An object or drawing which appears to have properties which are physically impossible, deceptive, or counterintuitive.
See also Benham's Wheel, Freemish Crate, Goblet Illusion, Hermann Grid Illusion, Hermann-Hering Illusion, Hyzer's Illusion, Impossible Figure, Irradiation Illusion, Kanizsa Triangle, Müller-Lyer Illusion, Necker Cube, Orbison's Illusion, Parallelogram Illusion, Penrose Stairway, Poggendorff Illusion, Ponzo's Illusion, Rabbit-Duck Illusion, Tribar, Tribox, Vertical-Horizontal Illusion, Young Girl-Old Woman Illusion, Zollner's Illusion
References
Ausbourne, B. ``A Sensory Adventure.'' http://www.lainet.com/illusions/.
Ausbourne, B. ``Optical Illusions: A Collection.''
http://www.lainet.com/~ausbourn/.
Ernst, B. Optical Illusions. New York: Taschen, 1996.
Fineman, M. The Nature of Visual Illusion. New York: Dover, 1996.
Gardner, M. ``Optical Illusions.'' Ch. 1 in
Mathematical Circus: More Puzzles, Games, Paradoxes and Other Mathematical Entertainments from Scientific American.
New York: Knopf, 1979.
Gregory, R. L. Eye and Brain, 5th ed. Princeton, NJ: Princeton University Press, 1997.
``Illusions: Central Station.''
http://www.heureka.fi/i/Illusions_ctrl_station.html.en.
Luckiesh, M. Visual Illusions: Their Causes, Characteristics, and Applications. New York: Dover, 1965.
Pappas, T. ``History of Optical Illusions.'' The Joy of Mathematics. San Carlos, CA: Wide World Publ./Tetra,
pp. 172-173, 1989.
Tolansky, S. Optical Illusions. New York: Pergamon Press, 1964.