706 Mr. H. G. J. Moseley on the 



nickel plates roughened with coarse emery-powder. The 

 only serious difficulty in the experiments is caused by the 

 heat produced by the cathode ray bombardment, and the 

 consequent liberation of gas and destruction of the surface of 

 the target. This makes it necessary to use the element in a 

 form which is not too volatile and prevents the employment 

 of a very powerful discharge. The total time of an exposure,, 

 including rests, varied from three minutes for a substance 

 such as ruthenium, which could safely be heated, to thirty 

 minutes for the rare earth oxides. The importance of using 

 an efficient high-tension valve may again be mentioned. 



The oxides of Sa, Eu, Gd, Er were given me by Sir Wil- 

 liam Crookes, O.M., to whom I wish to express my sincere 

 gratitude. For the loan of the Os and a button of Ru I am 

 indebted to Messrs. Johnson Matthey. The alloys were- 

 obtained from the Metallic Compositions Co., and the oxides 

 of La, Ce, Pr, Nd, and Er from Dr. Schuchardt, of Gorlitz. 



Almost every line was photographed in two different 

 orders, and the double angles of reflexion measured as before- 

 to within o, l and sometimes 0°"05. In some sets of experi- 

 ments a small error caused by the crystal surface not being 

 exactly on the spectrometer-axis gave rise to a systematic 

 discrepancy in the results obtained from reflexion in different 

 orders. It was found that this error, which never changed 

 the reflexion-angle by more than o, 05, could be measured 

 more accurately from the amount of the discrepancy than 

 from direct observation of the crystal. A more serious cor- 

 rection was necessary when using the long wave-length 

 apparatus. In this case the slit and photograph are not 

 equidistant from the crystal, and the position of the spectrum- 

 lines on the plate is no longer independent of the angle at 

 which the crystal is set. The necessary corrections were 

 calculated geometrically, and verified by photographing the 

 same line for both right-handed and left-handed reflexions 

 and with the crystal set at various angles. 



In the work on the very short wave-lengths, the reflexion 

 of the general heterogeneous radiation gave some trouble. 

 This is always an important part of the radiation from an 

 X-ray tube, but with a hard tube it is analysed by reflexion 

 mainly into constituents of very short wave-length, and so 

 usually does not interfere with the line-spectra. It is only 

 with an extremely soft tube, combined with precautions 

 against absorption by the air, that constituents reflected at 

 large angles become prominent. When examining such a 

 spectrum as that of Ag in the K series, the general reflexion 



