The impressions made by the luminous rays remain for a certain time, and are then gradually effaced; it is plain then, that if the action is reproduced at shorter intervals than the duration of the impressions, the brain perceives, not a series of sensations, but a continuous one. Thus in the rapid rotary movement of a burning coal, the eye perceives only a luminous circle, and when a wheel revolves rapidly, the spokes seem to approach each other and form a continuous surface. The impression of colour persists as well as of form; and if we cause a circle divided into party-coloured sections to revolve rapidly, they produce the sensation formed by a blending of them together; red and blue, for example, look like violet, and a great variety of different shades produce an impression as of gray. According to M. Plateau, the duration of impressions on the retina is about half a second.

This persistence of impressions has given rise to the construction of an apparatus, which is at the same time an object of amusement and a curious philosophical instrument Such is, for example, the phenakistiscope. It was upon the same principle that the beautiful experiments were founded, by the aid of which Wheatstone measured the duration of lightning flashes.