Delboeuf illusion

English

The two circled dark discs are the same size, but the left disc seems smaller.

Etymology

Discovered in 1865 by the Belgian scientist Joseph Remi Leopold Delboeuf (1831–1896).

Noun

Delboeuf illusion (plural Delboeuf illusions)

1. An optical illusion of relative size perception, usually illustrated by two discs of identical size, one of which is surrounded by a ring.