Zöllner illusion

The Zöllner illusion is a line distortion illusion first published by the astronomer Johann Zöllner in 1860. The diagonal lines, although parallel, appear not to be. The illusion was one of a series specifically designed to cause errors in optical equipment of that time. They did cause errors, and also great concern among scientists, over the validity of all human observations.

Zollner illusion