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 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.

Related Topics:
Stereoscopy - 19th century - Charles Wheatstone - 20th century

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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.

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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.

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The stereoscope offers several advantages:

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• 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.

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Stereoscope: Fine arts stereoscopy ►