The theory that the mind retains an image for a split second. This accounts for the fact that when a motion picture flashes a series of progressive images, instead of the mind seeing the flashing of a series of images, it sees the illusion of motion.
Through experience in the early days of film innovation, it was determined that any rate less than 16 frames per second caused the mind to see flashing images. Today's theatrical film--when the "film" is in fact film, that is "celluloid"--runs at 24 frames a second. Digital Video, or DV, records at an equivalent to 29.97 "frames" per second; television in the USA displays a complete new image at just under 30 times a second and at a comparable rate in other countries.