J. M. Adams — Spectrum of the Rontgen Hays. 91 



Art. IX. — A Spectrum of the Rontgen Rays from a Focus 

 Tube, and the Relatively Selective Absorption of Rontgen 

 Rays in Certain Metals. A preliminary note ; by John 

 Mead Adams. (With Plate I.) 



In the course of a research upon the transmission of Ront- 

 gen rays through metallic sheets, it became necessary to test 

 by direct experiment Rontgen's theory that an ordinary beam 

 of Rontgen rays is heterogeneous and that substances show 

 selective absorption toward the different kinds of rays ; and 

 furthermore to ascertain whether the selective absorption, if it 

 exists, follows the same law for all substances ; in other words, 

 whether the absorption of different substances is relatively 

 selective. 



To obtain a direct answer to these questions a spectrum of 

 Rontgen rays was sought by the -following method : A Ront- 

 gen-ray tube (see figure 1, Plate I) was prepared, like an ordi- 

 nary focus tube in all essential respects except for the target. 

 The target ' consisted of a strip of platinum 6'3 0ms long by 

 l*3 cms wide, bent into a circular arc of 5 cms radius and placed 

 in the tube in the position indicated by T in the figure. A 

 thick lead screen was set up in front of the tube (that is, facing 

 the concave side of the target) in a plane parallel to the axis of 

 the tube, and about 18 cms distant from it. At a point opposite 

 to the target this screen was pierced by a small hole about 

 015 cm in diameter with bevelled edge. A photographic plate 

 or a fluorescent screen could be placed parallel to the lead 

 screen and about as far in front of it as the axis of the tube 

 was behind it. When the tube was in operation under these 

 circumstances, an observer at the fluorescent screen perceived 

 a bright spot upon it — the image of the spot on the target 

 where the cathode discharge from the electrode C focused, 

 formed according to the j)rinciple of the pin-hole camera. A 

 magnetic field was then applied to the tube in the neighbor- 

 hood of the electrode C, in direction perpendicular to the 

 plane of the paper, and of such magnitude that the cathode 

 discharge was spread into a spectrum* along the concave sur- 

 face of the target. The bright spot upon the fluorescent 

 screen was now drawn out into a band, and it was to be 

 expected that the Rontgen rays at different points along this 

 band would show different properties, since they were pro- 

 duced by cathode particles which differed from one another, 

 presumably in velocity. 



That a more or less complete separation of the Rontgen rays 

 into a spectrum was actually effected was made plain by divid- 



*Birkeland, Comptes Rendus, cxxiii, p. 492, 1896. 



