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Fraser's Spiral
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As impossible as it may seem, Fraser's Spiral (also known as the twisted cord illusion) is not a spiral but a series of concentric circles. Click on the reveal button to confirm this. The effect is created by adding an unaligned design (the barber's pole stripes) to a regular pattern (the concentric circles).

ReferenceReferences

[1] K. Fraser J, "A New Visual Illusion of Direction," British Journal of Psychology, vol. 2, pp. 307–320, 1908.
[2] H. Varenne, "Fraser, James: 'A new visual illusion of direction.'" [Online]. Available: http://varenne.tc.columbia.edu/bib/texts/fraser.html. [Accessed: Mar 11, 2010].
[3] M. Barile, "Fraser's Spiral," in MathWorld--A Wolfram Web Resource, E. W. Weisstein. [Online]. http://mathworld.wolfram.com/FrasersSpiral.html. [Accessed: Mar 11, 2010].