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This optical illusion is amazing from many points of view. Its most widely spread version consists of a white disk with black arcs on it (usually known as Benham's disks or Benham's tops). When the disk is spinned, you can see the arcs in different colours. Arcs of different radii are seen in different colors! The colors may also vary with the angular velocity of the disk. And, what is even more intriguing, different people see different colors in the same top under the same conditions!
This phenomenon was first observed by Gustav Theodor Fechner in 1834, and was popularised in the form of a disk or a top by Charles Benham in 1894 - he even published an article in Nature about it [1]. Yet, almost 200 years later, this intriguing optical illusion is not well understood. You can try it yourself. See what happens when you spin it faster, slower, and try it both clockwise and counter-clockwise. Compare what you see with what other people see. If you want to do it yourself try with any of the variations displayed here or here (this last one includes DIY instructions). For a web-based demonstration check this site or the wikipedia entry on Benham's top.
[1] C.E. Benham, Nature 51, 200 (1894)
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