The Ebbinghaus illusion is an optical illusion of relative size perception. Two circles of identical size are placed close to each other. One circle is surrounded by a ring of smaller circles, the other by a ring of larger circles. In the first case, this makes the middle circle appear larger, in the second case smaller.

The illusion is named after its discoverer Hermann Ebbinghaus (1850-1909), a German psychologist. Ebbinghaus is best known for his work on the memory capacity of the human brain, but he also worked on visual perception.
The perceived difference in size arises from the change in size of the surrounding circles and the interpretation of the visual stimuli in the brain.