STKUCTTJItE OF CRYSTALS — WYCKOFF. 



201 



that are thus obtained is shown in figures 2 to 9. 5 Their symmetry 

 is seen to be directly related to the symmetry of the diffracting cry- 

 stal ; for instance, the cubic crystal of magnesium oxide gives a four- 

 fold pattern (fig. 9) when the X rays pass along a tetragonal^ axis, a 

 trigonal crystal such as calcite furnishes a pattern (fig. 7) that shows 



Fig. 10. 



a threefold symmetry when the X rays are traveling parallel to the 

 principal axis. 



The "Reflection" of X rays. — In Laue's experiment the crystal 

 may be thought of as behaving toward X rays as if it were a three- 

 dimensional diffraction grating. The mathematical treatment of 

 such a grating 6 

 presents very con- 

 siderable difficul- 

 ties. Fortunately, 

 h o w e v e r, W. L. 

 Bragg has pointed 

 out that these dif- 

 fractions of X 

 rays by the atoms 

 of crystals can 

 just as satisfac- 

 torily, and vastly 

 more simply, be treated as reflectioTis from planes of atoms within 

 the crystal. 7 



This point of view suggests that X rays ought to be " reflected " 

 from the faces of crystals. If a parallel beam of X rays of a single 



8 Figs. 2 and 3 are obtained from photographs by P. Rinne, Ber. Verb.. K. Sachs. Ges. 

 Wiss., 67, 303, 1913. The others have been prepared by the writer. 

 6 M. Laue, W. Fiiedrich, u. P. Knipping, op. cit. 

 7 VV. L. Bragg, Proc. Camb. Phil. Soc., 27, 43, 1913. 



Fig. 11. 



