The Hering illusion is an optical illusion discovered by the
German physiologist Ewald Hering in 1861. The two horizontal lines are both
straight, but they look as if they were bowed outwards. The distortion is
produced by the lined pattern on the background, that simulates a perspective
design, and creates a false impression of depth. Orbison illusion is one of its
variants. Wundt illusion produces a similar, but inverted effect.