Cylindrical film 



Flat film 



Figure 9-8. Schematic diagram of single-crystal layer-line positions in cylindrical- and 

 flat-film cameras. 



/ = 



n X 



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

 equivalent point to the next along the axis of rotation (Friedman, 1945). 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. The diameters (or 

 radii) of all lines in the pattern are measured and recorded in centimeters or 

 millimeters. If the X-ray pattern is recorded on flat film, the Bragg angle is 

 obtained from the tangent relationship, namely, 



tan 20 = 



line radius 



sample to film distance 



With the Bragg angle 6 known, the interplanar distances, d, can then be readily 

 calculated by means of Bragg's law, 



163 



