Optical Illusion

This is the coolest optical illusion I have ever seen. It's called the Lilac Chaser. For the full effect you have to use the larger image, but here is what happens:

When one stares at the cross in the center of the image for 10–20 seconds, two effects will appear in order: One, the moving empty space between dots will appear as a green dot. Two, the moving green dot will appear to wipe out the purple dots, until only the green dot is visible. A separate effect appears if the eyes move away from the center, showing a ring of green dots.