CONTEMPORARY ADVANCES IN PHYSICS 



413 



They involve seven variables: the wave-length, the direction-cosines 

 of the primary or oncoming beam, the direction-cosines of the scattered 

 or outgoing beam. 



5 

 GLANCING 



60 

 ANGLE 



Fig. 14 — Diffraction-peaks obtained with X-rays and a revolving crystal. The 

 X-rays contained very intense waves of several different wave-lengths, and the col- 

 lector was shifted continually so that it would capture a certain strong diffraction- 

 beam produced by each of these in turn. (D. L. Webster.) 



Now that we have this family of equations, the features of the 

 four great methods of crystal analysis can be restated in few words. 

 In the methods of Laue and of Davisson (II and III) the crystal and 

 the primary beam are fixed in space, which amounts to prescribing 

 values for a\, olo, az\ then only four equations are left {6d has fallen 



