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XLII. Notices respecting New Books. 



Vorlesungen uher tlieoretisclxe Phgsik. H. VON Helmholtz. 



Band V. ElektromagnetiscJie Theorie dcs Lichts, herausgegeben 



von Arthur Konig und Carl Eunge. Leipzig : Leopold 



Voss, 1897. 

 HTHE publication of Helmholtz's course of lectures on theoretical 

 physics will be welcomed by physicists of every nationality. 

 To be students under a Maxwell or a Helmholtz is a privilege 

 accorded to the comparatively few, and of these fortunate disciples 

 only two or three may feel themselves able to undertake the 

 presentation of the thoughts and ideas of their master in their 

 original form. We heartily rejoice that in the present case such 

 volunteers have been found as Prof. Runge and Dr. Konig ; these 

 will be assisted in the preparation of subsequent volumes by 

 Dr. Krigar-Menzel. The volume now issued as a first instalment 

 of the work is not the initial part of the course, but it was more 

 easily edited than the rest because one of the students took down 

 the lectures in shorthand at the time of their delivery. Nearly 

 half the volume was revised by Helmholtz himself. 



After a short introduction dealing with the emission and wave 

 theories of light and leading up to the mention of Hertzian 

 vibrations, the equations of plane transverse and longitudinal 

 waves are briefly discussed. Maxwell's equations of the electro- 

 magnetic field are then obtained in a very simple manner, starting 

 from the experimental facts that magnetic lines of force form 

 closed curves round an electric current, and that a change of linear 

 magnetization gives rise to electric currents in closed circuits 

 surrounding it ; the co-existence of electric and magnetic polari- 

 zations is thus established. The equations are afterwards trans- 

 formed and applied to the case of a disturbed aether, with or 

 without conducting matter, and it is shown that the waves set up 

 correspond in type to the transverse waves in an elastic medium, 

 the electric and magnetic displacements being at right angles to 

 each other. The transition to optics is effected by means of 

 Huyghens's principle, which Helmholtz deduces from electro- 

 magnetics by a generalization of Green's theorem, introducing time 

 as a fourth variable in addition to the three space-coordinates. 

 Diffraction, interference, reflexion, and refraction are treated as in 

 ordinary optics, the author returning to electromagnetics in the 

 discussion of polarization and dispersion. 



A connexion between aether and matter is necessary in order 

 to explain dispersion and magnetic rotation of the plane of 

 polarization ; the author assumes that the molecules of matter 

 contain two ions with equal positive and negative charges, and that 

 under the influence of the electric field these ions may be drawn 

 asunder or rotated round their centre of mass, which remains 

 fixed. In an alternating electric field some of the energy of the 



Phil. Mag. S. 5. Vol. 43. No. 263'. April 1897. 2 A 



