The Müller-Lyer Illusion
A depth-processing explanation 
of the Müller-Lyer illusion 
suggested by Richard Gregory: 
the test line in the photograph 
on the left is processed as the 
edge of a convex corner and the 
one in the photograph on the 
right as the edge of a concave 
corner.

The best known of all illusions 
is the one created roughly a 
century ago by F. C. Müller-Lyer.