

with a Liquid Mercury Anticathode. 423 



focus spot. The second is that one gets the same intensity 

 whether the focussing of the cathode rays on the anticathode 

 is sharp or not. These rather important points seem some- 

 times to be overlooked by designers of X-ray bulbs for 

 spectroscopic work. 



The rocksalt crystals which have been used in the present 

 work have been tested by taking photographs of the copper 

 K« doublet. The two lines a and a! appeared very clearly 

 on the plates, and the following values for their wave-length 

 difference dX have been found in different series of measure- 

 ment : — 



dX in 10 - 11 cm. 



3-81 

 381 



3-68 



3-82 



The average value for these four groups of observations is 

 d\ = 3*78 X 10 -11 cm. wild is in fair agreement with the value 

 3'79 x 10" 11 given by Siegbahn*, and 3*857 by Stensonf. 



Another test was carried out by measuring the wave- 

 length of the copper K/3 L line. Siegbahn's t value A 0l = 

 1537*36 x 10 ~ n cm. for the copper Ka line was taken as 

 standard. The following values resulted from a series of 

 photographs : — 



^Cu/3 X 



1389-5x10-11 cm. 



1389-1 



1389-2 



1389-9 



The average of these values 1389*4 + 0*2 is in close 

 agreement with Siegbahn's I.e. value 1389*53 for the copper 

 Kp x line. Two other values have been obtained with the 

 same crystal which had been removed and replaced again. 

 This gave \ ft = 1389*0 and 1388*5 X 10" 11 cm. The average 

 does not agree very well with the first. A similar discre- 

 pancy exists between Siegbahn's value and the value 1388*87 

 obtained recently by Hjalinar§ in Siegbahn's laboratory. 



4. Ihe Electric Equipment. 

 The bulb was run with a Butt coil. The spark-gap between 

 points varied between 7 and 12 cm. The current was 1 to 2 

 milliamps. A mercury gas break was used in the experi- 

 ments. The inverse current which became rather considerable 



* Siegbahn, Phil. Mug. xxxvii. p. 601 (1919). 

 t Stenson, ZeitscAr.f. Physik, iii. p. 60 (1920). 

 % Siegbahu, loc. cit. 

 § Hjalmar, Phil. Mag. xli. p. 675 (1921V 



