C.90 



Dr. A. E. H. Tutton 



[March 14, 



sectors, due to 8. P. Thompson, which between crossed Nicols shows a 

 black cross, the arms of which are deviated one sector to the right or 

 left when the thin quartz plate is interposed, in accordance with the 

 right- or left-handed nature of the plate. Also the phenomenon of the 

 bi-quartz, and of the natural biquartzes often produced by twinning of 

 right and left individuals, may be illustrated, terminating with some 

 beautiful cases of repeated twinning, including amethyst. The imi- 

 tation of the phenomena of the two varieties of quartz, by crossing 

 strips of mica in a right-handed or a left-handed pile, as shown by 

 Reusch, may also be referred to and demonstrated, as affording some 



Fig. 13. — Apparatus of Friedrich, KNl]■PI^'o and Laue for passing 

 X-Rays through Crystals and Photographing the Effect. 



definite proof that the mineral does possess an analogous helical 

 structure, complementarily opposite in its two varieties. 



Now these two crystalline minerals, zinc blende and quartz, have 

 been chosen advisedly as examples of crystal structure. For a 

 remarkable series of experiments have recently been carried out by 

 Lane, Friedrich and Knipping at Munich, where the lecturer had the 

 advantage of seeing some of the first photographic results last 

 summer. In these experiments X-rays were passed through crystals 

 of various substances, notably zinc blende, and, in more recent 

 experiments by Laue at Zurich, quartz. The issuing rays were 

 received on a photographic plate, on which they recorded a pattern 

 of spots having the symmetry (full holohedral) of the space-lattice 



