Zöllner illusion

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 J. C. Poggendorff, editor of Annalen der Physik und Chemie, who subsequently discovered the related Poggendorff illusion.

In this figure the black lines seem to be unparallel, but in reality they are parallel. The shorter lines are on an angle to the longer lines. This angle helps to create the impression that one end of the longer lines is nearer to us than the other end. This is very similar to the way the Wundt illusion appears. It may be that the Zöllner illusion is caused by this impression of depth.

It is interesting to see what happens when the colours in this illusion are changed. 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.