This book is based on a course of lectures given more than once at Oxford. Some who attended it have expressed a wish that its substance might be given a more permanent form. I cannot boast that it contains much which a reader could not have found out for himself if, at every hard place in the old books, he had turned to commentators, histories, encyclopaedias, and other such helps. I thought the lectures worth giving and the book worth writing because that method of discovery seemed to me