EACH TIME A PHOTON hits light receptors on the retina, it triggers a Rube-Goldbergian chemical reaction that takes tens of milliseconds to reset. We don't notice this interruption—our brains smooth it over into an apparently fluid stream of visual information—but the delay provided just the opening animators like Walt Disney needed.
Animators, of course, were not the first to notice this perceptual quirk, often called persistence of vision. Aristotle found that when he stared at the sun, the burned-in image faded away slowly. Roman poet Titus Lucretius Carus described a dream in which a sequence of images presented rapidly before him produced the illusion of motion. By then the Chinese had invented the chao hua chih kuan (“the pipe that makes fantasies appear”), a cylindrical contraption that, when spun in the wind, displayed a succession of images. It gave “an impression of movement of animals or men,” writes Joseph Needham in Science and Civilisation in China.