Delboeuf illusion

The Delboeuf - deception is one by the Swiss mathematician and psychologist Joseph Remi Leopold Delboeuf (1831-1896) called geometrical- optical illusion. In a circle again shown as the inner circle of a two circles ring. The other time is set the same circle as the outer circle. In the first case, the same circle is displayed smaller than in the second case.

In the Delboeuf Bluff two concentric circular rings apparently change their size: they differ by a fraction of their diameter, then they approach seemingly in their perceived size ( assimilation ). The maximum of the illusion is a frame ratio ( framing ratio) of about 1.4 to 1.5. The frame ratio here is the outer diameter of the black ring divided by the outer diameter of the red circle. If the black ring much larger then the inner circle has been reduced apparently again (contrast effect).