This is the coolest thing ever: Wiggle 3D stereoscopy.
Stereoscopy is the art of making and viewing 3D images on 2D media (on paper, film, or screens), but usually this requires special glasses or viewing techniques. The basic idea is that we humans perceive 3D because we have two eyes that are spaced about 4 inches apart, and because each eye sees a slightly different view, this gives the impression of 3 dimensions and of depth. 3D glasses, whether they’re the red-and-green anaglyph type, or the modern polarized kind (which you may have used to watch 3D movies in the cinema) work on this principle, by feeding each eye a slightly different image, recreating the natural 3D effect.
If you don’t have special 3D glasses, it’s still possible to view some stereoscopic images by the cross-eye technique. This is popular among chemists for some reason. In high school I learned much of my chemistry from Linus Pauling’s General Chemistry, but my favorite thing about that book were the cross-eye stereograms. Paleontologists also seem to like the cross-eye stereogram; when I took vertebrate paleontology in college, our professor had some fun showing us how to view stereograms (although he used the parallel-eye technique, rather than the cross-eye technique. The parallel-eye technique has the advantage of working even though you are viewing something on a screen. The cross-eye technique usually requires you to put your face really close to the paper.)
Wiggle 3D is somewhat different, though, and doesn’t require special glasses. It creates the 3D effect by animation, wiggling back and forth between the two layers of the stereogram. What’s particularly amazing to me is that the 3D effect is still there even when you are only viewing it with one eye. Try it! Close one eye and look at the wiggle GIF image, and you’ll still feel like you’re viewing 3D. Physiologically that doesn’t make sense: how can one eye by itself have depth perception? Somehow the wiggle effect manages to trick the brain into imagining depth perception.
This is so cool! What’s even cooler: vintage stereograms from the New York Public Library can be converted to Wiggle 3D GIFs using an online tool developed by the library. Stereograms were really popular in the early days of photography. There’s plenty of vintage ones out there that have been digitized by libraries or public collections. It really does bring one closer to the world of the past when we can see it in 3D and in color.