between and around the projected figures. Wertheimer used the Greek letter φ (phi) to designate illusions of motion and thought of the high-frequency objectless illusion as a "pure phi phenomenon", which he supposed was a more direct sensory experience of motion. Wertheimer's work became famous due to his demonstrations of the phi phenomenon, while the optimal motion illusion was regarded as the phenomenon well-known from movies. In 1913, Friedrich Kenkel defined different types of the motion illusions found in the experiments of Wertheimer and subsequent experiments by Kurt Koffka (who had been one of Wertheimer's test subjects). Kenkel, a co-worker of Koffka, gave the optimal illusion of motion (with the appearance of one figure moving from one place to the next) the designation "β-Bewegung" (beta movement). Confusion about phi phenomenon and beta movement Wertheimer's pure phi phenomenon and beta movement are often confused in explanations of film and animation, but they are quite different perceptually and neither really explains the short-range apparent motion seen in film. In beta movement, two stimuli, and , appear in succession, but are perceived as the motion of a single object, , into position . In phi movement, the two stimuli and appear in succession, but are perceived as the motion of a vague shadowy something passing over and . There are many factors that determine whether one will experience beta movement or the phi phenomenon in a particular circumstance. They include the luminance of the stimuli in contrast to the background, the size of the stimuli, how far apart they are, how long each one is displayed, and precisely how much time passes between them (or the extent to which they overlap in time). References Optical illusions