IN the spring of 1906 I had the honour to deliver before the Royal Institution of Great Britain a lecture 011 Recent Progress in Magneto-optics. A distinguished astronomer told me two or three years later that he had read the account of this lecture several times over. I have no doubt that his remark was meant as a compliment, but it signified also a too condensed exposition of the subject. At about the same time came the proposal of the Editor of this series of monographs to supplement my discourse and to give an expansion of it in book form. Con sideration of the two communications, both acting in the same direction, easily induced me to prepare the present volume. A few historical facts in the history of the relations between magnetism and light may be recalled here.