Delboeuf illusion

The two black circles are exactly the same size; however, the one on the left may seem larger or smaller.

The Delboeuf illusion is an optical illusion of relative size perception. 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.^[1]

Eponym

It was named for the Belgian Philosopher, Mathematician, Experimental Psychologist, Hypnotist and Psychophysicist, Joseph Remi Leopold Delboeuf (1831 – 1896), who created it sometime in 1887 – 1888.

References

1. Roberts B, Harris MG, Yates TA. (2005). "The roles of inducer size and distance in the Ebbinghaus illusion (Titchener circles)". Perception 34 (7): 847–56. doi:10.1068/p5273. PMID 16124270.