The two red lines look as if they curve in toward the middle. They are perfectly straight and parallel.
Try this
- Change Number of slanted lines: more lines usually make the bow stronger.
- Tick Hide the slanted lines to see them straight.
Why it happens
This is the mirror image of the Hering illusion. In the Hering figure the lines fan out from the center and the parallels seem to bulge outward; here they fan in from the sides and the parallels seem to pinch inward. In both cases the angles where the slanted lines cross the straight ones are misjudged, and the error flips with the direction of the slant. Gerald Westheimer's review groups both with other geometrical-optical illusions in which the orientation of nearby lines shifts where the brain places an edge.
Did you know?
Wilhelm Wundt, often called the founder of experimental psychology, described it in the 19th century as a counterpart to Ewald Hering's 1861 figure.
References
- Westheimer G. (2008). Illusions in the spatial sense of the eye: Geometrical-optical illusions and the neural representation of space. Vision Research, 48, 2128-2142. doi:10.1016/j.visres.2008.05.016
- Background on Wikipedia
The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.
Frequently asked questions
What is the difference between the Wundt and Hering illusions?
They are mirror images: in Hering the lines look bowed outward, in Wundt bowed inward.
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