260 



PROF. W. H. BRAGG ON X-RAYS AND CRYSTAL STRUCTURE. 



the angles of reflection, that the very roughest measurements of relative intensity 

 give us all that we want. This is at any rate the case with some of the simpler 

 crystals.* 



Let us take one or two examples. The spectra of the (100), (HO) and (ill) planes 

 of rock salt are shown diagrammatically in fig. 9, the positions of the vertical lines 

 showing the relative magnitudes of the sines of the angles at which the various 

 reflections occur and the heights the relative intensities on an arbitrary scale. 



The sines of the angles of the first order reflections are as 1 : ^/2 : \/3/2. Hence 

 we know that the fundamental lattice is that which gives rise to the face-centred 

 cube. 



The actual values of these sines agree with the supposition that one atom 

 of Na (or Cl) is associated with each point of the face-centred cube. The calculation 

 has been already published, t and it will not be necessary to repeat it. 



100 



no 



111 



vz 



2/3 



Fig. 9. 



We can imagine a face-centred lattice of sodium atoms, and an exactly similar 

 lattice of chlorine atoms which is at first coincident with it, but is then moved out 

 from it in a direction and to an extent which we must gather from the intensity 

 observations. Each chlorine atom is then related to the sodium atom from which it 

 came, as regards distance and orientation, in identical fashion throughout the 

 crystal. 



Considering the diagram we see that there is a rapid and steady decline in 

 the intensities of the spectra of the (100) and (110) planes as we proceed to higher 

 orders. From experience of many cases we believe this to be normal, that is to say, 

 it always occurs when all the reflecting planes are similar and equally spaced. We 

 conclude that the chlorine atoms have so moved that they still lie in the (100) and 

 (110) planes of the sodium lattice. But the (ill) spectra are not of the same type. 

 A marked alternation of intensity, odd orders weak and even orders strong, is 



* W. L. BRAGG, ' Roy. Soc. Proc.,' 89, p. 478. 

 t 'Roy. Soc. Proc.,' vol. 89, p. 246 or p. 276. 



