Pulfrich effect

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A visual illusion that is seen when a pendulum such as a length of string with a weight attached to one end is swung from side to side in a plane perpendicular to the line of vision. If it is viewed from a distance with a lens from a pair of sunglasses or a similar dark filter over one eye but with both eyes open, and with attention focused on the centre of the swing, the pendulum appears to move in an ellipse parallel to the floor, clockwise as seen from above if the dark lens is over the left eye and anticlockwise if it is covers the right eye. Any object moving across the line of vision is similarly affected, so that if the dark filter covers the left eye, objects moving from left to right on a television screen appear to recede and objects moving from right to left appear to be displaced forward. Since 1966 this effect has been exploited periodically by companies marketing expensive spectacles that are claimed to make ordinary television images three-dimensional but that consist merely of one dark and one plain lens. See also kinephantom. [Named after the German psychologist Carl Pulfrich (1858–1927) who first described it in 1922, although he was born blind in one eye and never saw the effect himself]

Subjects: Psychology.

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