In 1895, an English man named Charles Benham sold a painted image of this pattern:
When the image is spun, an illusion of colors (Fechner colors) are seen:
Not everyone sees the same colors, and the phenomenon is not understood. The phenomenon may have to do with how the color receptors in our eyes respond at different rates to the RGB colors.
This black-and-white disk is being determined as to whether or not it is a good diagnostic tool for eye diseases.