Smithsonian Report, 1920. — Wyckoff. 



Plate 5. 



Fir,. 12.— An X-ray spectrometer. G is a divided circle upon which the positions of the crystal and ioniza- 

 tion chamber may be read: Fis a microscope and scale for following the gold leaf of the electroscope (E); 

 i/is an earthed shield for the wire connecting the chamber and the electroscope. 



Fig. 13. — The front slit (s) is variable. The second one ($') is fixed at a considerable width so that the 

 beam of X rays striking the crystal mounted on the rotating table (C) is somewhat divergent. The 

 photographic film is curved to the arc of a circle along F. 



