On the apparent orbit of the pulfrich pendulum

Abstract

Some misconceptions have appeared in recent literature concerning quantitative aspects of the Pulfrich stereophenomenon, in particular the relation of the transverse axis of the apparent elliptical orbit of the pendulum bob to the actual physical path of the bob. On the basis of a concise mathematical analysis of the expected orbit a possible source of the misconceptions is indicated. The general solution of the equations shows that the other conic sections (parabola and hyperbola) as well as ellipse and circle may be possible orbits. An experiment in which the corresponding conditions were produced with a cathode-ray-tube display of the simple harmonic motion failed to reveal parabolas or hyperbolas. A factor which appeared to prevent their recognition was the blurring of the monocular perceptions due to the finite duration of retinal responses.