91. Before Roentgen published in his second paper of Mar. 9, ’96, an account of his focus tube, the Kings College published a description of an exactly similar one, represented in the cut. See _Elec. Rev._, Lon., Mar. 13. ’96, p. 340. The cathode is concave and the anode is formed of platinum and is plane and at such an angle that the X-rays generated, § 63_b_., on diffusion of internal cathode rays, will be thrown out through the thin walls of the bulb. § 55 and 57. As the rays emanate from a point, the shadows are much clearer, especially in conjunction with powerful rays permitting several feet between the object and the tube. Mr. Shallenberger was among the first, and was the first as far as the author knows (_Elect. World_, Mar. 7, ’96, see cut reproduced) to originate the use of an X-ray focus tube. [Illustration: TYPICAL FOCUS TUBE. ] 91_a_. APPARATUS EMPLOYED.—Prof. Roentgen paid tribute to Tesla, by alluding to the advantages resulting from the use of the Tesla condenser and transformer. In the first place, he noticed that the discharge apparatus became less hot, and that there was less probability of its being pierced. Again the vacuum lasted longer, at least in the case of his particular apparatus. Above all, stronger X-rays were produced. Again careful adjustment of the vacuum was not as necessary as with the Ruhmkorff coil. 92. X-RAYS AND LONGITUDINAL VIBRATIONS.—Prof. Roentgen did not consider X-rays and ultra-violet rays to be of the same nature, although they produced many common effects. The X-rays, as he found, by the above related experiments, behaved quite differently from the ultra-violet rays, which are highly refrangible, practically all subject to reflection, capable of being polarized, and absorbed according to the density of the absorbents. For valid reasons, the X-rays cannot be infra-red rays. While he does not affirm any theory, yet he suggests the theory of longitudinal waves for explaining the properties of X-rays. (This was not suggested again in his second announcement.) He stated that the hypothesis needs a more solid foundation before acceptance. The reason why Roentgen termed the energy X-rays is simply because X in algebra represents an unknown quantity. [Illustration: SHALLENBERGER APPARATUS AND FOCUS TUBE. § 91. ]