There are three types of eye movement: smooth pursuit, vergence, and saccades. Smooth pursuit describes the parallel, fluid movement of the eyes as they follow a moving object. Vergence describes the movement of the eyes in opposite directions — in order to focus nearer or farther. Saccades are rapid movements of both eyes in the same direction, jumping from target to target in the visual field or moving incrementally such as when one reads or watches a passing train. Each book shows a pair of related images joined by a video transition. Each frame of the transition is output to a page.