Figure 9-6. Flat-film camera modified to study thin sections of minerals. 



Platy or fibrous crystals may become oriented in the cement during rolling 

 of the rod. The orientation changes the circular lines of the pattern to arcs, 

 especially the lines formed at a small angle to the beam. On film patterns 

 orientation effects usually do not cause any difficulty where qualitative identi- 

 fication is the objective, so long as the film is wide enough to include all 

 orientation arcs. On the other hand, these effects frequently are advantageous 

 in that they give some idea concerning the orientation of the crystallographic 

 planes producing these arcs. Moreover, because the orientation arcs are darker 



158 



