the Absorption of X- Rays. 183 



of very hard rays (X = O360-0*U5) Hull and Rice 



recently have determined — for Pb and Cu, in which 



P 

 case the following formulae have been found to hold : — 



^ pb = 0*12 + 430 X 3 , 



r 



^ Cu = 0-12 + 150X 3 . 



If ^Pb/Cu * s ca lc u lated from these formulae for the mean 

 wave-lengths used by the author (Table I.), we obtain 



*Tb/Cu : ~ L IL IIL IV - Mean - 



according to Hull and Rice. 925 925 922 918 922 

 „ Table VIII. . 855 898 945 950 912 



The mean values found according to the said formulae 

 and the mean values from my experiments agree very 

 closely; but by comparing the original values, it is seen 

 that whereas in my experiments tfp b / C increases with a 



decreasing wave-length, these values according to the 

 Hull and Rice formulae are fairly constant. After Barkla's 

 opinion, however, the absorption coefficient cannot be 



exactly expressed by the formula — = C + k\ b , because 



neither C nor b is perfectly constant within a large wave- 

 length range. In my experiments, it is true, the wave- 

 length range has extended markedly on both sides of 

 the mean wave-lengths stated above ; but this can hardly 

 account for the difference between the values found 

 according to the Hull and Rice formulae and those in 

 my experiments. On. the other hand, the deviations can 

 possibly be explained by the circumstance that C has not, 

 as these physicists have suggested, the same value in all 

 elements, but is greater for Pb than for Cu. In section IV. 

 I shall return to this question. 



If by the Hull and Rice formula for aluminium, 



£ M = 0-12 + 14-9 X 3 , 



jc K ,, n is calculated, we obtain the values stated in the 

 table below. In Barkla and White's paper values are 



given of - for a very wide wave-length range in regard to 



A\ and Cu. By interpolation the necessary values may be 



02 



