The Poggendorff Illusion is an optical illusion that involves
the brain's perception of the interaction between diagonal lines and horizontal
and vertical edges. It is named after Johann Poggendorff, who first described it
in 1860.
In the picture to the left, a straight black line is obscured by a red
rectangle. The black line appears disjointed, although it is in fact straight;
the second picture illustrates this fact.