Stereoscopes, depending on the optical device they use to convey to each eye the light coming from one only of the pair of images, are called mirror stereoscopes or refraction stereoscopes.
The mirror stereoscope, invented in 1832 by Charles Wheatstone (1802-1875), consists of a straightedge, which in the original model was wood, at the ends of which there are two vertical screens perpendicular to them on which the two reproductions of the object to be viewed are attached; at the centre of the straightedge there are two flat, vertical mirrors at right angles to each other, with the common edge lying on the plane perpendicular to the straightedge.
By looking at the angle formed by the mirrors it is possible to see, separately and simultaneously, the image fixed to the screen on the right with the right eye and the one on the left screen with the left eye.
The distance and positions of the mirrors and their screens are adjusted so that the images supplied by the two mirrors overlap behind the straightedge.
In refraction stereoscopes the two reproductions, placed side by side in a single frame, when they are on glass, transparent paper or film, are observed with transmitted light and are illuminated from behind through a plate of frosted glass which covers the front face of the instrument almost entirely; when the images are on opaque paper, they are viewed in diffused light and are illuminated by means of a mirror hinged to the upper part of the instrument. ...continues