612 SCIENCE PROGRESS 



single plane, but the distance between two adjacent planes of 

 the series of parallel planes in question. Of course, part of 

 a heterogeneous radiation will be reflected at any angle. In 

 Bragg's explanation of the Laue patterns the position of the 

 spots depends on the existence of several series of parallel 

 planes rich in atoms ; the incident radiation is supposed hete- 

 rogeneous, and from this the planes pick out the wave-length 

 required for reflection to take place for the fixed angle of 

 incidence. 



More recently Moseley has examined, using the method of 

 the X-ray spectrometer described, the characteristic radiation 

 from all the metals whose atomic weights lie between 40 and 

 65, by employing them successively as the targets in an X-ray 

 tube ; the characteristics are excited by the fast cathode rays. 

 The metals were mounted on a little truck, so that they could 

 be brought at will in the path of the cathode rays ; the X-rays 

 produced were reflected from a crystal of potassium ferro- 

 cyanide and detected photographically. The X-ray, or high 

 frequency, spectrum of each element he found to consist of 

 two lines, one stronger than the other, which he calls the a 

 (strong), and the fi line ; Bragg also found two lines for the 

 rhodium spectrum. The wave-length of each line was found 

 in terms of the spacing of the planes in the rocksalt crystal, 

 known from Bragg's researches, and it was found that the 



quantity Q = s\J 3— (where v is the frequency of the a radiation, 



v a constant, the fundamental frequency of ordinary line 

 spectra) was a whole number, increasing by one for each 

 successive element taken in the order of their atomic weights. 

 If N be the atomic number, the number, that is, of the place 

 occupied by the element in the periodic system (H = I, He = 2, 

 Li = 3, . . . , Ca = 20, etc.), Moseley found that Q = N — I. or v, 

 the frequency, varies as (N — I) 2 . This suggests that the atomic 

 number is perhaps more important for physical processes 

 than the atomic weight; the point will be referred to again 

 later. 



Rutherford and Andrade are investigating the 7 rays from 

 radium by the method of reflection from crystals, and have in 

 a preliminary note announced that they have photographed 

 groups of lines given by the 7 rays from radium B and 

 radium C ; hence the 7 radiation from these substances is not 



