PREFACE 
M ANY books have been written on the quantum theory 
and on its applications to special branches of physics, 
the work of Arnold Sommerfeld on Atomic Structure and Spectral 
Lines, in particular, being indispensable to all concerned with 
these subjects. The present volume is not a treatise on quantum 
theory, but an attempt to deal with the baffling problem of 
the nature of the quantum. Let it be said at once that no final 
solution of this problem has yet been reached ; indeed some 
investigators would maintain that in the last analysis of all 
physical problems we must rest content merely with mathe 
matical formulation. It is not at present possible to bridge 
the gap between the undulatory theory of light, which pictures 
the disturbance as a spreading wave, and the quantum theory 
of radiation which supposes the energy to be concentrated in 
bundles or “ light units.” It is still true, as Sir William Bragg 
said at the Robert Boyle lecture in 1921, that “ we are obliged 
to use each theory as occasion demands.” There seems hope, 
however, that some type of electromagnetic model, probably 
subject to suitable restrictions expressing quantum conditions, 
may serve to correlate the known facts of electricity, magnetism, 
and radiation. This anticipation receives support from the new 
undulatory or wave mechanics associated with the name of 
Schrodinger, of which an account is given in this volume. 
It is becoming more and more evident that the electron 
must be regarded as the seat of a periodic process, and that 
particles of matter may themselves be spoken of as “ waves.” 
While this volume was passing through the press this view 
has been confirmed by the experiments of Davisson, who exam 
ined the reflection of electrons from a crystal, and of G. P. 
Thomson, who studied the scattering of a beam of electrons 
in passing through a thin crystalline plate. C. G. Darwin has 
discussed the theoretical aspects of the electron as a “ vector 
wave.” 
The quantum theory has revealed an atomicity in nature of 
a kind previously unsuspected. This aspect of the theory, 
emphasized by Dr. J. H. Jeans in his first Report on Radiation 
and the Quantum Theory (1914), retains its mystery. But the 
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