Experimental studies of the perception of motion in the past, especially of visual motion, have failed to resolve the old puzzles or to yield any kind of general explanation. The root of the trouble may be a persistent misconception of what gives rise to the perception—an erroneous but plausible assumption about the stimulus. The optical motions corresponding to the types of viscous or elastic motion of a surface have been little studied. Motion perspective has been analyzed for the surface of the earth from horizon to horizon, as it applies in aviation, and gradients of velocity have been analyzed for the slant of a plane surface. The visual motion of an extremity is somewhere between a subjective movement and an objective motion. Most of the known experiments and demonstrations concerned with visual motion presuppose the retinal image displacement hypothesis and the general theory of sensations.