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.
In the 19th century Europeans developed their own animated pictures in the form of spinning disks and zoetropes featuring sequential drawings visible through a slit, says Donald Crafton of the University of Notre Dame and author of Before Mickey. The first animated film, Phantasmagoria, came out in 1908, depicting the decapitation of a clown and other slapstick in a series of 700 drawings, which took two minutes to show. It was a visual tour de force, though choppy by today's exacting standards.
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Science didn't catch up to the animators until 1912, when Max Wertheimer, in Experimental Studies on the Seeing of Motion, revealed that it takes 25 frames per second to fool the human eye. It's a good thing people don't have the vision of fruit flies, which need more than 200 frames per second to succumb to the illusion of motion.