146_a_. REFLECTED AND TRANSMITTED X-RAYS COMPARED.—In order to carry on a series of investigations, Mr. Tesla constructed a complete special apparatus represented in Fig. 2, p. 149, and embodied in it also an idea which he attributed to Prof. William A. Anthony, which consisted in arranging for sciagraphs to be produced by the rays transmitted through the reflecting substance as well as by the reflected rays themselves. The figure serves to show at a glance the construction and, therefore, the explanation need be but brief. It consisted of a _T_ tube, having three openings, those at the base and side being closed by photographic plates in their opaque holders, which carried on the outside the objects _o_ and _o´_ to be sciagraphed. At an angle to both plates, and centrally located, was a reflecting plate, _r_, which could be replaced by plates of different materials. At the upper opening of the plate _B_ was a discharge tube, _b_, placed in a heavy Bohemian glass tube, _t_, to direct the scattered rays downward as much as possible from the electrode, _e_, to and through the thin end of the discharge tube. The objects to be sciagraphed, namely _o_ and _o´_, were exact duplicates of each other. No statement could be found as to the thickness of the tested plates, _r_, except that they were all of equal size. The distance from the bottom of the discharge tube to the reflecting plate, _r_, was 13 inches, and from the latter to each photographic plate about 7 inches, so that both pencils of rays had to travel 20 inches in each instance. One hour was taken as the time of exposure. After a series of experiments with a great many different kinds of metals, they arranged themselves as to their reflecting power, in an order corresponding to Volta’s electric contact series in air. § 153. The most electro-positive metal was the best reflector, and so on. For exhaustive investigations upon the discovery of Volta, see _“Experimental Researches” of Kohlrausch, Pogg. Ann., ’53_, and Gerland, _Pogg. Ann., ’68_. The metals Tesla tested were zinc, lead, tin, copper and silver, which were, in their order, less and less reflecting, and the order is the same in the electro-positive series, zinc being the most positive, and the others less and less so, in the order named. For a complete list of the metals arranged by the Volta series, see any standard electrical text-book. He could not notice much difference between the reflecting powers of tin and lead, but he attributed this to an error in the observation. He tried other metals, but they were either alloys or impure. Those named in the list above were the pure metals. However, he carried on experiments with sheets of many different substances, and arrived at the following table, which shows particularly the relative transmitting and reflecting powers of the various substances in the rough. Impression by Impression by Reflecting Transmitted Rays. Reflected Rays. Body