At school, I remember a craze in 3D stereograms. Even a physics teacher would have a collection of posters in his classroom.
It’s hard to see them at first, but once you get the hang of it, well, it’s a bit like riding a bike. The more you do, the more you find it easier and you never forget how to do it. Looked at a stereogram for the first time in years this afternoon and it worked beautifully.
There are many ways to get started, but the one that works for me is to take off my glasses (also can work for non-shortsighted people), put my nose up to the picture, relax the eyes and vision and very slowly pull back. Then, eventually, the picture will appear.
Some people just stare at the picture, but the trick is not to try too hard to see a picture, it will eventually just ‘pop’ up.
Anyway, hope you enjoy it.