The magnitude of the Pulfrich stereophenomenon as a function of binocular differences of intensity at various levels of illumination.

The present experiment is concerned with a phenomenon associated with an hypothesized visual latent period. The effect was first described and analyzed by Pulfrich in 1922 and now bears his name. The phenomenon had been observed frequently, but as merely a peculiar disturbance, by those working with the stereocomparator and stereoautograph. When the stereoscopic plates of a star, for example, were adjusted in a stereocomparator to produce an image which appeared to coincide in space with the stereoscopic image of the "distance indicators" (Messmarke), it was frequently noted that if the plates were quickly moved laterally, the image of the star seemed to move either in front of or behind its original position. It was first believed that the disturbance resulted from a loose connection in the plate-bindings. Subsequent investigation showed, however, that it was not a change in separation between the stereoscopic plates which produced the stereo-effect, but that the spatial displacement was caused by a difference in brightness between the two plates.