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In this illusion, the vertical lines in the above figure are parallel, but appear to be tilted at an angle. In 1860, F. Zöllner sent his discovery in a letter to physicist and scholar J. C. Poggendorff, editor of Annalen der Physik und Chemie, who subsequently discovered the related Poggendorff illusion.
IllusionWorks.
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S. "Some Visual Illusions Occurring in Interrupted Systems." http://members.tripod.com/~modularity/interr.htm.Pappas,
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Joy of Mathematics. San Carlos, CA: Wide World Publ./Tetra, p. 172,
1989.Weisstein, Eric W. "Zöllner's Illusion." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ZoellnersIllusion.html