A visual illusion in which a pair of parallel straight lines appear to be bowed outwards when superimposed on a series of straight lines radiating like spokes of a wheel from a central point between the parallel lines (see illustration). It is closely related to the Wundt illusion. [First reported in 1861 by the German psychologist and physiologist Ewald Hering (1834–1918)]
Hering illusion. The vertical lines are straight and parallel.