Stereopticon
A stereopticon is a slide projector or relatively powerful "magic lantern", which has two lenses, usually one above the other, and has mainly been used to project photographic images. These devices date back to the mid 19th century, and were a popular form of entertainment and education before the advent of moving pictures. Magic lanterns originally used rather weak light sources, like candles or oil lamps, that produced projections that were just large and strong enough to entertain small groups of people. During the 19th century stronger light sources, like limelight, became available. For the "dissolving views" lantern shows that were popularized by Henry Langdon Childe since the late 1830s, lanternists needed to be able to project two aligned pictures in the same spot on a screen, gradually dimming a first picture while revealing a second one.