Two variations on the same theme. For each image, fix your eyes on the central spot, relax your eyes slightly, and then move your head rapidly back and forth, away from and toward the computer screen. You will see the two broken rings surrounding the spot rotate in opposite directions. The illusion is quite strong. You will find many other variations of this same illusion, all over the Internet, but usually with no explanation of what you have to do to see the illusion, or what the illusion is! This is properly called the Pinna-Brelstaff Illusion, and as usual Prof. Michael Bach has a very nice animated version.

[Thanks to Prof. Akiyoshi Kitaoka, of the Department of Psychology, Ritsumeikan University, Kyoto, Japan.]