PART I 
FUNDAMENTAL FACTS AND 
PRINCIPLES 
CHAPTER I 
THE QUANTUM THEORY: PRELIMINARY 
SURVEY 
The quantum theory . . . represents a complete departure from 
the old Newtonian system of mechanics. 
We, as physical machines, are built on a scale which is large 
compared with the scale of light waves and electrons, from which 
it has resulted that our first physical experiments, as a race, have 
been concerned with matter also on a scale very large in comparison 
with its ultimate structure. The Newtonian laws have undoubtedly 
been found adequate to explain the whole series of what we may call 
large-scale phenomena, but no adequate reasons have, so far, been 
given for asserting that they must also be the laws which govern 
small-scale phenomena. The fact seems to be that the old laws are 
not, so to speak, fine-grained enough to supply the whole truth with 
regard to small-scale phenomena. 
J. H. Jeans 
To be living in a period which faces such a complete recon 
struction of our notions as to the way in which aether waves are 
absorbed and emitted by matter is an inspiring prospect. The 
atomic and electronic worlds have revealed themselves with beauti 
ful definiteness and wonderful consistency to the eye of the modern 
physicist, but their relation to the world of aether waves is still to 
him a profound mystery for which the coming generation has the 
incomparable opportunity of finding a solution. 
R. A. Millikan 
1. Origin of the Quantum Theory 
r HE quantum theory, which has proved of such great 
importance in the study of Physics in the present century, 
riginated in an attempt to account for the characteristic pro- 
erties of the heat radiated from a hot body. We shall suppose 
lat in the interior of such a body is a cavity, and that the body 
l