Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. Much of the work involved in the writing of this book was done in fulfillment of the requirements for the degree of Master of Arts at Columbia University. This Work, largely in the nature of research, was under the direction of Dr. Charles C. Trowbridge, of the Department of Phy sics, to whom the author is naturally indebted for many valuable suggestions and much friendly aid.