The advance of photography : its history and modern applications (1911)

348 THE ADVANCE OF PHOTOGRAPHY vacua, the rays do not come to a focus quite so soon as when using the higher vacua, and the object is to make the area of the anode struck by the kathode rays as small as possible. At the same time it is just as well that the rays should not be brought to a focus on the anode, as the amount of heat generated is so great that the anode is soon melted at the place struck. Gardiner1 has carried out some experiments in which the kathode raj's are deflected by a weak magnet, so that different parts of the anti-kathode come under their influence. In this way the time of usefulness of the tube is lengthened, and it is found that the photographs obtained do not suffer any lack of distinctness. When the anode is struck by the rays from the kathode, the so-called X or Rontgen-rays are produced, and the part of the bulb facing the anode then becomes of a bright green colour. * From recent experiments made by Dr Kaye,2 it appears that the 45° position of the anti-kathode enjoys no real advantage over any other position relative to the direction of the kathode-rays. He found that the fluorescence of the bulb, which is due to the secondary kathode rays from the anti-kathode, increases very markedly as the deviation from the normal incidence increases, but that the Rontgenrays do not show any corresponding order of variation. According to his researches, it would appear that the ideal Rontgen-ray tube should have a concave tantalum (this he preferred to aluminium) kathode, directly facing, and parallel to, a heavy bowl or trough-shaped tantalum anti-kathode. The kathode should be cooled in some manner, as this greatly reduces the tendency of the bulb to harden when it has been in use for a long time. The reason tantalum is preferred for use in such tubes 1 Rontgen Soc. Journ., 5, pp. 80-81, May 1909. 2 Hoy. Soc. Proc, Ser. A, 83, pp. 189-194, Jan. 7, 1910.