Wundt Illusion

The Wundt illusion is a geometrical-optical illusion in which the perceived properties of simple lines or shapes differ from their true geometric properties; in its classic form, two straight, parallel vertical lines appear to bow inward toward each other, even though they remain perfectly straight, because of a background pattern of lines that radiate outward from points near the two vertical lines and intersect them. The German psychologist Wilhelm Wundt first described the effect in 1898, as part of a broader program of research into geometrical-optical illusions, and the illusion persists even for a viewer who already knows, and can verify, that the two lines are actually straight, distinguishing it from illusions that vanish once their true geometry is pointed out. The Wundt illusion belongs to a family of similar illusions discovered during the middle and later nineteenth century, including the Hering illusion and the Muller-Lyer illusion.