Delboeuf illusion

Though the two circled dark discs are the same size, the left disc seems smaller than the right one.

The Delboeuf illusion is an optical illusion of relative size perception: In the best-known version of the illusion, two discs of identical size have been placed near to each other and one is surrounded by a ring; the surrounded disc then appears larger than the non-surrounded disc if the ring is close, while appearing smaller than the non-surrounded disc if the ring is distant. A 2005 study suggests it is caused by the same visual processes that cause the Ebbinghaus illusion.[1]

  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. S2CID 26626773.