camera for taking stereoscopic views. Oliver Wendell Holmes invented the kind of stereoscope that, together with a collection of stereoscopic views, became a popular instrument of home entertainment in the United States until the advent of the home phonograph and the radio. The principle of the stereoscope is applied in binocular field glasses and binocular microscopes.