From The Collaborative International Dictionary of English v.0.48 [gcide]:
4. (Opt.) The apparent enlargement of a bright object seen upon a dark ground, due to the fact that the portions of the retina around the image are stimulated by the intense light; as when a dark spot on a white ground appears smaller, or a white spot on a dark ground larger, than it really is, esp. when a little out of focus.
From WordNet (r) 3.0 (2006) [wn]:
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