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