NOTE 



I HAVE avoided the overloading of this volume with 

 references to authorities, but my debt to many contem- 

 porary writers will be sufficiently clear to the reader. I 

 must, however, express my very special obligations to my 

 colleagues, Dr. A. Wolf and Dr. T. Percy Nunn, both 

 of whom carefully criticised the manuscript and suggested 

 many important modifications. Dr. Wolf has also read 

 the whole of the proofs, and Dr. Nunn has given me an 

 expert's aid in the chapter in which I have had to refer to 

 mathematical method. Neither of my friendly critics, 

 however, are responsible for the final form of the text, and 

 any errors or failings that are detected in it must be 

 imputed to myself. 



L. T. H. 



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