The Hering Illusion is an optical illusion in which two straight and parallel lines appear to be bowed outwards. It was first described by the German physiologist Ewald Hering in 1861 and is still used today to study visual perception. The illusion is created by the use of two sets
From Wikipedia
The Hering illusion is one of the geometrical-optical illusions and was discovered by the German physiologist Ewald Hering in 1861. When two straight and parallel lines are presented in front of a radial background, the lines appear as if they were bowed outwards. The Orbison illusion is one of its variants, while the Wundt illusion produces a similar, but inverted effect.