The German psychologist Walter Ehrenstein is remembered for studies two illusions in particular both of which are known variously as the Ehrenstein illusion
The Ehrenstein square illusion is an optical illusion in which the sides of a square placed inside a pattern of concentric circles take an apparent curved shape and the square appears to be a trapeziod.
The ends of the dark segments produce the illusion of circles. The apparent figures have the same color as the background, but appear brighter. A similar effect is obtained in the Kanizsa triangle, the Ponzo illusion and the Orbison illusion