Apparent Depth and the Müller—Lyer Illusion

The Müller–Lyer illusion was presented stereoscopically as either a flat or a three dimensional figure. When the flat figure was made to appear closer or further than a comparison line, the illusion persisted along with large changes in overall size due to classical size constancy. When the fins of the figure were tilted in depth the illusion was somewhat reduced for both forward and backward tilts. It is argued that the size constancy theory of illusions should predict an enhancement when stereopsies and 'the typical perspective view' reinforce each other, and abolition or reversal of the illusion when they are in opposition. These results therefore pose some problems for the theory.