The Ehrenstein illusion is an optical illusion studied by the
German psychologist Walter Ehrenstein in which the sides of a square placed
inside a pattern of concentric circles take an apparent curved shape.
Ehrenstein illusion (2) Sometimes the name Ehrenstein is also associated with the figure on the right, one of the illusory contour figures.
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.