A visual illusion in which two lines of equal length appear unequal when drawn horizontally at different heights between a pair of converging lines (see illustration). It is also called the railway illusion, because it is often illustrated with a picture of converging railway lines, and it is closely related to Ehrenstein's square illusion. See also corridor illusion, Orbison illusion, perceptual constancy, size constancy. [Named after the Italian psychologist Mario Ponzo (1882–1960) who first published it in 1913]
Ponzo illusion. The horizontal lines are equal in length.