42 



BELL SYSTEM TECHNICAL JOURNAL 



illumination of a given section are suf&cient to determine the nature of the 

 four possible twins in the section. The four corresponding photographic 

 views of the section have been arranged in a special manner to simplify their 

 explanation. This arrangement, as shown in Fig. 5.22A, B, C and D, is 

 equivalent to what would be observed if one looked down on a single, sta- 

 tionary section, and illuminated the section from the four different direc- 



FROM PLANE 

 or I AND R 



Fig. 5.22— Reflection patterns of the twinned, Z-cut sections shown in Figs. 5.23. 5.24, 

 5.25 and 5.26 were obtained by the means shown in E. A, B, C, and D are a key to the 

 four equivalent directions of illumination of a single stationary section. 



tions shown in the figure. For each direction of illumination there is a cor- 

 responding view, the outline of the section (and any cracks, chips or other 

 flaws) being identically positioned in each view. However, when the four 

 types of twins are present in a given section, each view will show a different 

 region, or regions, of brightness. For each view, the interpretation of 

 handedness and electrical sense of the bright portion of the view is according 



