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.