576 ] 



XLVIII. Note on a Spectrum of tlie Rontgen Rays from a 

 Focus Tube, and the relatively Selective Absorption of Rontgen 

 Rays in certain Metals. By John Mead Adams *. 



[Plate XIV.] 



~Z N the course of a research f upon the transmission of 

 JL Rontgen rays through metallic sheets, it became neces- 

 sary to test by direct experiment Rbntgen'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 

 Rontgen-ray tube (see fig. 1, PL XIV.) was prepared, like an 

 ordinary ifocus-tube in all essential respects except for the 

 target. The target consisted of a strip of platinum 6*3 cms. long- 

 by 1*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 0*15 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 focussed, formed according to the principle 

 of the pin-hole camera. A magnetic field was then applied 

 to the tube in the neighbourhood of the electrode C, in direc- 

 tion perpendicular to the plane of the paper, and of such 

 magnitude that the cathode discharge was spread into a 

 spectrum % along the concave surface 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 



* Communicated by Professor J. Trowbridge, 

 t Adams, Proc. Amer. Acad. xlii. p. 671 (1907). 

 t Birkeland, Comptes Bendus, cxxiii. p. 492 (1896). 



