Delboeuf Illusion
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The red circles have the same radius. However, their sizes seem to vary, depending on the sizes of the blue circles. This illusion was first discovered by Franz Joseph Delbouf, a Belgian philosopher and expert on hypnosis.
Contributed by: Michael Schreiber (March 2011)
Open content licensed under CC BY-NC-SA
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"Delboeuf Illusion"
http://demonstrations.wolfram.com/DelboeufIllusion/
Wolfram Demonstrations Project
Published: March 7 2011