Ann._, vol. 48, p. 785. [Illustration: DIAGRAM OF LENARD’S APPARATUS. pp. 53 to 69. ] ------------------------------------------------------------------------ CHAPTER VI ------- 65. LENARD’S EXPERIMENTS. CATHODE RAYS OUTSIDE OF THE DISCHARGE TUBE. _Wied. Ann._, Jan., ’94, Vol. LVI., p. 225; _The Elect._, Lon., Mar. 23 and 30, ’94, Apr. 6, ’94; and _Elect. Rev._, Lon., Jan. 24, ’96, p. 99.—Of more importance in connection with X-rays is the consideration of Lenard’s experiments than any others. The reader must bear in mind that his exhaustive investigations resulted from his discovery (founded upon a hint from Hertz) that the cathode rays might be transmitted to the outside of the generating discharge tube. His interest, therefore, in the discovery was so great that his researches extended to the minutest details. Passing from these introductory remarks, the characteristics of the tube that he employed will be explained first. Reference may now be made to the accompanying Fig. A. He employed several different kinds of tubes, but finally settled upon one of which the essential elements are shown in the said figures. It was permanently connected to the pump, § 53, so that the pressure within could be varied. Opposite the cathode, which consisted of a thin disk of aluminum, the end of the tube was provided with a thick metal cap, having a perforation, which in turn was closed by a thin aluminum sheet secured by marine glue in an air-tight manner, and called a window. The anode was a heavy brass cylinder, shown in section, within the discharge tube and surrounding the leading-in wire of the cathode. The anode and the aluminum window were connected to each other, electrically, and to earth, as well as two a secondary terminal of an induction coil, whose electrodes were in shunt to those of the discharge tube, in order that the operator might adjust the sparking distance which rapidly increased with the exhaustion. The induction coil had a mercury interrupter. 65. PROPERTIES OF CATHODE RAYS IN OPEN AIR.—In all directions around the window upon the outside and in the open air, a faint bluish glow (§ 11 and 140) extended and vanished at a distance of 5 cm., as indicated by dotted lines in Fig. B at beginning of this chapter. The degree of luminosity may be judged by saying that it was not sufficient to admit of investigation by the ordinary pocket spectroscope. A new window was void of luminosity; but with use, bluish gray and green and yellow spots occurred thereon.