Historical information
Stereoscopy (also called stereoscopic or 3-D imaging) refers to a technique for creating or enhancing the illusion of depth in an image by presenting two offset images separately to the left and right eye of the viewer. Both of these 2-D offset images are then combined in the brain to give the perception of 3-D depth. Three strategies have been used to accomplish this: have the viewer wear eyeglasses to combine separate images from two offset sources, have the viewer wear eyeglasses to filter offset images from a single source separated to each eye, or have the lightsource split the images directionally into the viewer's eyes (no glasses required).
Stereographic cards and the stereoscope. Two separate images are printed side-by-side. When viewed without a stereoscopic viewer the user is required to force his eyes either to cross, or to diverge, so that the two images appear to be three. Then as each eye sees a different image, the effect of depth is achieved in the central image of the three.
Stereograms cards are frequently used by orthoptists and vision therapists in the treatment of many binocular vision and accommodative disorders. Source: www.wikipedia.org
Physical description
Wooden viewer with two glass "windows", separating divider between them. Sliding cross-piece which holds the photocard between wire holders. Turned handle.
