similar to those thrown off by an electrode in a highly exhausted enclosure. This seems to be at this moment still a matter of controversy.” Roentgen, in his first announcement, showed that the phosphorescent spot was the source of the X-rays. § § 79 and 80. All the different opinions and theories, therefore, indicated that phosphorescence by sun-light might possibly emit X-rays. Probably few had sufficient belief in the matter, one way or the other, to try the experiment in an extreme manner. The author was curious to prove the question, but he only obtained negative results. It cannot be conceived how the matter could have been more severely tested, for he concentrated the light of the sun nearly to a focus by a large lens, namely 5 in. in diameter, together with a reflecting funnel. The maximum phosphorescence was therefore obtained by placing suitable chemicals at the opening in the funnel. The sciascope showed absolutely no X-rays present. Photographic plates were not in the least acted upon, even after hours of exposure, the same having opaque covers of aluminum. See _Elect. Eng._, N.Y., Apr. 8, ’96, p. 356. If X-rays are emitted from the sun, they are all absorbed by the atmosphere of the earth, or are overcome by some other force. [Illustration: COOLING DISCHARGE TUBE. EDISON. § 135. ] ------------------------------------------------------------------------ CHAPTER XI 137. TESLA’S EXPERIMENTS. _Elec. Rev._, N.Y., March 11, ’96, page 131, March 18, page 147, April 1, page 171, and April 8, page 183. KIND OF ELECTRICAL APPARATUS FOR OPERATING DISCHARGE TUBES FOR POWERFUL X-RAYS. § 106, 109, 114, 131. The experiments performed by Nikola Tesla were particularly noteworthy for the magnitude and intensity of the rays generated by his apparatus, under his skilful manipulation of the adjustments and circuits particularly as to resonance. The remarkable results that he obtained are not surprising when we learn that he employed his well-known system for producing exceedingly enormous potential and unusually high frequency. § 51. The primary electrical generator as he indicated and as apparent from his system referred to in the above section, could be either a direct or alternating current generator, or other form. If the first is employed, of course an interrupter is necessary in order that there may be a current induced in the secondary. [Illustration: SCIAGRAPH OF RAT, TAKEN BY OLIVER B. SHALLENBERGER WITH FOCUS TUBE (CUT AT p. 81) AND TESLA SYSTEM. § 137, pp. 136 and 138. ]