Joseph Plateau, who invented the phenakistoscope, the first device capable of producing the illusion of a moving image from a sequence of still images, provided a definition of the persistence of vision in 1829. In his research into visual physiology, he described the effects of light stimuli and colors on the retina, determining their duration to equal one tenth of a second. Yet this value is not constant: it increases as the eye adapts to darkness, hence the sensation of movement which is characteristic of a film projection. Plateau thought that the eye sees at a rate of ten images per second and that, when they are superimposed, the brain links them into a single, continuous, moving image.