Max Wertheimer proved in 1912 that test subjects did not see anything in between the two different positions in which a figure was projected by a tachistocope at the speed that was ideal for the perception of one moving figure. He used the Greek letter φ (phi) to designate illusions of motion. At higher speeds, when test subjects believed to see both of the fast blinking figures more or less simultaneously, a moving objectless phenomenon was seen between and around the projected figures. Wertheimer supposed this "pure phi phenomenon" was a more direct sensory experience of motion. Some of these experiments involved beta movement. However, Wertheimer's work became famous (indeed it launched Gestalt psychology) due to his demonstrations of the phi phenomenon—a different illusion of apparent motion.