A visual illusion in which a line, area, or volume appears larger if it is occupied by a number of distinct elements than if it is empty (see illustration). The first written reference to it is to be found in Book 16, Chapter 7 of Problems, a work often spuriously attributed to the Greek philosopher Aristotle (384–322bc), probably written by one of his followers. The illusion is there presented the wrong way round, and the explanation that is offered to explain it is therefore beside the point: ‘Why is it that magnitudes always appear less when divided than when taken as a whole? Is it because, though things which are divided always possess number, in size they are smaller than that which is single and undivided?’ The first systematic investigation of the illusion was reported in 1860–1 (not 1854–5 as often stated) by the German physicist Johann Joseph Oppel (1815–94). Also called the filled and unfilled extent illusion, the Oppel illusion, or the Oppel-Kundt illusion. See also Kundt's rules. Compare filled-duration illusion.
Filled-space illusion. The distance between A and B is the same as that between B and C.