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windmill illusion

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A visual illusion first described by the English mathematician Robert Smith (1689–1768) in his book A Compleat System of Opticks (1738), in which the blades of a windmill, seen from a distance silhouetted against the sky, appear to reverse their direction of rotation. The direction of rotation sometimes becomes indeterminate, and Smith pointed out that the illusion can also be observed in weather-cocks and flags. It should not be confused with the wagon wheel illusion but is closely related to the kinetic depth effect.

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