Surround a circle with big circles and it suddenly looks smaller than it is

Two identical circles can look like completely different sizes depending only on what surrounds them — ringed by large circles, one looks smaller; ringed by small circles, the other looks bigger. The effect holds up even when the viewer knows the trick and could measure the circles themselves.

— Hermann Ebbinghaus (traditional attribution); described by Edward Titchener, 1901, Ebbinghaus illusion