Refers to double pictures of the same scene that produce the effect of three dimensionality when viewed through a stereoscope. They were first envisioned in 1832 by the English physicist Charles Wheatstone, who described this as a uniquely photographic art form, since a draftsman could not draw two scenes in exact perspective from viewpoints separated only 2 1/2 inches, which is the normal distance between human eyes necessary for the three-dimensional effect. Wheatstone's mirror stereoscope was not practical for use with photographs, and the invention was not popular until the 1850s, when Sir David Brewster, a Scottish scientist, designed a simpler viewing instrument. The introduction of the collodion process, which simplified exposure and printing techniques, allowed three-dimensional photographs to become a popular craze. They may be daguerreotypes, negatives, or other forms of photographs. For images in the form of photographic prints on cards, use the more specific term "stereographs."
