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A stereograph, also known as a stereogram or stereo view, is a double photograph that appears three-dimensional when viewed through a stereoscope. Scientist Charles Wheatstone invented a reflecting stereoscope in 1838 as a laboratory instrument. Some photographers did use this instrument to exhibit photographs, but it was not until the development of the lenticular stereoscope in 1850 by Sir William Brewster that stereographs became popular. They reached their height of popularity between 1870 and 1890 but continued to be created until as late as 1940.
The term "stereograph" is said to have originated with Oliver Wendell Holmes who, in addition to being an author, poet, physician, and lecturer, invented a hand stereoscope in 1859. Holmes also published two essays about the cards in the Atlantic Monthly: "The Stereoscope and Stereograph" (June 1859, volume 3, issue 20) and "Sun-Painting and Sun-Sculpture" (July 1861, volume 7, issue 43), both of which can be accessed through Cornell University Library's Making of America site.
By the mid-1850s, several companies were engaged in the mass-production and sale of stereographs to hundreds of retailers throughout Europe and the United States, where European views were also very popular with American tourists. The mass-produced cards could be sold through photographers' studios, opticians, or art shops, or by publishers or mail-order businesses. When a photographer died or went out of business, the negatives were then sold to other photographers or to publishers. In some instances, identical copy negatives were sold to several publishers.
William Culp Darrah, who calls the stereograph the "first visual mass media," estimates that between 6,000 and 12,000 stereographers were producing images between 1860 and 1880 and that at least 5 million images had been produced. Their affordability, availability, and range of subjects, including city views, railroads, landmarks and scenic views, agriculture, the United States Civil War and other wars, exhibitions and expositions, costumes, disasters, and group compositions, among countless others, brought the experiences of travel and current events to the general population. To modern viewers, they serve as "a primary source for the study of nineteenth-century social history, reflecting social conventions and cultural values." (Fife)
A number of websites offer tips on viewing stereographs without the use of a stereoscope. One example is Instructables.