When I started collecting optical illusions for these pages, I naturally searched on the Internet. And what I found was quite depressing... as usual with the Internet. Not only did I find the same illusions over and over, but the majority of them did not "work"... the posters had not even bothered to find or create a version that was deceptive. There are endless versions of the purported illusion seen above on the Internet, and yet the image above is the only one I have found that actually shows the illusion! And what is the illusion? Look at the drawing again. You should see blue spots of four different sizes. OK, then what is the illusion? There are actually only three different sizes of spots. The central spot of each of the two arrangements is actually the same size! Why does it look so much larger in the arrangement on the left? This is properly called, I think, the Titchener-Ebbinghouse Illusion.

A good interactive flash version of the illusion can be found here.

By the way, the illusion also works quite well with squares instead of circles, as you can see below:

Jump to Illusion 25, 50, 75, 100, 110, 120, 130.

An extremely nice collection of very colorful illusions, many of which were new to me.

Individual Project LITE pages are linked elsewhere on these illusion pages, but here's the full directory.

Individual ZONEFLASH animated illusion pages are linked elsewhere on these illusion pages, but here's the full directory.

Although Akioshi Kitaoka's illusion pages are randomly distributed, like the ones you are looking at now, there actually is an index, here.

A long article on illusions, with dozens of animated illustrations.