Smisthsonian Museum. Washington DC stereoscope card, 19th cenrury.

Washington DC stereoscopic card.
Robert Dennis's stereographs collection includes more than 72,000 stereoscopic views organized primarily by geography. The collection bears the name of the native New Yorker who assembled it over a period of more than six decades, Robert N. Dennis (1900-1983).
Stereographs consist of two nearly identical photographs or photomechanical prints, paired to produce the illusion of a single three-dimensional image, usually when viewed through a stereoscope. Stereographs were produced from the 1850s to the 1940s, with the bulk between 1870 and 1920.