The Fraser spiral is a distortion illusion in which overlapping black arc segments appear to form a spiral but are in reality a series of overlapping concentric circles. To demonstrate this, simply follow one of the curves around with your finger. The illusion is named after the British psychologist James Fraser (1863–1936) who first published it in 1908.¹

 

Reference

1. Fraser, James. "A new visual illusion of direction." British Journal of Psychology, 2: 307–337 (1908).