﻿X-ray Wave-lengths and their Absorption Coefficients. 409 



possible to obtain such clear-cut results as in the case of 

 rhodium, because the bulbs were of an old pattern and the 

 anticathodes were surrounded by raised rims. The rays 

 leaving at a grazing angle could not be used, and there was 

 no longer a "line-source." The images were therefore 

 blurred, more especially in the case of palladium. 



Nevertheless the figure makes it clear that the spectra of 

 all three metals are of exactly the same kind. Diamond was 

 used instead of calcite in order to effect the separation of 

 a x from « 2 by the use of the high resolving powers of the 

 third order spectrum. In order to separate ft from y it was 

 found best to use the first order spectrum ; the distance 

 between ft and <y is twice as great as that between a x and « 2 > 

 and the two can just be resolved in that order. In the third 

 7 is too weak. It is certainly remarkable that the spectra of 

 these three substances should resemble each other so closely. 

 It will be of much interest to know how far the resemblance 

 extends to the spectra of other substances. 



The angle of reflexion of a 2 in the first order of calcite is 

 6° 11'. The structure of calcite has been given by W. L. 

 Bragg (Proc. Roy. Soc. lxxxix. p. 486). Assuming the 

 density of the crystal to be 2*71 and the mass of the H atom 

 to be 1*64 x 10~ 24 , it can be calculated that the spacing of 

 the (1 1 1) planes is 2*845 A.U. 



This gives 



A, = 2x2-845xsin6°ir, 



= 0-613 A.U., 



agreeing well with the value 0*614 A.U. previously found 

 by the use of diamond *. 



The values of all the wave-lengths in the three spectra 

 may be readily calculated from the results illustrated by the 

 figure. A value for the a line of Pd, 0*576 A.U. was given 

 in a ! paper published in the Royal Society Proceedings, 

 November 1913 ; the crystal used was rocksalt. Moseley f, 

 using potassium ferrocvanide calibrated by reference to 

 rocksalt, found 0"584 for Pd and 0*560 for Ag. Recently 

 Maimer J gives for the a. lines of Pd and Ag 0*590 and 

 0*564, and for the ft lines of the same substances 0*522 

 and 0*499. He also used rocksalt. As far as the particular 

 crystal has influence, diamond and calcite are to be preferred 

 to rocksalt, for the reasons already given. The following- 

 table gives the wave-lengths in Angstrom units calculated 

 from the experiments described in this paper. The values 



* Phil. Mag. May 1914. t Phil- Mag. April 1914. 



\ Phil. Mag. December 1914. 



