February 6, 1914] 



SCIENCE 



211 



Jones, identical with that in liis book, " The 

 Siege of Charleston." Burt G. Wilder 



Brookline, Mass., December 12, 1913 



SCIENTIFIC BOOKS 

 Researches in Magneto-Optics, With Special 

 Reference to the Magnetic Resolution of 

 Spectrum Lines. By P. Zeeman. (Mac- 

 millan's Science Monographs.) London: 

 Macmillan and Co., Ltd. 1913. Pp. xvi 

 + 219 + viii plates. 



Since the discovery by Zeeman in 1896 of 

 the resolution of spectrum lines in the mag- 

 netic field, works have appeared at intervals 

 which summarized the development of the 

 subject to the date of publication. Each of 

 these has been needed when it appeared, partly 

 by reason of differences in treatment by the 

 several authors, but chiefly because of the con- 

 tinuous output of new matter, both on the ex- 

 perimental and theoretical sides; so that an 

 author, by the time his book was off the press, 

 would welcome an opportunity to add numer- 

 ous footnotes or an extensive appendix. 



The investigation of the Zeeman effect dur- 

 ing these seventeen years impresses one as 

 having been very ably conducted. The im- 

 mediate development of the elementary theory 

 by Lorentz gave the phenomenon the place in 

 relation to the theories of light and of elec- 

 tricity which it has ever since maintained. 

 Although the demands on instrumental equip- 

 ment are severe, the rich field and the close 

 connection with theory caused investigations 

 to be taken up in many laboratories and the 

 requirements have greatly stimulated the de- 

 velopment of optical methods. In some cases, 

 theory has predicted a result which at once 

 appeared when the experiment was tried. On 

 the other hand, if one compares the original 

 explanation of the normal triplet with the 

 involved mathematical treatments employed 

 to account for the complex resolutions, the 

 pressure exerted on the theorists by the labora- 

 tory results is quite apparent. The develop- 

 ment of the theory, however, has been one of 

 growth from a beginning still regarded as 

 sound. 



In the book under review, Professor Zeeman 



has given us an account, simple in language, 

 largely historical in arrangement, and occa- 

 sionally touched with personal reminiscence, 

 which records in a highly attractive manner 

 the main features of the investigations started 

 by his discovery. It is in keeping with the 

 title and with the series of monographs to 

 which this book belongs that the author de- 

 votes his closest analysis to those features of 

 the phenomenon which have been studied in 

 his own laboratory. This involves the cor- 

 relation of his results with those of others on 

 these subjects, but other important lines of 

 investigation, such as the application to 

 spectral series, are not omitted. 



The first chapter is devoted to the instru- 

 mental means employed in the study of the 

 Zeeman effect, especially as regards the effi- 

 ciency of different spectroscopes in giving the 

 high resolving power required. Emphasis is 

 laid on the three requirements of great resolv- 

 ing power, high magnetic field-strength, and 

 sharpness of spectrum lines for the best results 

 in this work. At the close of the chapter we 

 are reminded of what has occurred to many 

 investigators, that we are near the limit of 

 field strength to be obtained from an iron- 

 cored magnet, and that the hope of great ad- 

 vance, both as to intensity and uniformity of 

 field, lies in the use of a large solenoid. Al- 

 though the construction of this would require 

 a larger expenditure than has ever been de- 

 voted to a single line of physical research, the 

 certainty of the results would seem to make 

 the adoption of the method only a question of 

 time. 



The early investigations on emission spectra 

 and the derivation of e/m from the separation 

 of the normal triplet are treated in the second 

 chapter, and the author passes next to the 

 " inverse effect," or the magnetic resolution 

 of absorption lines. This branch of the study 

 must be regarded as still in a preliminary 

 stage. A decided stimulus has been given to 

 the examination of the inverse effect by the 

 discovery of the magnetic field in sun-spots, 

 and much important work, described in a 

 later chapter, has been done by Zeeman him- 

 self. The methods are quite different from 



