322 K W. G. Wyckoff— Crystal Structures of 



reflecting the X-rays are plotted in the gnomonic projec- 

 tion. Both the crystal and the eye are considered to be 

 at the center of the sphere of projection. The plane of 

 the projection is a tangent-plane normal to the direction 

 of the incident X-ray beam (fig. 4). Thus B is the 

 projection corresponding to A. In the gnomonic projec- 

 tion, 11 zones of planes lie along straight lines and if the 

 rays pass parallel (or nearly parallel) to an axis of the 

 crystal, the indices of any plane can be obtained directly 

 from the co-ordinates of its position on the projection. 



Photographic Plate 



Fig. 4. — The method of representing the reflection of a plane (in a Laue 

 photograph) in gnomonic projection. 



The following procedure has been found to reduce the 

 time required for reading a Laue photograph to at least 

 one-third or one-fourth of that needed if a stereographic 

 projection is prepared. The accuracy under ordinary 

 conditions of working is also appreciably greater. 



An exact reproduction of the positions of the spots in 

 the photograph is prepared in the center of the paper 

 upon which the projection is to be made. A spot and the 

 projection corresponding to it lie on a straight line which 

 passes through the central spot (C, G ', fig. 4). The dis- 

 tance from a spot to the center C is the product of tHe 



11 The gnomonic projection, its properties and uses to the erystallographer, 

 will be found in V. Goldschmidt, "Ueber Projektion u. Krystallberechnung > ' 

 (1887), or in H. E. Boeke, "Die Gnomonische Projektion" (1913). A 

 simple discussion is given by C. Palache in Am. Mineralogist, 5, 67, 1920. 



