The Zoellner Illusion is an optical illusion that involves two circles of the same size and color, but one appears to be larger than the other. This illusion was discovered by German physicist Adolph Zoellner in the late 19th century and is still studied today. The illusion is created
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.