22Oct/070
Now you see ‘em, now you don’t
Concentrate your gaze on the '+' symbol in the center. The purple dots will fade away and disappear. Move your gaze and they reappear! Just one of the many tricks our eyes play on us...
If you want to go beyond being entertained, check out the science behind the illusion at the original site. Michael Bach has done a superb job of putting up a bagful of illusions on his site with in-depth analysis and tweaks.