PREFACE TO THE FOURTH EDITION 



A CERTAIN number of additions have been made since 

 the first edition was published ; these were enumerated 

 by my husband in the prefaces to the second and third 

 editions of the book, but it is not necessary to re- 

 capitulate them. 



In revising the book for a fourth edition, the object 

 has been to make the fewest and smallest changes 

 consistent with giving a true idea of the present state 

 of our knowledge. In many cases progress has been 

 made since the author revised the book for the third 

 edition, but except when this progress has rendered 

 the text in any way misleading, it has been thought 

 best to make no change, and merely to add references, 

 either in footnotes or at the ends of the chapters, to 

 books or papers in which the later work is described. 

 Material alterations or additions, and footnotes other 

 than simple references, are enclosed in square brackets. 

 The only considerable alterations to the text are on 

 pp. 200-202, 269-271, and 275-279. 



I wish to express my warmest thanks to Dr. Leonard 



Doncaster, F.R.S., for his great kindness in revising 



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