My thanks are due to the many kind friends who have helped me in one way or another; to my professors at University College, Dublin, without whose help and encouragement this book would never have been written; to Dr. L. C. Purser of Trinity College, Dublin, who gave me welcome advice at the beginning of my studies; to my tutors at Oxford, especially to Mr. J. G. C. Anderson of Christ Church and to the Delegates of the Clarendon Press for undertaking the publication of this book.
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