PREFACE vii 



ing the year which has elapsed since the delivery of the 

 lectures I have made many efforts to shape this and other 

 parts of the book into something with which I might feel 

 better content. I release it now with more diffidence than 

 I have felt with regard to former books. 



The conversational style of the lecture-room is gen- 

 erally considered rather unsuitable for a long book, but 

 I decided not to modify it. A scientific writer, in for- 

 going the mathematical formulae which are his natural 

 and clearest medium of expression, may perhaps claim 

 some concession from the reader in return. Many parts 

 of the subject are intrinsically so difficult that my only 

 hope of being understood is to explain the points as I 

 would were I face to face with an inquirer. 



It may be necessary to remind the American reader 

 that our nomenclature for large numbers differs from 

 his, so that a billion here means a million million. 



A. S. E. 

 August 192S 



