Lilac chase or Phi phenomenon

• Described by Max Wertheimer in his 1912 Experimental Studies on the Seeing of Motion
• In this Lilac chaser illusion, the omitted space appears to be filled with a green circle when viewed long enough

On the beginning you will see 11 pink (lilac) points on a gray background. There is also a gap which moves every tenth of a second (0.1s) in the clockwise direction.
After a while, the gap will take the green color. This is due to an interesting effect in which the colors of the lilac points are inverted in the optical illusion to a green color. See the image below, that green is right opposite to pink. If the points were black, one would see white where the gaps were.

And in the end, while still focusing on the cross, the points will disapper and you will see just the grey background.
An interactive version of the illusion may be found here: http://www.michaelbach.de/ot/col_lilacChaser/index.html 
This version allows viewers to adjust the color, saturation, and timing of the disks.

Yellow Lilac Chase Illusion:

[Click to view original illusion]

Credit:

Lilac Chaser by Michael Bach and Jeremy Hinton.