While experimenting with a highly exhausted vacuum tube on the conductivity of electricity through gases, Dr. Roentgen noticed that a paper screen covered with potassium platinocyanide—a phosphorescent substance—which chanced to be lying nearby, became fluorescent under action of some radiation emitted from the tube, which at the time was enclosed in a box of black cardboard. Professor Roentgen then found, by experiment, that this heretofore unknown radiation had the power to pass through various substances which are impenetrable to ordinary light-rays. He found that if a thick piece of metal—a coin, for example,—were placed between the tube and a plate covered with the phosphorescent substances, a sharp shadow was cast upon the plate. On the other hand, thin plates of aluminum and pieces of wood cast only partial shadows. Thus was it demonstrated that the rays which produced the phosphorescence on the glass of the vacuum tube could penetrate bodies quite opaque to ordinary light-rays. Like ordinary light, these rays affected a photographic plate; but owing to their peculiar behavior in regard to reflection and refraction, Roentgen was led to put forward the hypothesis that the rays were due to longitudinal, rather than to transverse waves in the “ether.” They will ionize gases, but they cannot be reflected, polarized or deflected by a magnetic or electric field, as are ordinary light-rays. (It has been shown that the _scattered_ secondary rays show polarization.) Being in doubt as to the real nature of these penetrating rays, Roentgen called them “X-rays.” In 1896 Professor Roentgen was the recipient of the Rumford Medal of the Royal Society. This honor was shared by his compatriot Philipp Lenard. Lenard was the discoverer of the rays emanating from the outer surface of a plate composed of (any) material permeable by cathode rays. By impinging on solids, the cathode rays (negative electrons) generate X-rays. “Lenard rays,” which are similar in all their known properties to cathode rays projected from the cathode of a vacuum tube, do not emanate from the cathode. (Unlike the X-rays, cathode rays may be deflected from their natural course along “straight lines” by the application of a magnetic or electric field.) Professor Lenard, as also Hertz, discoverer of the now well-known “wireless waves,” had already demonstrated that a portion of the cathode rays could pass through a thin film of a metal such as aluminum. When Roentgen rays (X-rays) are allowed to fall upon any substance, the matter emits cathodic (or secondary Roentgen) rays. “The characteristic secondary radiation may be compared with the phosphorescence produced by ultra-violet light, and the cathodic secondary rays with the photoelectric effect” (Sir J. J. Thomson).[2]