.' August 12, 1909] 



NATURE 



185 



afforded by a few judiciously selected illustrations, but 

 these the author considers undesirable. 



The large number of drugs dealt with renders the 

 book extremely valuable for reference, but the defects 

 alluded to militate against its efficiency as an educa- 

 tional work for students. Henry G. Gree.sish. 



MAGNETO- AND ELECTRIC-OPTICS. 

 (i) Magneto- tind Elektro-optik. By Dr. Woldemar 



Voigt. Pp. xiv+396. (Leipzig : B. G. Teubner, 



igoS.) Price 14 marks. 

 (2) Coiirs de Physique. Cinquieme Partie. Elcc- 



troptique. Ondes Hertziennes. By Prof. H. 



Bouasse. Pp. 426. (Paris : Delagrave, n.d.) Pricj 



14 francs, 

 (i) ' I HIS is a work on one of the most fascinating 

 J- branches of modern science by one of those 

 who have done most towards its development. To 

 all students o4 physics the volume is to be specially 

 recommended as giving an admirable account of a 

 subject which has the most intimate bearing on 

 present theories of the constitution of matter ; to those 

 who are themselves working in the same field the 

 name of the author will be sufficient to indicate that 

 the book is indispensable. 



The subject dealt with is not the electromagnetic 

 theory of light in general, but, in Prof. \'oigt's own 

 words, " that special branch of optics which deals with 

 the action of a magnetic or an electric field on the 

 optical properties of bodies." The study of this special 

 branch of optics may be said to have begun in 1845 

 with Faraday's experimental demonstration that the 

 plane of polarisation of a beam of polarised light is 

 rotated by the action of a magnetic field having its 

 lines of force parallel to the direction in which the 

 light is travelling. Since that date an enormous 

 amount of attention has been given to the pheno- 

 mena due to the action on light of a magnetic or an 

 electric field. In 1876 Kerr succeeded in detecting the 

 rotation of the plane of polarisation when light is 

 reflected normally from the surface of a ferromagnetic 

 metal in a strong magnetic field. In 1896 Zeemann 

 showed that the periods of free vibration of the light 

 emitted by a flowing gas are changed bv the direct 

 actfion of a magnetic field, and that there is a corre- 

 sponding action on an absorbing medium. In 1898 

 Voigt himself showed that a body in a magnetic field 

 becomes doubljr refractive in the direction perpen- 

 dicular to the lines of force, in the same manner 

 as a uniaxial crystal perpendicular to its axis. In 

 the section of electro-optics, the advances made are 

 less marked. Kerr's demonstration that an isotropic 

 insulating substance becomes doubly refractive in the 

 direction perpendicular to the lines of electric force may 

 be taken as fundamental. An army of workers has fol- 

 lowed along the main lines thus indicated, and to-day 

 the mere marshalling of the known facts of observation 

 is a task of no small difficulty, while there are many 

 points of which theory yet fails to give any con- 

 vincing explanation. 



Prof. Voigt's book is founded on lectures given by 

 him at Gbttingen L'niversity The material thus 

 NO. 2076, VOL. 81] 



available, however, has clearly been greatly extended 

 and amplified, with results which may be regarded 

 as exceptionally satisfactory. Thus the elementary 

 and fundamental facts of observation, as well as all 

 the more important experimental appliances and ap- 

 paratus, are very fully and clearly described, while, 

 on the other hand, a large amount of detail as to 

 the latest developments, both in experiment and 

 theory, is included. In a branch of knowledge so 

 necessarily subject to modification under the influence 

 of new ideas, it is usually to be expected that a volume 

 will be already almost antiquated by the date of its 

 publication, but one of the chief claims of the present 

 work to attention is the extent of the information 

 given as to the most recent work. 



The first two chapters are devoted to setting forth 

 the fundamental observations and the principal experi- 

 mental methods and developments in connection with 

 the Faraday effect and the Zeeman effect. The two 

 chapters following, covering some 120 pages, are con- 

 cerned with the explanation of these observations on 

 the electron theory. The chief feature of Voigt's own 

 work is the establishment of a simple connection 

 between the emission and absorption phenomena and 

 the rotatory effect of the magnetic field, and the 

 development of the theory here given is founded on 

 the series of papers published by the author between 

 1898 and igo2 in the Aiiualcn dcr Physik. The 

 theory is, howev'er, here further extended, and in- 

 cludes, in chapter v., a discussion of Becquerel's 

 more recent experiments on crystals of compounds of 

 the rare earths. 



The remaining chapters are devoted to the dis- 

 cussion of the " magnetic " Kerr effect, and the con- 

 sideration of the fundamental phenomena of electro- 

 optics. In the field of electro-optics the experimental 

 difficulties have rendered progress slow. Insulation 

 breaks down under very high voltages, and conduction 

 is never entirely absent, and more refined methods of 

 observation are necessary before the indications of 

 theory can be verified. The subject is, however, of 

 great theoretical interest and importance, and the 

 publication of the present work will no doubt do much 

 in stimulating further research. 



(2) The general plan of the " Cours de Physique," 

 of which the present volume is the fifth part, is some- 

 what unusual. The volumes previously issued bear 

 the titles: — (i) " Mecanique Physique"; (2) " Ther- 

 modynamique — Theorie des Ions"; (3) "Electricity 

 et Magnetisme"; (4) " Optique. Etude des Instru- 

 ments," the part here considered being devoted to 

 the general discussion of electromagnetic radiations, 

 including Hertzian waves. Part vi. is to be entitled 

 "Etude des Symetries " ; from the references in the 

 present volume we gather that this might best be 

 translated " Crystallography." We are by no means 

 clear that the arrangement thus outlined is satisfac- 

 tory. In the volume before us especially, the inclu- 

 sion of the discussion of Hertzian waves, though en- 

 tirely logical and perfectly defensible on theoretical 

 grounds, seems to us unfortunate. The questions in- 

 volved are essentially practical and of special interest 

 to the electrician, and the early chapters devoted to 



