The theory that electrochemical processing delays in the human eye or human brain create the illusion of continuity of motion if a series of images is presented in rapid succession.The theory is based on the discoveries of Paul Nipkow, a German engineer who developed the first true television mechanism in 1884. Nipkow used a scanning disk, lenses, mirrors, a selenium cell, and electrical conductors to transmit images in rapid succession to a lamp that changed in brightness according to the strength of the currents received. Using this mechanical scanning technique, Nipkow demonstrated that portions of a full image viewed in rapid succession created the illusion of viewing the full image. It later was discovered that viewing 15 or more images per second created the illusion of full motion.The theory of persistence of vision is controversial. See also phi phenomenon.