80. SOURCE OF X-RAYS MAY BE AT POINTS WITHIN THE VACUUM SPACE.—In one case, he employed a Lenard tube, and found that the X-rays were generated from the window which was in the path of the cathode rays. § 67. Different bodies within the discharge tube were found to have different quantitative powers of radiating X-rays when struck by the cathode rays. He stated “If for example, we let the cathode rays fall on a plate, one half consisting of a 0.3 mm. sheet of platinum and the other half a 1 mm. sheet of aluminum, the pin-hole photograph of this double plate will show that the sheet of platinum emits a far greater number of X-rays than does the aluminum, this remark applying in every case to the side upon which the cathode rays impinge.” On the reverse side, however, of the platinum, no rays were emitted, but a large amount was radiated from the reverse side of the aluminum. § 67. He admitted that the explanation was simple; but, at the same time, he pointed out that this, together with other experiments, showed that platinum is the best for generating the most powerful X-rays. One form with which he experimented is illustrated in Fig. J, in principle, being described as a bulb in which a concave cathode was opposite a sheet of platinum, placed at an angle of 45° to the axis of the curved cathode, and at the focus thereof. [Illustration: J ] 81. REFLECTION OF X-RAYS.—He emphasized the knowledge that there is a certain kind and a certain amount of reflection, such as that produced upon light and, as pointed out by Lenard, upon cathode rays, by certain turbid media. The following quotation sets forth the exact experiment to show slight reflection at metal surfaces. “I exposed a plate, protected by a black paper sheet 1 to the X-rays (_e.g._ from bulb J) so that the glass side 2 lay next to the discharge tube. The sensitive film was partly covered with star-shaped pieces (4 slightly displaced in the Fig.) of platinum, lead, zinc and aluminum. On the developed negative the star-shaped impressions showed dark (comparatively) under platinum, lead and more markedly, under zinc; the aluminum gave no image. It seems, therefore, that the former three metals can reflect the X-rays; as, however, another explanation is possible, I repeated the experiment with only this difference, that a film of thin aluminum foil was interposed between the sensitive film and the metal stars. Such an aluminum plate is opaque to the ultra-violet rays, but transparent to X-rays. In the result the images appeared as before, this pointing still to the existence of reflection at metal surfaces.”