determine the degree of novelty, assuming that some one else must have conceived the idea of employing a self-induced current to operate Geissler tubes; but nothing nearer than Poggendorff’s experiment § 42 could be found, and therefore the following claim (in patent 548576, Oct. 22, ’95,) was granted among a hundred or so relating to developments and details and particularly covering the vacuum interrupter. “The method of producing luminous effects, consisting in converting a current of low potential into one of high potential, by rapidly and repeatedly interrupting the low potential current in its passage through a self-inductive resistance, and passing the former current through a Geissler tube, thereby producing light within the tube.” [Illustration: EDISON’S BENEFICENT X-RAY EXHIBIT, § 82, p. 71, and § 132, p. 126. Calcic tungstate screen at center, sciascope near right. ] ------------------------------------------------------------------------ CHAPTER V ------- 53. CROOKES’ EXPERIMENT. DARK SPACE AROUND THE CATHODE. _Lect. Brit. Asso., Shef., Eng._, Aug. 22, ’79.—According to Lenard (_The Electr., Lond._, Mar. 23, ’94) Hittorf discovered the cathode rays, and Varley, § 61_a_., and Crookes studied them. The pressure of the residual gas was 1 M. of an atmosphere. Prof. Crookes, F.R.S., maintained the evacuated space in communication with the air pump and with an absorbent material. Before his time most experimenters worked with a vacuum not much less than 30,000 M. The first experiment is illustrated in diagram, at Fig. 6 p. 17, but the vacuum was not the highest in this type. The tube was cylindrical and was provided with electrodes at the ends. Another electrode was located at the centre and was made the cathode, while the two terminal electrodes were made the same pole; namely, the anode. Upon connecting the tube in circuit with the secondary of a large induction coil, the luminosity did not extend either continuously or in striae throughout the length of the tube. Former investigators had likewise noticed the dark space. The space and glass on each side of the central cathode were dark. The dark space extended for about one inch on each side of the negative pole. It is not intended here, any more than in former cases, to present theories in explanation further than to briefly allude to any conclusion at which the experimenter himself arrived. Crookes’ explanation of the phenomena has not been universally accepted, nor has it been proved otherwise. The knowledge of the existence of rays, now known as Roentgen rays, will assist in formulating theories upon the Crookes’ phenomena and may either confirm some of his views or overthrow them. Crookes considered that the residual atmosphere was in such a state as to be as different in its properties from gas, as gas is from liquid and liquid from solid, and therefore he named the attenuated