Size Constancy is the tendency to perceive an object as being the same size regardless of whether it is close or far away. The Müller-Lyer illusion is an optical illusion consisting of a stylized arrow. When viewers are asked to place a mark on the figure at the midpoint, they invariably place it more towards the "tail" end. It was devised by Franz Carl Müller-Lyer (1857–1916), a German sociologist, in 1889.