﻿408 Prof. W. H. Bragg on the Relation between certain 



a few of the readings taken in the regions between the 

 " lines "" and on either side of them are shown in the figure. 

 It will he observed that the spectrum is practically a pure 

 line-spectrum. In fact, no general or " white " radiation 

 can be found — a condition which is no doubt due to a 

 particular state of the bulb. The small readings of the 

 figure, which lie between the lines, are due to various minor 

 causes, including scattering of X rays. If a diamond is 

 employed, they are proportionally far less. With the 

 spectrometer- slits opened to two or three millimetres so as 

 to obtain the maximum effect, the electroscope-leaf moved 

 260 divisions in five seconds when the glancing angle 

 (between incident rays and diamond) was 8° 30' and the 

 reflected portion included the strong a line. When the 

 angle was half this, the reading was not more than one 

 division in the same time. 



Calcite is an accurately built crystal, resembling diamond 

 in this particular. It is a far better crystal for accurate 

 work than rocksalt, which may be compared to a badly ruled 

 diffraction-grating giving false images or " ghosts/' The 

 rhodium anticathode was placed so that the rays left it at a 

 grazing angle, and the source of rays was therefore, in effect, 

 a line parallel to the slits. The pencil of rays was limited 

 just before incidence on the crystal by a slit 0*2 mm. wide. 

 With this combination of circumstances, the lines of the 

 spectrum are well separated from each other. 



The spectrum contains four lines. The two of longest 

 wave-length constitute the doublet which has already been 

 observed and examined *. In combination they compose 

 the strong line which Moseley f has observed to be given 

 by a great number of substances, constituting in fact the 

 principal part of the K series of characteristic radiations. 

 The term " doublet " is not really justified, for there is no 

 reason to suppose the two constituents stand in any special 

 relation to each other. They may be spoken of separately 

 as <x x and « 2 , which terminology will be in touch with 

 Moseley's. 



The line near to 11° in the spectrum is the other well- 

 known constituent of the characteristic radiations of the 

 K series ; it is known as the ft line. There is also a fourth 

 line which has not been noted before, so far as I am aware. 

 It is marked 7 in the figure. 



Examination has also been made of the rays from bulbs 

 having anticathodes of palladium and of silver. It was not 



* Nature, March 12, 1914 ; Phil. Mag. May 1914. 

 t Phil. Mag. April 1914. 



