The actions and movements of figures on the cinema screen are optical illusions. The screen really registers thousands of individual still photos. The illusion of motion is created because the eyes cannot register each picture separately, the speed of release per second being too high for its own power to do so. Thus the sense organ deceives us into thinking that the actors are moving, when really each and every photo shows them still. If the reels of film were turned just slowly enough to depict each photo separately, the illusion of living movement would disappear altogether.
-- Notebooks Category 19: The Reign of Relativity > Chapter 1: The Cosmos of Change > # 47