viii PREFACE 



I have been working for more than three years, I am indebted 

 for the most cordial assistance. Both of them have given me 

 bibliographical information of a most useful character. 



I beg Professor G. Unwin, who kindly helped me with 

 the language, to accept my most sincere thanks. 



To Professor Niels Bohr, Copenhagen, I am indebted for 

 useful remarks about the mathematical part. 



Finally, I have to express my great obligation to Mr W. D. 

 Evans, M.A., who has read through the manuscript and 

 proofs of the mathematical part and who has assisted me in 

 various ways. 



Julius MacLeod. 



The University, 

 Manchester. 2isi December 1918. 



