some Carbonates of the Calcite Group. 



319 



At the same time the distance of each spot from the 

 central image and the approximate intensity of each reflec- 

 tion were noted. The pattern obtained, when the X-rays 

 are passed not qnite normal to the (111) plane, is shown 

 diagrammatically in the center of fig. 2. The pattern 

 given in 1ig. 3 is obtained when the plane is (100). 



The Method of Projection. — Because the gnomonic pro- 

 jection 8 seems to offer, for this purpose, a number of ad- 

 vantages 9 over the familiar stereographic 10 projection, 

 the manner of its application will be briefly mentioned. 



A P 



Fig. 1. — A!=ioriginal crystallographic axes; A 2 =axes used in the com- 

 putations; A 3 =axes of the true unit of structure. 



This form of projection has two more or less obvious 

 advantages. The indices of the reflecting planes are much 

 more easily obtained and the method is capable of a gen- 

 eral routine application not only to all systems of crystals 

 but equally to various orientations of a crystal. This sec- 

 ond fact is especially important in the present instance 

 because patterns which are somewhat unsymmetrical are 

 of greatest value. A decided saving in time is thus 

 effected by being able to treat such photographs by rou- 

 tine methods. 



The positions of the normals to the planes which are 



8 F. Kinne, Ber. Verhandl. K. Sachs. Ges., 67, 303, 1915. 



9 Thanks are due to F. E. Wright, of this Laboratory, for pointing out 

 some of the advantages to him. 



10 W. L. Bragg, Proc. Koy. Soc. A, 89, 248, 1913. 



