112. SOURCE ON INNER SURFACE OF THE DISCHARGE TUBE DETERMINED BY PIN-HOLE IMAGES. Reference may now be made to S. and M.’s Experiment, Fig. 2.—The discharge tube has, as before, a cathode on one side, and the phosphorescent spot during operation on the opposite side. Lead plates were provided in positions indicated by the heavy black straight lines, there being a pin hole in each one. Behind these lead plates, measured from the discharge tube, were the covered photographic plates, as indicated. By measurement, it was afterwards determined that practically all the X-rays started from the phosphorescent spot. The electrode was put in an oblique position, as indicated, so that the same would not obstruct any X-rays trying to pass through the pin hole in the uppermost plate. The experiment served specifically to show that the X-rays started from the inner surface of the glass, because images produced on the upper and lower plates were equally strong. Perrin also found that the X-rays are developed at the interior sides of the tubes. (_Comptes Rendus_, Mar. 23, ’96. From trans. by L. M. P.) The rays, in producing each image, had to pass through an equal thickness of glass. If the rays had come from the outer surface, for example, two thicknesses would have been traversed by the rays striking the upper plate, and no thickness by those impinging upon the lower plate. That no rays came from any other portion or element of the discharge tube was evident, because a picture of the phosphorescent spot was the only one produced, and this picture was inverted, as usual, with pin hole cameras. (A pin-hole camera is the same as any other, with the lens replaced by a very small hole, which acts as a lens.) In the way of further evidence, if not enough already, Meslans early determined that the phosphorescent spot on the glass is the source of X-rays (_Comptes Rendus_, Feb. 24, ’96. From Trans. by Mr. Louis M. Pignolet). JEAN PERRIN’S EXPERIMENTS. THE ORIGIN OF X-RAYS. _Comptes Rendus_, Mar 23, ’96. From Trans. by Louis M. Pignolet.—He also confirmed that X-rays radiate from the phosphorescent spot. 112_a_. DE HEEN’S EXPERIMENT. THE ANODE BELIEVED TO BE THE SOURCE OF X-RAYS. _Comptes Rendus_, Feb. 17, ’96. From trans. by Louis M. Pignolet.—A lead screen, pierced by several holes, was placed between the discharge tube and the photographic plate. The shadows of the holes on the plate indicated that the rays emanate from the positive pole of the tube. As both Thomson (E.) and Rowland, as well as De Heen, at first concluded likewise, is it not probable that the anode was struck by the cathode rays (see § § 113, 116)? For it was fully admitted that the anode, otherwise, does not emit X-rays. [Illustration]