The Ebbinghaus illusion, also known as Titchener circles, is an optical illusion on size perception as influenced by the sizes of surrounding objects or shapes. The most usual example is a comparison of two identical circles.
For instance, circle A is surrounded by much bigger circles while circle B is surrounded by much smaller circles. Even if circles A and B have the same size, circle A appears smaller due to its marked contrast with the bigger circles around it. This was introduced by Hermann Ebbinghaus, a German psychologist, and popularized by Edward Titchener, an English psychologist, in the English-speaking countries.