280 Prof. Barkla and Miss White on the 



of the K fluorescent X-radiation ; similarly there are the 

 J, L, M, . . . absorptions associated with the emission of 

 J j L, M, . . . electrons, and J, L, M, . . . fluorescent (cha- 

 racteristic) radiations *. 



The total mass absorption coefficient of X-rays associated 

 with fluorescence in any element may thus be written 



l = (ll + 15 + 15+...), 



P \P P P J 



where r K /p is the absorption coefficient definitely associated 

 with the emission of the K characteristic radiation, tl//o is 

 the absorption coefficient associated with the emission of 

 L radiation, and so on for each of the terms. 



Each of the terms Tj, tk, t l , &c. is zero when the wave- 

 length of the primaiy radiation is greater than that of the 

 constituent of shortest wave-length in the corresponding 

 characteristic radiation ; as the wave-length of the primary 

 radiation becomes shorter, the term tk or r L suddenly rises 

 and soon begins to diminish again with the wave-length, 

 continuing to do so without limit. As all the K absorption 

 curves are similar in form, T K /p is a function of XJX K , thus 

 for any element t k //o = &/k(\/\k), the constant k depending 

 on the particular element. Similarly, t l //o==Z/l(A/\ l ). Pre- 

 sumably a similar relation may be found for Tj/p. 



The functions /j,/k,/ l , ... though similar in features do 

 not appear to be identical. Corresponding functions, however, 

 are the same in different elements, and the relative values of 

 the coefficients^', #, I, ... appear to be the same for different 

 elements. It should, however, be pointed out that the simi- 

 larity of corresponding functions and equality of ratios of 

 the corresponding coefficients have not been observed for 

 substances differing widely in atomic weight. It may be 

 found that these observed laws are not perfectly general. 

 The particular form of these functions of XJX K cannot be 

 simply expressed ; indeed, the exact form is not known in 

 the regions where X is slightly less than Xk 7 owing to lack of 

 perfect homogeneity in the radiations upon which expe- 

 riments have been made. It has been shown, however, to 

 be the same for each element, for the form has been found 

 to be the same when instead of perfectly homogeneous 

 radiations similar beams of radiations of neighbouring wave- 

 lengths have been employed. 



* For more complete account see Bakerian Lecture 1916, Phil. Trans. 

 A. 217. pp. 315-360. 



