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. The present edition has been carefully revised. The recent discoveries of importance have been included, as well as the best of new theories. The authors thank fully acknowledge the receipt of valuable suggestions from teachers as to better methods of presenting certain portions of the subject, which they have freely used. The book is believed to be not only adapted to class use, but reliable and thoroughly modern.