In the section of his Farbenlehre dealing with ‘physiological colours’, Goethe devotes by far the most space to the so-called ‘afterimages’ which appear in the eye as the result of stimulation by external light, and persist for some little time. To create such an afterimage in a simple way, one need only gaze at a brightly lit window and then at a faintly lit wall of the room. The picture of the window appears there, but with the light-values reversed: the dark cross-bar appears as light, and the bright panes as dark.