Direction of ejection of photo-electrons by polarized x-rays.-Stereoscopic photographs were obtained, by Wilson's cloud expansion method, which show the ionized tracks of photo-electrons ejected by plane polarized x-rays. The polarized x-rays, scattered by a paraffin block at 90° to an unpolarized primary beam of hard x-rays, were directed horizontally through the expansion chamber of a Wilson cloud apparatus in which they produced the photo-electrons. Exploded tungsten wires furnished the instantaneous illumination of the droplets. The photographs, taken with the plate at 90° to the polarized beam, show two types of asymmetry in the direction of ejection of the photo-electrons. Lateral asymmetry. There is a strong concentration of photo-electrons ejected nearly in the direction of the electric vector of the plane polarized radiation performing the ejection. Longitudinal asymmetry. Stereoscopic examination of the photographs shows one-sixth of the photo-electrons ejected with a component opposite to the beam, one-third ejected approximately at right angles to the beam, and one-half ejected with a component along the beam. Theoretical interpretation according to the classical and quantum theories. The results are in accord with the classical theory. To explain them on the quantum theory we must assume that the quantum is a vector bundle of energy, for it explodes, so to speak, at right angles to its direction of motion.