A visual illusion in which a straight line extending between the most distant points on the circumferences of two circles appears shorter than a line of the same length extending between the nearest points on the circumferences of two similar circles (see illustration). It is closely related to the Baldwin illusion and possibly also the Müller-Lyer illusion. [Named after the Italian psychologist Mario Zanforlin (born 1934) who first described it in 1967]
Zanforlin illusion. The horizontal straight lines are equal in length.