Fraser spiral

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A twisted-cord illusion in which a series of concentric circles composed of cord made by twisting dark and light strands together, or of thin strips of diagonal stripes resembling such a twisted cord, appear as a spiral when presented against a chequered background composed of radiating curved bands (see illustration). The term is a misnomer, given that it is not actually a spiral. [Named after the Scottish physician and psychologist James Fraser (1863–1936) who published it together with other twisted-cord illusions in the British Journal of Psychology in 1908]

Fraser spiral. What appears to be a spiral is in fact a series of concentric circles, as can be confirmed by tracing all the way round any of them.

Subjects: Psychology.

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