[ 39 ] 



IV. On the High-Frequency Spectra (L-Series) of the 

 Elements Tantalum- Uranium. By Manke ISiegbahn, 

 Dr.phiL, and ElNAR FRISIAN, Lic.phil. * 



[Plate I.] 



Introduction. 



IN a former communication f the writers have given an 

 account of some preliminary researches on the high- 

 frequency spectra of the elements gold-uranium. We then 

 followed the line called a, being the one best determined 

 from the measurements by Moseley $ of the other elements. 



This research contains a somewhat complete representation 

 of the L-series of the heaviest elements (from tantalum to 

 uranium). We have succeeded in finding at least 11 dif- 

 ferent line-groups. The measurements also indicate some 

 rudimentary groups, the existence of which further investi- 

 gations may decide. 



We have also examined the elements polonium and radium. 

 In the case of polonium, two of the characteristic lines a x and 

 fii and several others were obtained. It is possible that some 

 of these are due to impurities. With radium, only a very weak 

 a-line could be photographed owing to the small quantity we 

 had at our disposal (0*1 mgr.). These measurements defi- 

 nitely confirm the ordinals of these elements as being 84 and 

 88 respectively. 



Experimental Arrangements. 



For these experiments the X-ray spectrometer, seen in 

 Plate I., was used §. The lead-slits, the clockwork with the 

 rock-salt crystal, and the plate-holder are mounted on a 

 marble plate furnished with three set-screws. The width of 

 the slits may be changed arbitrarily. In this case the first 

 slit was 0*1 mm. wide, and the second about 2 mm. The 

 crystal, mounted on a small table, was adjusted by two 

 screws, which respectively displaced the crystal and turned 

 it about a horizontal axis. With the aid of the clockwork 

 the crystal is turned round at a constant speed of 15° in an 



H 



* Communicated by the Authors, 

 t Phil. Mag. xxxi. p. 403 (1916) 

 % Phil. Mag. xxvii. p. 703 (1914). 



§ Some of the elements were examined with a similar apparatus of 

 wood. 



