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An Illusion in which the eye alternately sees two black faces, or a white goblet.
References
Fineman, M.  The Nature of Visual Illusion.  New York: Dover, pp. 111 and 115, 1996.
 
Rubin, E.  Synoplevede Figurer.  Copenhagen, Denmark: Gyldendalske, 1915.
 
What's Up with Kids Magazine.  ``Reversible Goblet.''
  http://wuwk.spurtek.com/COI_reversible_goblet.htm.