Published in the 1990s, the Magic Eye series consisted of picture books filled with autostereograms.
On the face of it, they were psychedelic and colourful 2D patterns, a bit like Jackson Pollock’s paintings.
But – and this was the clever part – if you stared at them really hard for a couple of minutes, and let your eyes sort of de-focus, then stared a little longer and a little harder, you eventually got to the point where you succeeded . . . in giving yourself a blinding headache.
Some people claimed to be able to see a 3D image materialise, almost floating off the page. The key technique was that even though you were staring, you had to look through the page and let the focus of your eyes relax, possibly to the point of going cross-eyed.
After all that effort, if you did eventually glimpse something, the 3D effect was novel for about ten seconds. Then the realisation struck that you had just spent a chunk of your life concentrating harder than an ancient Greek philosopher in order to catch a brief glimpse of a 3D elephant.