PREFATORY NOTE 



THE following paper was written for an audience the 

 majority of whom could be expected to possess little 

 or no previous knowledge of the subject. My en- 

 deavour was therefore rather to give an exposition 

 of matters already known and views previously ex- 

 pressed than to add anything novel. The paper has 

 been reprinted in the hope that such an exposition 

 may be of service to others. The form in which it was 

 delivered has been retained, with the exception of 

 minor corrections : to explain a phrase which occurs 

 here and there it may be added that copies of the 

 tables were in the hands of the audience. 



G. U.Y. 



ST JOHN'S COLLEGE, 



1920. 



