I have this on my
personal original Stickings90 website Select Oddities from the menu on the left hand side and scroll down to opticla illusions. This one is D
This illusion works because as you stare at it, parts of your eyes (rods and cones) get tired. When you look at another surface, like the celing, the fact they have switched off causes the illusion.
Or something like that . I did physics not biology if I have slipped up in my explanation here.