THURSDAY, NOVEMBER 13, 1913. 
THE ZEEMAN EFFECT. 
Researches in Magneto-Optics. With Special 
Reference to the Magnetic Resolution of 
Spectrum Lines. By Prof. P. Zeeman. (Mac- 
millan’s Science Monographs.) Pp. xvi+219 
+viii plates. (London: Macmillan and Co., 
etd) 1913.)) Price 6s. net.. 
HIS synthesis of our knowledge in an im- 
portant and fundamental branch of physics 
-—opened up by our author in 1896, and after- 
wards cultivated so zealously and fruitfully by 
many workers, but by himself far in front of all 
others—will be most welcome to all who wish to 
keep abreast of the advancing tide of electrical 
and optical discovery. Prof. Zeeman has paid us 
the compliment of writing his book in English; 
and nowhere, perhaps, will he have more attentive 
readers than here. Though occasional slight 
differences of idiom betray that the work is not 
composed in his native language, yet the clear- 
ness and directness of statement, and the con- 
ciseness of exposition, enable him to cover a large 
field, so to speak in a single view, in a manner 
which will make the book a permanent companion 
of all who are interested in the progress of the 
marvellous subject which is indissolubly asso- 
ciated with the name of the Professor of Physics 
of Amsterdam. 
In the early days of this research it could 
scarcely have been anticipated that it would grow 
almost into a separate science. The present 
writer well remembers the earliest announcement 
in this country of the first phase of Prof. Zeeman’s 
discovery, which was contained in a single 
sentence in NATuRE in December, 1896, imbedded 
in the midst of an abstract of proceedings of 
the Amsterdam Academy of about a month before ; 
its importance was, however, at once grasped, 
and the experiment was promptly repeated and 
verified by Lodge. The idea of a spectrum line 
being widened by a magnetic field had in fact 
been thought of; but a rough estimation had 
shown that if the ions concerned are comparable 
‘in mass to atoms, the effect would be far too 
slight for practical detection. The actual small- 
ness of the inertia of the electron, only 1/1500 of 
that of the hydrogen atom, which made all the 
difference in this regard, could not have been 
anticipated. But when Zeeman’s full paper came 
to hand, it was found to include much more; not 
only Lorentz’s brilliant and decisive test of a 
magnetic influence, and its verification, viz., the 
"circular polarisations of the edges of ihe widened 
spectral line: it also contained the establishment | 
NO. 2298, VOL. 92] 
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of an actual splitting of each line into sharp 
components, which had been suggested as pos- 
sible, though one would imagine scarcely likely, 
by the special circumstances of Lorentz’s simple 
illustration of a single electron revolving round 
a centre of force. 
This latter very remarkable result, the sharp 
multiplication of the line instead of a mere general 
broadening, remains the theoretical crux of the 
subject, and at the same time is that feature of 
definiteness which makes and will make the 
Zeeman phenomenon so effective a probe as 
regards the inner physical structure of the 
individual molecules of matter. 
The value thus found for the ratio of charge 
to inertia, for the negative optical electron, fell 
at once into line with the value belonging to the 
free corpuscles of J. J. Thomson—the Crookes- 
Stokes torrent of charged particles which carry 
the kathode current in vacuum tubes—as an- 
nounced by the discoverer of free electrons, and 
of their stupendous translatory velocities, in the 
earlier part of the same year. Thus the electron 
theory, which already embraced in its theoretical 
scope all electric phenomena as well as all effects 
of radiation, was raised, by convergence from both 
its aspects in the same year, from a mental con- 
structive synthesis to the rank of tangible experi- 
mental fact. Special electron theories could thus 
in future be launched out in detail, into regions 
of tentative speculation hitherto almost regarded 
as fanciful, as the test of experiment became 
applicable more and more as a check on their 
exuberance or an indication for their fruitful 
modification. 
The earliest general comparative study of the 
phenomena of resolution, for the various spectral 
lines of the same chemical element and of related 
elements, was made in the two following years by 
Preston, working within the circle of FitzGerald’s 
influence in Dublin, who was able, as it happened, 
to turn to account a powerful Rowland grating 
that had just previously been established at the 
Royal University. The circumstances which pre- 
vented Zeeman himself, for nearly ten years, from 
proceeding with the full exploration of his own 
subject in this direction—namely, his transfer 
from the Leyden laboratory to a lectureship at 
Amsterdam University, and the very imperfect 
spectroscopic equipment which he found there— 
are recorded here not without pathos, at the be- 
ginning of chap. iv., in explanation of his occupa- 
tion during those years mainly on side problems 
| which could be attacked with small optical powers. 
The rule announced by Preston, and now appro- 
| priately known by his name, as it arose out of his 
| last piece of work before the premature termina- 
M 
