Stereoscope



A stereoscope is a device for viewing stereographic cards, which are cards that contain two separate images that are printed side-by-side to create the illusion of a three-dimensional image. This is an example of stereoscopy. When stereographic cards are 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. This is the oldest method of stereoscopy, having been discovered in the mid-19th century by Charles Wheatstone. In the late 19th and early 20th century "stereoviews",stereo cards, stereo pairs or stereographs were popularly sold. The cards had a pair of photographs, usually taken with a special camera that took the pair of images from slightly separated views simultaneously. Cards were printed with these views (often with explanatory text); when the cards were looked at through the double-lensed viewer, called a stereoscope or a stereopticon (a common misnomer), a three-dimensional image could be seen.



A simple stereoscope is limited in the size of the image that may be used. A more complex stereoscope uses a pair of horizontal periscope-like devices, allowing the use of larger images that can present more detailed information in a wider field of view. The stereoscope is essentially an instrument in which two photographs of the same object, taken from slightly different angles, are simultaneously presented, one to each eye. Each picture is focused by a separate lens, and the two lenses are inclined so as to shift the images toward each other and thus ensure the visual blending of the two images into one three-dimensional image.

A moving image extension of the stereoscope has a large vertically mounted drum containing a wheel upon which are mounted a series of stereographic cards which form a moving picture. The cards are restrained by a gate and when sufficient force is available to bend the card it slips past the gate and into view, obscuring the preceding picture. These coin-enabled devices were found in arcades in the late 19th and early 20th century and were operated by the viewer using a hand crank. These devices can still be seen and operated in some museums specializing in arcade equipment.



The stereoscope offers several advantages:
 * Using positive curvature (magnifying) lenses, the focus point of the image is changed from its short distance (about 30 to 40 cm) to a virtual distance at infinity. This allows the focus of the eyes to be consistent with the parallel lines of sight, greatly reducing eye strain.
 * The card image is magnified, offering a wider field of view and the ability to examine the detail of the photograph.
 * The viewer provides a partition between the images, avoiding a potential distraction to the user.

A transparency viewer is a type of stereoscope that offers similar advantages.

Fine arts stereoscopy
Graphic artists have and continue to produce original artwork to be viewed using stereoscopic devices (stereoscopes).

Several fine arts photographers are producing and marketing photographic stereoscope cards, but these seem to fall within a rather narrow genre, appearing to be mostly high-quality erotica.