The Delboeuf illusion is an optical illusion of relative size perception. It was named for the Belgian Philosopher, Mathematician, Experimental Psychologist, Hypnotist and Psychophysicist, Joseph Remi Leopold Delboeuf, who created it sometime in 1887 – 1888.
In the best-known version of the illusion, two circles of identical size have been placed near to each other and one is surrounded by an annulus; the surrounded circle then appears larger than the non-surrounded circle if the annulus is close, while appearing smaller than the non-surrounded circle if the annulus is distant. A 2005 study suggests it is caused by the same visual processes that cause the Ebbinghaus Illusion.