Persistence of Vision
Persistence of vision is the eye's ability to retain an image for roughly 1/20 of a second after the object is gone. For example, with a thaumatrope, he eye continues to see the two images on either side of the thaumatrope shortly after each has disappeared. Consequently, as it spins, the series of quick flashes is interpreted as one continuous image. Roget further describes how objects will appear to be moving if there is a brief interruption between the display of each image.