Hering illusion

The Hering illusion is an optical illusion due to the physiologist Ewald Hering (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.

The Orbison illusion is one of its variants,

while the Wundt illusion produces a similar, but inverted effect.

The Orbison illusion is an optical illusion that was first described by the psychologist William Orbison in 1939. The bounding rectangle and inner square both appear distorted in the presence of the radiating lines. The background gives us the impression there is some sort of perspective. As a result, our brain sees the shape distorted.

The Wundt illusion is an optical illusion that was first described by the German psychologist Wilhelm Wundt in the 19th century. The two red horizontal lines are both straight, but they look as if they are bowed inwards. The distortion is induced by the crooked lines on the background, as in Orbison’s illusion. (In some illustrations the Wundt illusion uses two vertical lines, but the inverted effect compared to the Hering illusion is more obvious with horizontal lines)

More of this in The Nature of Visual Illusion by Mark Fineman.