January 19, 1922] 



NATURE 



69 



cause a confused broadening (see the Electrician 

 of February 26, 1897, vol. 38, top of 

 p. 569). I was still too much influenced by the 

 idea of random atomic motions, instead of precise 

 electronic orbits. 1 The real effect, as perceived in 

 advance by Lorentz and realised clearly by Zeeman, 

 was much sharper and more beautiful than that, 

 and my suspicion of a more complex or mixed effect 

 was unnecessary ; the simple Lorentz theory served, 

 just as cited in that same article of mine in the 

 ♦ Electrician (except that I made a slip and gave a 

 ^alue for the perturbed frequency which I correct 

 in the second following issue, p. 643 of the same 

 \o\. 38. Some remarks on e\m, interesting from 

 an historical point of view, will also be found 

 in that issue, and an extremely short formulation 

 of the theory, thus : " Magnetised change of centri- 

 petal force, eYirm^mrdu)^, whence do)=^e'H.f2m." 

 Here also is the record of a temporary slip about 

 the sign of the effect, made by Zeeman himself, 

 for he at first announced that the radiating particle 

 was a positive charge). 



At the same time, a pure doubling or a tripling, 

 characteristic of a truly circular or elliptic orbit per- 

 turbed by an apsidal or a precessional motion, is not 

 the last word; for, though this is the standard of 

 simplicity, each line in the spectrum is liable to have 

 peculiarities of its own, depending on the nature of 

 the electronic orbit which is magnetically perturbed ; 

 and hence there is found, not indeed mere broaden- 

 ing, but a quadrupling, sextupling, and other 

 varieties of effect, such as are now well known, but 

 which I confess surprised me when first I saw their 

 indications. 



In the Phil. Mag. for December, 1897, is an 

 important theoretical paper by Sir Joseph Larmor, 

 ' ' On the Theory of Magnetic Influence on Spectra, 

 and on the Radiation from Moving Ions." Towards 

 the end of this paper he deduces his fundamental 

 expression for radiative power, as proportional to 

 square of charge and square of acceleration. Non- 

 radiation from charges moving at uniform speed 

 seems to be indicated — which would be very con- 

 venient in making unperturbed atoms permanent^- 

 but unfortunately centripetal acceleration seems 

 equally destructive to their constitution, were it not 

 for the modern device Ox quanta. 



In this same volume at the P/iil. Mag. (vol. 44, 

 pp 55 and 255) are two additional papers by 

 Zeeman, also reproduced in the book tmder review, 

 constituting the real publication of the occurrence 

 of definite doublets and triplets in the magnetic 

 field ; and they are followed in this book by another 



1 Incidentally and generally, it cannot be unknown, but it seems to be 

 s(>met(mes overlooked, that every regularUy tends to evade the equipirtition 

 ot energy difficulty : for Maxwell's proof requires the motions to be not 

 only interchangeable, but also completely random. 



NO. 



2725, VOL. 109] 



one giving metrical results obtained photographic- 

 ally. 



Photographic records of the effect were, indeed, 

 tried for by other experimenters, though without 

 success (see Nature, vol. 56, p. 420). In Nature, 

 vol. 57, p. 173, however, Thomas Preston reports 

 complete success, in Dublin, with a fine grating be- 

 longing to the Royal University of Ireland, mounted 

 in accordance with Rowland's geometrical-slide 

 design. But on p. 192 of the same volume a meet- 

 ing-report shows that Zeeman had exhibited speci- 

 mens of similar photographs at the Amsterdam 

 Academy a month earlier; and he was now able to 

 apply the photographic method to the obtaining of 

 more exact measurements, as mentioned in the P/iil. 

 Mag. for February, 1898, p. 197. See also Preston, 

 p. 325 of the same volume (vol. 45), by whom a 

 plate showing the various appearances with great 

 distinctness is submitted. A few pages further on 

 (p. 348) is a communication, which still further em- 

 phasises complexities and individual peculiarities in 

 the magnetised lines, by Prof. Michelson, who here 

 begins to apply to them his remarkably powerful 

 " visibility " test, which was first elaborated in the 

 Phil. Mag. for September, 1892 (vol. 34, p. 280), as 

 a sequel to his earlier more theoretical paper in April, 

 1 89 1, and is now employed with such skill and 

 brilliant success at Passadena to measure the dia- 

 meter of stars. 



In further papers by Zeeman the spectrum of 

 iron is specially examined, and a lack of sym- 

 metry detected in some of its lines. And presently 

 the resolving power of the Michelson echelon is 

 pressed into the service fpr the further examination 

 of details, with results which are described and 

 expounded through the remaining 200 pages of this 

 interesting memorial volume. 



The extreme importance of Zeeman 's great dis- 

 covery, and the admirable way in which he worked 

 it out with the inspiring theoretical assistance of 

 Prof. H. A; Lorentz — so that theory and experi- 

 ment went hand in hand, as it is to be wished they 

 did more often — may be allowed to justify and 

 excuse this somewhat personal welcome of its 

 twenty-fifth anniversary by an English physicist. 



Oliver Lodge. 



The Kaiser Wilhelm Institute. 



Festschrift der Kaiser Wilhelm Gesellschaft zur 

 Forderung der Wissenschaften zu Ihrem zehn- 

 jdhrigen Jubildum Dargehracht von ihren In- 

 stituten. Pp. iv-f282. (Berlin: Julius Springer, 

 1921.) 100 marks. 



HE Kaiser Wilhelm Gesellschaft zur 

 Forderung der \\'^lssenschaften ovi^es its 

 origin to the action of certain leading industrialists 



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