MATTER, SPACE, AND TIME 241 



scratches on a diffraction grating act towards 

 light, separating the waves, and spreading them 

 out into a spectrum. 



Now X-rays are produced by the impact of 

 cathode rays on solid obstacles, and it is found 

 that the wave-lengths of the X-rays so produced 

 depend on the nature of the target exposed to the 

 cathode rays. Generally there is a certain amount 

 of diffuse radiation, mixed with rays of definite 

 frequency and wave-length corresponding to the 

 line spectra of visible light. 



The spectra from these characteristic X-rays 

 were examined in 191 3 and 19 14 by H. G. J. 

 Moseley, who directed the rays from different 

 elements, when used as cathode ray targets, 

 successively on to the surface of a large crystal 

 of potassium ferrocyanide to serve as a grating. 

 The resulting line spectra when photographed 

 and measured showed a surprising regularity. 

 Similar groups of lines are found in the spectra- 

 of different elements, and Moseley discovered that 

 the square roots of the frequency of vibration of 

 the chief lines in each X-ray spectrum increased 

 regularly by a constant amount as he passed from 

 element to element in the periodic table. By 

 adjusting the constants, this constant difference 

 can be made equal to unity, and Moseley was thus 

 able to assign to each element an atomic number, 

 representing its true place in the periodic table 

 which begins with hydrogen = i. 



On the nuclear theory of the atom, the 

 frequencies of vibration must depend on the 

 electric charge on the nucleus, and Moseley 

 concluded that the atomic number also repre- 

 sented the number of electric units in the charge 



R 



