chaPTer The Film system the basIcs of the fIlm system The essentials of the film system have changed very little since its invention in the 19th century. Film is a mechanical and photochemical motion picture sys- tem. It creates the illusion of motion through the rapid presentation of a series of sequential photographic images fixed to a flexible and transparent strip of cellulose. At a rate of 24 frames per second each image is projected onto a screen and held stationary long enough for the viewer to register the image before it is quickly replaced with the subsequent still image, which is again held for a fraction of a second, and so on and so on. The viewer perceives this rapid presentation of still images as motion through the perceptual phenom- enon known as short-range apparent motion. 1 Simply put, when shown a rapidly changing series of sequential still images in which there is only slight difference from image to image, humans processes this visual stimuli with the same perceptual mechanism used in the visual processing of real motion. 8