A good source of information on Gilbert is Dr. Duane H. D. Roller's doctoral thesis, written under the direction of Dr. I. B. Cohen of Harvard University. Dr. Roller, at present Curator of the De Golyer Collection at the University of Oklahoma, informed me that an expanded version of his dissertation will shortly appear in book form. Unfortunately his researches were not known to me until after this article was completed. One criterion as to the book's significance in the history of science can be applied almost immediately. A number of historians have pointed to the introduction of numbers and geometry as marking a watershed between the modern and the medieval understanding of nature. Thus A. Koyre considers the Archimedeanization of space as one of the necessary features of the development of modern astronomy and physics.[2] A. N. Whitehead and E. Cassirer have turned to measurement and the quantification of force as marking this transition.[3] However, the obvious absence[4] of such techniques in _De magnete_ makes it difficult to consider Gilbert as a founder of modern electricity and magnetism in this sense. [2] Alexandre Koyre, _Etudes galileennes_, Paris, 1939. [3] Alfred N. Whitehead, _Science and the modern world_, New York, 1925, ch. 3; Ernst Cassirer, _Das Erkenntnisproblem_, ed. 3, Berlin, 1922, vol. 1, pp. 314-318, 352-359. [4] However, see M: pp. 161, 162, 168, 335. [Illustration: Figure 1.--WILLIAM GILBERT'S BOOK ON THE LOADSTONE, TITLE PAGE OF THE FIRST EDITION, FROM A COPY IN THE LIBRARY OF CONGRESS. (_Photo courtesy of the Library of Congress._)]