Subsurface Laboratory Methods 



229 



If single-crystal-rotation patterns taken perpendicular to each of the 

 three crystallographic axes are available, one dimension of the unit cell 

 can be calculated from each of the three patterns. For cylindrical pat- 

 terns the angle «„ is calculated from the tangent function of the distance 

 measured on the pattern between the and nth layer line and the film ra- 

 dius. For flat patterns it is calculated from the distance measured between 

 the layer and the apex of the nth-layer-line hyperbola and the sample-to- 



Cylindrical film 



Flat film 



Figure 98. Schematic diagram of single-crystal layer-line positions in cylindrical- 



and flat-film cameras. 



film distance (See fig. 98). This value is then substituted in the equation: 



/= 



nX. 



sin Un 



to obtain the identity period, /, or the distance between planes from one 

 equivalent point to the next along the axis of rotation.^^ 



On the other hand, if the diffraction data were obtained by the powder 

 method, a technique considerably simpler than the single-crystal rotation 

 method, the procedures of measurement and calculation are different. 



'* Friedmaii, H., op. cit., chap. 5. 



