Every student of physics remembers the old-time lecture room experiments in which the Geissler tubes, with their beautiful play of colored lights, illustrated the action of the electrical discharge from the glass plate machine or the Ruhmkorff coil, on rarified gaseous media. Electrical experiments in high vacua by Sir William Crookes, and by Hittorf and Lenard, have greatly added to the present knowledge in this field, and paved the way to the discovery of Prof. Roentgen. It was known that a vacuum tube, variously called after the names of these scientists, as a Crookes, Hittorf, or Lenard tube, having platinum electrodes sealed in its ends, would, under the static discharge of electricity through it, give peculiar manifestations of light. One of the conducting terminals of such tubes was called, in electrical parlance, the “anode,” from the Greek ανα (up) ὁδος (way), meaning the way up or into the tube, and referring to the entering path of an electric current, or its positive pole; while the other was called the “cathode,” from κατα (down), ὁδος (way), meaning the way down or out, and referring to the outgoing path of an electric current, or its negative pole. When such glass tube, partially exhausted of air, received through its anode and cathode terminals a discharge of static electricity, a peculiar manifestation of light is seen between the anode and cathode terminals. At the anode it appears as a peach blossom glow, and at the cathode it appears as a bluish green light. If the exhaustion of the air in the tube is carried very high, approaching a perfect vacuum, or to about one millionth of the atmospheric pressure, the glow light at the anode disappears, and that at the cathode increases until it fills the entire tube with its characteristic light. This is called the “cathode ray,” or “cathodic ray,” an illustration of which is given in Fig. 215, where the cathode ray is seen in a Crookes tube emanating from the negative pole N or cathode _a_, and casting a shadow of the Maltese cross _b_ into the end of the tube, as seen at _d_. Many of the characteristics of the cathode ray had been observed prior to Prof. Roentgen’s discovery, which, briefly stated, grew out of the following observation: He noticed that when a vacuum tube illumined by the cathode ray was completely masked or covered up by an external shield of black paper, so that no illumination of the tube was visible to the eye, there still passed through it certain subtle rays of light, invisible to the eye, but which would instantly illuminate a sheet of paper coated on one side with barium platino-cyanide, even at a distance of two yards or more, and that these invisible light rays were capable of passing through many substances opaque to ordinary light. He also discovered that these rays could be made to take a shadow photograph on a sensitive plate without even exposing the plate in the usual way, the X-Rays