Preface vii 



suggested ; not in the belief that it will prove to be true, but in the hope 

 that it may in some degree help others ultimately to find the truth. This 

 scheme will be found to contain nothing fundamentally new ; it consists only 

 of a patchwork of parts of existing theories. This is perhaps Jiardly sur- 

 prising; so many cosmogonical conjectures have been made that it is 

 unlikely that any really novel hypothesis remains to be put forward. In any 

 case a theoretical investigation such as that of the present book is necessarily 

 destructive rather than constructive ; primarily it serves to test and eliminate 

 existing theories rather than to indicate new possibilities. 



It is a pleasure to thank many friends who have helped me in various 

 ways. First I must thank the great number of astronomers who have 

 allowed me to draw on their stores of astronomical knowledge. I have to 

 express my obligation and cordial thanks to Professor Hale, Professor Ritchey 

 and Mr F. G. Pease of Mount Wilson Observatory for permission to repro- 

 duce the very fine photographs which enrich my book. Finally it is a 

 pleasure to express to the officials and staff of the Cambridge University 

 Press my appreciation of their unfailing courtesy and the care they have 

 bestowed on the printing of the work. 



J. H. JEANS. 



Dec. 18, 1918. 



