ARTICLES 



RECENT WORK ON THE SPECTRA 



OF X-RAYS 



By PROF. W. H. BRAGG, C.B.E., D.Sc, F.R.S., 

 University College, London 



In the years immediately preceding the war the study of 

 X-ray spectra had just been begun. A new line of research 

 had been opened up, and it was seen that it must lead to 

 results of very great importance. The war necessarily inter- 

 fered with the progress of the work, arresting it almost com- 

 pletely in the countries that were most deeply involved. But 

 in some of the countries that were neutral, and in America 

 until the energies of the researchers were diverted into the 

 work of the war, considerable progress was made. 



It is proposed to give here a sketch of the present position 

 as it is now constituted after recent advances. 



When we approach once more the subjects from which our 

 attention has been diverted for the past few years, we are all 

 in the position of learners again and it may be convenient to 

 restate the first principles of the subject. 



Every natural element may be made to emit X-rays 

 characteristic of the element, the rays constituting a bright 

 line spectrum in which the lines have much shorter wave- 

 lengths than in visible light. They range from two or three 

 Angstrom units (io~ 8 cm.) to a fraction of a unit, whereas in 

 ordinary light the lengths are many thousands of times greater. 

 In his classical survey of the X-ray spectra Moseley showed that 

 their general characteristics varied very little, if at all, from 

 element to element ; only in the frequency of any representative 

 line was there a steady march from one element to the next 

 in the natural order of the periodic table. See, for example, 

 fig. 3 below, p. 576, which shows the K series in the spectra 



of silver, palladium, and rhodium. If the frequency of a 



569 



