The illusion that the two ends of a straight line segment passing behind an obscuring rectangle are offset when,
in fact, they are aligned. The Poggendorff illusion was discovered in 1860 by physicist
and scholar J. C. Poggendorff, editor of Annalen der Physik und Chemie,
after receiving a letter from astronomer F. Zöllner. In his letter, Zöllner
described an illusion he noticed on a fabric design in which parallel lines intersected
by a pattern of short diagonal lines appear to diverge (Zöllner's
illusion). Pondering this illusion, Poggendorff noticed and described another
illusion resulting from the apparent misalignment of a diagonal line; an illusion
which today bears his name (IllusionWorks).
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[sic]." http://www.illusionworks.com/html/poggendorf.htmlLucas,
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