Preface ix 



a further service of great value by drawing a consider- 

 able number of the diagrams. I am also indebted to 

 Mr. C. E. Inglis, of this College, for fig 81 ; and I have 

 to thank Mr. W. H. Wesley, of the Royal Astronomical 

 Society, for various references to the literature of the 

 subject, and in particular for help in obtaining access to 

 various illustrations. 



I am further indebted to the following bodies and 

 individual astronomers for permission to reproduce photo- 

 graphs and drawings, and in some cases also for the gift 

 of copies of the originals : the Council of the Royal Society, 

 the Council of the Royal Astronomical Society, the Director 

 of the Lick Observatory, the Director of the Institute 

 Geographico-Militare of Florence, Professor Barnard, 

 Major Darwin, Dr. Gill, M. Janssen, M. Loewy, Mr. E. 

 W. Maunder, Mr. H. Pain, Professor E. C. Pickering, 

 Dr. Schuster, Dr. Max Wolf. 



ARTHUR BERRY. 



KING'S COLLEGE, CAMBRIDGE 



