Fraser's spiral is an optical illusion named after British psychologist James Fraser, who first studied the illusion in 1908 (Fraser 1908).
The illusion is also known as the false spiral, or by its original name, the twisted
cord illusion. While the image appears to be a spiral
formed by a rope containing twisted strands of two different colors, it actually
consists of concentric circles of twisted cords.
The visual distortion is produced by combining a regular line pattern (the circles) with misaligned parts (the differently colored strands). Zöllner's
illusion and the café wall illusion
are based on a similar principle, like many other visual effects, in which a sequence
of tilted elements causes the eye to perceive phantom twists and deviations.
