Hering illusion

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The Hering illusion is an optical illusion discovered by the German physiologist Ewald Hering in 1861. The two vertical 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. The Orbison illusion is one of its variants, while the Wundt illusion produces a similar, but inverted effect.