The Windmill Illusion is a visual illusion in which the blades of a windmill appear to reverse the direction of their rotation. The illusion was first described by Robert Smith, an English mathematician (1689–1768), in the book A Compleat System of Opticks (1738).
Smith noted that the illusion can also be observed in weather-cocks and flags. The windmill illusion should not be confused with the wagon wheel illusion, though it is closely related to the same kinetic depth effect
