78 QUANTUM THEORY 
increased; because then the Z electrons of the atom give rise to scattered 
waves more or less in the same phase; thus the amplitude of the resultant 
is proportional to Z and the intensity is proportional to Z 2 . 
We do not suppose that free electrons scatter continuously in the 
manner here investigated. Actually some electrons scatter whole quanta 
and others are idle. But by the correspondence principle (53-1) is the 
limit to which the true formulae must tend when each electron is scattering 5 
many quanta; and it is customary to entertain the optimistic view that togei 
such formulae will apply without grave inaccuracy a long way short of or o 1 
the limit. with 
For very short wave-lengths (e.g. y rays) the scattering coefficient is resul 
less than 0 - 2 . This is associated with the fact that the electron acquires A 
a velocity comparable with the velocity of light when it scatters a quantum the g 
of such radiation; its inertia increases, and other complications of the react 
problem ensue. force 
surfa 
the i 
L 
Since 
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T 
r, sin 
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when 
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T 
equal 
expre 
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A 
attrai 
whicl 
55 
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