The Zollner Illusion is an optical illusion that was first discovered by German psychologist Gustav Theodor Fechner in the 1860s. It is a classic example of an ambiguous figure, in which the same image can be interpreted in two different ways. The illusion consists of a series of alternating
From Wikipedia
The Zöllner illusion is an 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 Zöllner's original drawing.