NOVEMBER 13, 1913] 
NATURE 345 
ot 
ment, simply by showing that a magnetic field 
restored visibility of the line when applied to an 
absorbing vapour between nicols crossed for ex- 
tinction of the light. 
In the theoretical procedure of Voigt the 
radiating molecule has thus disappeared from 
the scene, or rather has become latent; the 
problem proposed is now to represent the effect 
of the medium in bulk heuristically, as well as 
may be, by introduction of appropriate new 
types of terms into the differential equations 
of propagation, new types which owe their 
justification, or at any rate their suggestion, to 
the general physical nature of the interaction of 
the molecules with the «ther in the magnetic 
field. The aim is thus coordination of phenomena 
rather than their explanation; and the procedure 
is specially appropriate to that philosophical view 
which restricts the sphere of physics to the adequate 
formulation of the relations subsisting between the 
tangible experimental data. The mode in which 
the interaction of the vibrating molecules gives 
rise in a general way to such terms in the 
equations of propagation, including the relation of 
reciprocity of the Zeeman to the Faraday effect, 
had been exhibited by FitzGerald, by means of 
simple illustrative systems, about the same time. 
All these converging activities show how ripe for 
the harvest ideas had become, through the pro- 
gress of the general theory of absorption and the 
related anomalous dispersion, first essayed by 
Young with imperfect means of analysis a century 
ago, and effectively developed in experiment and 
theory by Kundt, Maxwell, Rayleigh, Sellmeier, 
Helmholtz, &c. in more recent days. 
Similarly, allusion has been made above to the 
circumstance that the times had been ripening, 
before Zeeman’s discovery, towards the under- 
‘Standing of the relations of a magnetic field to 
the vibrations of the molecules which take part 
in the emission or transmission of radiation. The 
most remarkable and even precise anticipation of 
all, and one which by good fortune incited Prof. 
Zeeman to enter upon his investigation, was an 
experimental attempt made by Faraday himself, 
which our author had come to know of, very 
appropriately, from a reference in a lecture by 
Clerk Maxwell. Then there was the additional 
fortunate circumstance that Prof. Lorentz was at 
hand at Leyden, to bring to bear his exact ideas 
on the nascent discovery and point out the path 
for further developments. These are opportunities, 
Seemingly merely born of concurrent chances, yet 
‘such as are only grasped by men worthy of them. 
. The skill in optical experimentation, which is re- 
_ vealed by the investigations recorded in 
m NO. 2208, vor. 92] 
treatise, connotes a long training for the tasks 
there undertaken: we are thus reminded of Prof. 
Zeeman’s early exact measurements on the Kerr 
effect in reflection of light from a magnetic pole 
(not mentioned in this book), by which he won 
his spurs at Leyden, doubtless in that problem 
also enjoying the stimulus of Lorentz’s advice and 
inspiration. 
Recently the centre of interest has shifted in 
this subject into a purely observational side, to 
the mountain peak in California where G. E. Hale 
and his associates, by refined and determined work 
with the very powerful special equipment of the 
Carnegie Observatory, have realised in marvellous 
ways, still awaiting closer interpretation, one of 
Zeeman’s anticipations in his earliest paper, the 
application of the method to the exploration of 
the magnetic phenomena of the sun, greatly ex- 
panding thereby our picture of the activities of the 
ultimate source of all our light and power. 
But we must stop: these topics, and many 
others of absorbing and often perplexing interest, 
may be followed up in the book itself. Less than 
twenty years ago the Zeeman effect was unknown, 
we may almost say unthought of. Already it 
permeates, as a method of coordination and dis- 
covery, all the most refined problems of electrical 
and optical science. We have now a handbook 
of the present state of the subject, of the right 
degree of detail, written from the experimental 
point of view without. undue occupation or dis- 
traction with theoretical speculations for which it 
yet arranges the material, with brief side exposi- 
tions recalling to mind succinctly such knowledge 
of related subjects, spectral resolving power, 
spectral series, &c., as is essential to the argu- 
ment: and this reasoned survey has come to us 
from the hands of the discoverer and chief experi- 
mental promoter of the Zeeman phenomenon. 
Joni 
P.S.—In the foregoing review of Prof. 
Zeeman’s monograph, which was written early in 
October, it is remarked that recent observations, 
especially by Paschen and Back, and afterwards 
by Fortrat, on the modification of the Zeeman 
effect in strong fields, give support to the theory 
advanced by Voigt, which postulates mutual influ- 
ence between the constituents of a close multiple 
line in the spectrum. The case may now (Novem- 
ber 4) be put stronger. The recent account by 
Fortrat of the magnetic resolution of a sodium 
doublet (Comptes rvendus, October 20,° 1913, p. 
635) seems to leave no room for doubt that the 
equations advanced by Prof. Voigt are of the 
this { essence of the matter.—J. L. 
