The Poggendorff illusion is a distortion illusion in which the two ends of a straight line passing behind a rectangle appear offset when, in fact, they are aligned. It was discovered in 1860 by the physicist J. C. Poggendorff, editor of Annalen der Physik und Chemie, after receiving a letter from the astronomer Friedrich Zöllner. In this letter, Zöllner described an illusion (see Zöllner illusion) he had noticed on a fabric design in which parallel lines intersected by a pattern of short diagonal lines appear to diverge.
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