vi PREFACE 
. v". •- •- ' 
An effort has been made to include everything judged important up to 
November 1925; but naturally the whole book could not be rewritten in 
the last month and late developments had to be grafted on to an earlier 
foundation. Further additions have been made in proof up to March 1926; 
footnotes in square brackets show information received too late to be used 
in the text. 
The question of notation has caused me much perplexity since branches 
of physics ordinarily remote from one another are here brought together. 
Apart from the inadequacy of the alphabet, the abolition of overlapping 
symbols and adoption of a consistent notation throughout the book has 
been deemed impracticable. There are limits to our toleration of change 
of familiar symbols. A rose by any other name would smell as sweet, but 
the equation pa> = U 2 — U 1 would lack the familiar savour of the quantum 
relation. The use of m for absolute magnitude and the mass of an electron, 
of R for Boltzmann’s constant and the radius of a star, of a for a radiation 
constant and the semiaxis of an orbit, of e for the charge of an electron 
and the Napierian base, may cause momentary confusion but, it is hoped, 
no serious difficulty. Similarly we refer to the astronomical equation of 
areas in its most recognisable form in sections where = fd is not likely 
to be misconstrued as a relation between Planck’s constant and molecular 
weight. I would suggest to the reader in difficulty that there is a chance 
that the symbol which puzzles him is included in the list of natural constants 
in Appendix I. 
I have derived help from many colleagues. Mr R. H. Fowler has 
generally been my referee in difficulties over points of theoretical physics, 
and I have similarly had recourse to Dr C. D. Ellis for experimental 
questions. I thank especially Prof. E. A. Milne who has read the proof 
sheets and eliminated a number of errors and obscurities. My acknow 
ledgments are also due to the staff of the University Press for their care 
and attention in the printing. 
A. S. E. 
July, 1926