From The Measure:
Forget about electricity, running water, modern sanitation and food preservation; what did people do before GIFs? Well, it turns out that some were busy entertaining themselves with stereoscopes, instruments that allowed people to view 3D images from two slightly different photographs superimposed on top of one another. The New York Public Library has a collection of more than 40,000 of these images, but it took an accidental discovery by NYPL patron Joshua Heineman to find that these stereographs could be made into a modern, internet-friendly form of entertainment—ah yes, the GIF.
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Direct to Stereogranimator (via NYPL Lab)