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Bidwell's ghost


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An afterimage trailing behind a moving spot of light that is observed with dark-adapted eyes and fixated gaze in a dimly lit room. Behind the moving spot, with a delay of about 0.2 second, there appears to follow a ghost light leaving a trail that glows for several seconds. If the original stimulus is white, blue, or green, then the ghost is violet; if the original stimulus is orange or yellow, then the ghost is blue or blue-green; and if the original stimulus is red, then no clear ghost appears. The effect can be demonstrated by adapting one's eyes in darkness for 15 minutes and then moving a spot of light from a small torch or flashlight rapidly over a light-coloured surface while keeping the gaze fixated rather than tracking the movement of the spot; if the spot is moved through a tortuous path, the ghost will follow the same tortuous path one-fifth of a second later. The term is also used loosely as a synonym for a static afterimage or a Purkinje image, but in careful usage it is reserved for the moving afterimage described by Bidwell. [Named after the English physicist and barrister Shelford Bidwell (1849–1909) who described it in his book Curiosities of Light and Vision (1899)]

Subjects: Psychology.


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