x Preface 
and hope I have been fair and courteous to those whose views I cannot 
accept. My own personal contributions to the subject represent the outcome 
of twenty-five years of fairly continuous thought and work, and a considerable 
number of my results are published for the first time in the present book. 
I have to thank many friends for help and courtesies of various kinds, 
and particularly Dr W. S. Adams, Director of Mount Wilson Observatory, 
for permission to reproduce a large number of photographs. My thanks are 
again due to the officials and staff of the Cambridge University Press for 
extending to the present book the consummate skill and unremitting care by 
which they transform a mass of muddled manuscript into a masterpiece 
of typography. 
J. H. Jeans 
Dorking, 
January 25, 1928. 
PREFACE TO SECOND EDITION 
AS the first edition had the good fortune to be rather speedily exhausted, 
t\ the preparation of a second edition has been a pleasant and compara 
tively light task. 
I have expanded the book by references to various observational and 
theoretical results which have appeared since the first edition was written, 
and have allotted space more liberally to certain problems and investigations 
which friendly critics thought I had dismissed too briefly in the original 
book. Finally I have corrected a number of minor errors and misprints, and 
have to thank many friends and correspondents for bringing these to my 
notice. 
J. H. Jeans 
Dorking, 
November 14, 1928.