The Excitation of soft characteristic X-rays. 1015 



the two components o£ H a . With ordinary resolving-power 



this appears as a single line, but the part due to 2p 1 — 5d is 

 absent below the ionization potential. 



The combination line 2p 2 — 2F, X 7587, has never been 

 observed in magnesium. If this represents a physically 

 possible type of interorbital transition, it should accompany 

 the two-line spectrum for, on referring to fig. 3, it is seen 

 that but three combinations could take place once an electron 

 is in the 2 P ring, viz. P5 S-2 P, 2p 2 -2 P, and 1\5 S-2/> 2 . 

 Certainly the conditions for the production of 2p 2 — 2P are 

 most favourable from tbe probability standpoint. Using 

 dicyanin-stained plates having high sensitivity at this wave- 

 length, X 758 7, the two-line spectrum was carefully photo- 

 graphed, but no trace of X 7587 was present, the lines X4571 

 and X 2852 appearing alone with pronounced intensity. 



In conclusion, the writers desire to express their sincere 

 thanks to Dr. Dushman of the General Electric Co., and to 

 Dr. Rentschler, Dr. Shackelford, and Mr. Holden of the 

 Westinghouse Lamp Company for furnishing them with 

 essential parts to the apparatus used in the present experi- 

 ment. 



Bureau of Standards, 



Washington, D.C., 

 MavlS, ly21. 



CVT. The Excitation of soft characteristic X-rays. By 0. W. 

 Richardson, F.R.S., Wheatstone Professor of Physics in 

 the University of London, and C. B. Bazzoni, Ph.TJ., Pro- 

 fessor of Physics in the University of Pennsylvania *. 



ri^HIS work was commenced by one of us (0. W. R.) 

 -L during the war as a development of our work on the 

 limiting frequency in the spectra of helium, hydrogen, and 

 mercury in the extreme ultra-violet f. It was planned to 

 extend the use of the photoelectric spectroscope there de- 

 scribed to the analysis of soft X-rays generally. However, 

 the work had to be abandoned temporarily, and it has not 

 been possible to resume it until recently. 



The apparatus was of quartz glass, essentially the same as 

 the one used in the work on helium already referred to. 

 The cathode was of tungsten wire, and the anode and other 

 electrodes included in the discharge chamber were of such 



* Communicated by the Authors. 



t Phil. Mag. vol. xxxiv. p. 285 (1917). 



