Astonishing optical illusion

There are a lot of optical illusions published on the net. Everybody knows the "which line is longer?" and the "are those lines parallel?" pictures. Yawn.

But the Spanish Castle is one I find really amazing. It's a picture of a castle in false colours with a black dot in the middle. Stare at the dot for about 30 seconds, then move your mouse over the picture. You'll see the castle in real colours -- but only as long as you don't move your eyes!

This reminds me of the "Magic Eye" pictures popular a few (gosh, I'm getting old) years ago. When I got the first Magic Eye book, I spent hours staring at the patterns trying to make the 3D shape appear...