The possibilities of stereoscopic roentgenography were appreciated by the early roentgenologists. Two roentgenograms of a patient made with the x-ray tube displaced a few inches at right angles to the direction of the central beams were viewed in a stereoscopic illuminator. Each roentgenogram was presented to the respective eye. The illumination intensities were high, and many of the shadows were clearly defined so that a clear perception of depth might be obtained. Since the stereoscopic studies often yielded valuable information, it is not surprising that roentgenologists promptly attempted to apply this technic to fluoroscopy. Mackenzie Davidson in 1897 obtained stereoscopic pictures on a fluorescent screen, and Eugene Caldwell made some improvements in this technic which he reported in 1901 (1). These workers employed a shutter in front of the two sight holes through which the fluorescent screen was viewed, with the mechanism rotating the shutter also actuating the switching of the x-ray tubes respons...