248 THE QUANTUM [xviii. 2 
the significance of the quantum of action for the elementary 
physical processes is made clear in every respect or may be 
demonstrated by some simple dynamical model, misunderstands, 
I believe, the character and the meaning of the hypothesis of 
quanta. It is impossible to express a really new principle in 
terms of a model following old laws/’ To illustrate this point 
we may take the suggestion that the quantum theory implies 
the existence of discrete tubes of magnetic induction. This 
supposition standing by itself is not sufficient to explain the 
quantum theory. For although the ordinary laws of electro 
magnetism be applied to the stationary state of an atomic 
system (and even in such a state some modification seems 
necessary), we are not entitled to apply them to the catastrophic 
change which takes place between two stationary states. We 
may, for instance, imagine that such a change involves the 
separation (or addition) of one or more discrete magnetic tubes, 
but the change cannot be expressed in terms of the classical 
electromagnetic laws. 
Again, Whittaker’s attempt to account for quanta by postulat 
ing the existence of a magnetic structure in the atom, although 
extremely suggestive, does not entirely overcome the difficulty 
pointed out by Planck. 
On the other hand there is the danger of departing too far 
from the older ideas, and in this connection Planck says : “ Since 
nothing probably is a greater drawback to the successful develop 
ment of a new hypothesis than over-stepping its boundaries, I 
have always stood for making as close a connection between the 
hypothesis of quanta and the classical dynamics as possible, 
and for not stepping outside of the boundaries of the latter until 
experimental facts leave no other course open.” For this reason 
Planck himself has not adopted the more radical assumption 
“ that any radiant energy whatever, even though it travel 
freely in a vacuum, consists of individual quanta or cells.” 
The strange conflict * between the two rival theories of light 
has been emphasized repeatedly in this volume. There appears 
to be direct contradiction between the wave theory and the 
quantum theory of radiation. The former is able to explain 
the behaviour of light when it is passing through a telescope or 
is concerned with interference phenomena; the latter is able 
to account for emission, absorption or photo-electric action. 
“ The energy of the radiation behaves as though it possessed at 
the same time the opposite properties of extension and 
localization.” f 
* See an article by O. D. Chwolson in Scientia, vol. 41, p. 31, 1927. 
t O- W. Richardson, Electron Theory of Matter (2nd edition), p. 507, 
1916.