An illusion due to Franz Joseph Delboeuf, a Belgian philosopher and expert on hypnosis. If the same circle is placed inside two different concentric circles, its size will apparently vary.
Delboeuf Illusion
See also
Titchener IllusionThis entry contributed by Margherita Barile
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References
Nagoya Visual Illusion Experts Group. "Delboeuf Illusion." http://humanities.lit.nagoya-u.ac.jp/~illusion/gallery/NVEG/standard/Delboeuf01_e.html.Referenced on Wolfram|Alpha
Delboeuf IllusionCite this as:
Barile, Margherita. "Delboeuf Illusion." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/DelboeufIllusion.html