"In normal use the eyes are rarely still for long. Apart from small tremors, their most common movement is the flick from one position to another called a saccade'. Saccades usually take less than a twentieth of a second, but they happen several times each second in reading and may be just as frequent when a picture or an actual scene is being inspected. This means that there is a new retinal image every few hundred milliseconds. -Ulric Neisser"
