A stereopticon is a slide projector

Slide projector

A slide projector is an opto-mechanical device to view photographic slides. Slide projectors were common in the 1950s to the 1970s as a form of entertainment; family members and friends would gather to view slide shows...

 or "magic lantern

Magic lantern

The magic lantern or Laterna Magica is an early type of image projector developed in the 17th century.-Operation:The magic lantern has a concave mirror in front of a light source that gathers light and projects it through a slide with an image scanned onto it. The light rays cross an aperture , and...

", which has two lenses, usually one above the other.
These devices date back to the mid 19th century, and were a popular form of entertainment and education before the advent of moving pictures. Americans William and Frederick Langenheim introduced stereopticon slide technology — slide shows of projected photographs on glass — in 1850. For a fee of ten cents, people could view realistic photographs with nature, history, and science themes. At first, the shows used random images, but over time, lanternists began to place the slides in logical order, creating a narrative. This "visual storytelling" directly preceded the development of the first moving pictures.
The term stereopticon has been widely misused to name a stereoscope. A stereopticon will not project or display stereoscopic / three-dimensional (3-D

Three-dimensional space

Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...

) images. The two lenses are used to dissolve between images when projected. All stereopticons can be classified as magic lanterns, but not all magic lanterns are stereopticons.