236 Messrs. R. W. James and N. Tunstall on the 



second lattice are all displaced from these corners along the 

 diagonals in the same direction by a distance equal to *074 

 o£ the length of a diagonal of one of the small cells, all the 

 observed facts can be explained. This structure is shown in 

 fig. 2, in which, for the sake of clearness, one only of the 

 small cells is shown. 



The Arrangement and Spacing of the Planes. 



(a) (111) planes. (BCD, E G H, fig. 2.) 



Parallel to the (111) face, the planes containing the atoms 

 occur in pairs. The planes belonging to one of the lattices- 

 divide the distance between those belonging to the other in 

 the ratio 0"389 : 0*611. The planes belonging to one lattice 

 form a first-order spectrum at a glancing-angle of 4° 26', but 

 those belonging to the second lattice add a contribution to 

 this spectrum, differing in phase by 27r x 0'389 or 140° from 

 that from the first set, thus reducing the intensity of the 

 first-order spectrum. On this assumption, taking the inten- 

 sities of a normal set of spectra to be in the ratio 



100 : 34 : 14 : 7 : 4, 



the intensities of the first four orders should be 



11-7 : 20-6 : 10*5 : 0'2 : 3"8. 



If the intensity of the second-order spectrum is put equal 

 to 100, the calculated ratios are 



57 : 100 : 51 : 1*4 : 18. 



The observed intensities have the ratios 



60 : 100 : 48 : : 15, 



which is quite as close an agreement as can be expected. 



(6) (100) planes. 



The planes occur in pairs containing equal numbers of 

 atoms. The spacing of the pairs is 0074 : 0'926. Thus the 

 contributions to the first-order spectrum from the two sets of 

 planes differ in phase only by about 36°. This will corre- 

 spond to a nearly normal series of spectra, the intensities 

 falling off rather more rapidly, which is, in fact, what ^as 

 observed. The intensities of the spectra from this plane- 

 show clearly that the structure cannot be similar to that of 

 the diamond. 



