Fig. 17. — Spot Photograph afforded by Zinc Blende. 

 Incident X-Rays perpendicular to an Octahedron Face, 



AND parallel TO A TeIGONAL AxIS OP SYMMETRY. 



• 



Fig. 20. — Spot Photograph afforded by Quartz. 

 Incident X-Rays parallel to the Trigonal Axis. 



