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The Zöllner illusion is a classic optical illusion named after its discoverer, German astrophysicist Johann Karl Friedrich Zöllner. In 1860, Zöllner sent his discovery in a letter to physicist and scholar Johann Christian Poggendorff, editor of Annalen der Physik und Chemie, who subsequently discovered the related Poggendorff illusion, in the original drawing of Zöllner.
One depiction of the illusion consists of a series of parallel, black diagonal lines which are crossed with short, repeating lines, the direction of the crossing lines alternating between horizontal and vertical. This creates the illusion that the black lines are not parallel. The shorter lines are on an angle to the longer lines, and this angle helps to create the impression that one end of the longer lines is nearer to the viewer than the other end. This is similar to the way the Wundt illusion appears. It may be that the Zöllner illusion is caused by this impression of depth.
If the illusion is printed in green on a red background and the red and green are equally bright, the illusion disappears.
This illusion is similar to the Hering illusion, the Poggendorff illusion and the Müller-Lyer illusion. All these illusions demonstrate how lines can seem to be distorted by their background.