NOTES 485 



waves are reflected from the alternately twinned planes of a 

 potassium chlorate crystal or by the successive layers of an 

 iridescent shell. The experiment was tried and its success 

 was first announced by W. L. Bragg in a letter which he wrote 

 to Nature in December 191 2. 



The reflexion theory vindicated, Prof. Bragg and his son at 

 once set themselves to design an instrument suitable for the 

 investigation of the spectra of the rays and the structure of 

 the reflecting crystals. Their X-ray spectrometer was the 

 result. The collimator of the ordinary spectrometer was 

 replaced by two narrow slits, and the telescope by a small 

 ionisation chamber connected to an electroscope and provided 

 with a narrow slit to admit the rays reflected on to it by 

 the crystal which takes the place of the prism. With this 

 instrument two lines of research were immediately obvious. 

 First, the investigation of the characteristic rays emitted 

 by different anticathodes and secondly the determination of 

 the exact arrangement of the atoms on the space lattices of a 

 crystal. To this more difficult problem they devoted their 

 chief attention, and the precise positions of the atoms in many 

 crystals had been settled, and many important side issues had 

 been opened up, before the war broke out. Since then some 

 progress has been made with the general question in Germany, 

 Sweden, Japan, and America, and in England also ; but not 

 much determination of crystal structure has been done. Never- 

 theless sufficient work has been carried out to show that the 

 X-ray spectrometer is as powerful a means of scientific inquiry 

 as its parent instrument. It has, for example, provided direct 

 proof of the thermal agitation of the crystal molecules and, on 

 the chemical side, has given a beautiful explanation of the 

 difference in the behaviour of diamond and graphite. But 

 the further investigation of these questions and the application of 

 the spectrometer to the problems^ of metallurgy must, at least 

 in the country of its origin, await in their turn the conclusion 

 of peace. 



At the outbreak of the war W. L. Bragg, who had already 

 served for four years as a trooper in King Edward's Horse, 

 applied for a commission and was attached to the Leicestershire 

 R.H.A. (Territorials). He is now employed on special duty 

 in France. His brother was in camp with the same cavalry 

 regiment at the beginning of August 1914, and was afterwards 



