m 



Fig. 15. — Spot Photogeaph afforded by 



Zinc Blende. 



Incident X-Rays perpendicular to a Cube-pace, and 



parallel to a tetragonal axis op symmetry. 



Fig. 16. — Spot Photograph afforded by Zinc Blende. 



Incident X-Rays also perpendicular to a Cube-face, and parallel 



TO A Tetragonal Axis of Symmetry, but Photographic 



Plate twice as far from Crystal. 



