Poggendorff Illusion
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Can your eyes deceive you? Adjust the position of the upper red line until you think it is aligned with the lower red line. Then check your selection by removing the rectangle.
Contributed by: Sándor Kabai (August 2008)
Open content licensed under CC BY-NC-SA
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"Poggendorff Illusion"
http://demonstrations.wolfram.com/PoggendorffIllusion/
Wolfram Demonstrations Project
Published: August 15 2008