Stereographs, all the rage in the 19th and early 20th centuries, consist of two nearly identical photographs—one for the left eye, one for the right. Viewing the side-by-side images though a stereoscope (think proto-ViewMaster) helps our brains combine the two flat images and "see" the illusion of objects in spatial depth. Inspired by an art project of Joshua Heineman, the New York Public Library has created Stereogranimator, a web-based app that allows users to transform its historical stereographs into animated GIFs. 3D afficionados can also create red-blue anaglyphs, which, with the right glasses, recreate the incredible depth effect of these images. Heineman likens the NYPL's stereograph archive to "a gold mine of some 40,000 ViewMaster slides, nearly all more than 100 years old."