Waterfall Effect (Rotating Spirals)

When this disc is rotated, the spiral patterns seem to turn both in and out. An amazing effect occurs when, after watching these illusory movements for some time, we turn our gaze to a stationary object. This now also appears to be moving - in exactly the opposite direction! What happens if we look at the spiral with one eye only, cover it and look at the stationary object with the other, previously closed eye? The result is surprising and confirms something important: seeing, perception, happens primarily in the brain and not so much on the retina of the eye!