Apparent path of a Pulfrich target as a function of the slope of its plane of motion: a theoretical note.

A theoretical equation is derived to predict the apparent path of a Pulfrich target moving with constant velocity in a plane that intersects the pupillary plane of an observer. Generally, the predicted apparent path is a hyperbola. However, if the angle between the two planes is zero, or they intersect at a pupillary center, then the predicted apparent path is a line. Other theoretical equations are derived that describe the apparent size and orientation of a line target that is parallel to the pupillary plane and receding from the observer.