4 NATURE 

effect which magnetisation exerts on the polarisation 
of reflected light, discovered by Kerr in 1878, and 
discussed immediately after on the basis of general 
theory by FitzGerald but only for transparent media. 
A magneto-optic constant had to be introduced for 
each metal, naturally of complex type, which might 
be regarded as continuous with the constant of the 
Hall effect for a steady field. Experimental research, 
based on his formule, was started in the laboratory 
of Prof. Kamerlingh Onnes by Sissingh in 1886, in 
collaboration from 1889 with Zeeman: and _ their 
results are finally reported in Archives néerlandaises, 
1894. Everything connected with magneto-optics 
excited great interest in England from the time of 
Faraday’s fundamental discovery, and the stimulating 
dynamical speculations of Kelvin (and Maxwell, 
“Elec. and Mag.” ii.) connecting it with a rotatory 
molecular theory of magnetism. The discovery of 
Kerr intensified the interest. The very exact material 
provided by Sissingh and by Zeeman was available as 
a test for a concentrated theoretical formulation. 
One may be permitted to claim that the most system- 
atic theoretical development and thorough verification 
of the subject, remarkably consistent on all sides, 
is in a Cambridge Fellowship dissertation by J. G. 
Leathem, Phil. Trans., 1897, pp. 89-127, which 
has scarcely received the attention that it deserves. 
This theory attains even to features of exact prediction, 
which had been anticipated in a dissertation in Dutch 
by C. H. Wind shortly before. 
About 1897 came the cardinal discovery of the effect 
of a magnetic field on spectra, by Zeeman, which was 
worked out in the early stages in the light of Lorentz’s 
theoretical guidance. As already remarked, the 
elementary illustration by a single vibrating ion under 
elastic control, which covers all the normal features of 
the Zeeman subdivision, had been used to illustrate 
optical dispersion long before. The results admit of 
easy extension to any system of electrons describing 
interacting free orbits, however complex, about a 
massive positive nucleus. When there are more than 
three components in a spectral line, the vibrating 
system must be more complex. The application of the 
theory of the small vibrations of general dynamical 
systems, which suggests itself at once, gave no help and 
it was scarcely to be expected that it would. Recent 
schematic solutions employing the language of quasi- 
periodic systems are said to cover thoroughly the whole 
ground: it would be most interesting to have Prof. 
Lorentz’s reasoned views on the promise held out by 
this rather inscrutable type of analysis. One observes 
that he uses here as elsewhere the well-tried method of 
discussion by mirror images, to fix the types of sym- 
metry (cf. Astrophys. J. 1899): the magnetic field is 
NO. 2775, VOL. III] 



[January 6, 1923 
reversed in the image in order to avoid change of signs" 
of all the charges, which would lead to negative nuclei 
and positive electrons. 
There is a paper of 1892 in Ann. der Physik on 
refraction across thin metal prisms, in which one 
discovers a discussion of an essential point often sought 
for, namely, the influence on the direction of propaga- 
tion by rays of the steep gradient of amplitude along 
the phase-front of the emergent train. The intro- 
duction to this paper is on lines now strangely familiar ; 
an investigation of what type of differential equations 
one is formally restricted to by the principle of invari- 
ance alone, in order to give rise to simple trains of 
damped undulations in an isotropic absorbing medium. 
We come now to the two famous memoirs “ La 
Théorie électromagnétique de Maxwell et son applica- 
tion aux corps mouvants,” Archives néerlandaises 
1892 (pp. 189) and “ Versuch einer Theorie der elek- 
trischen und optischen Erscheinungen in bewegten 
K6rpern,” 1895 (pp. 139), both published as separate 
treatises. Both of them proved to be very difficult, 
in comparison with previous memoirs on cognate 
matters, partly on account of the strangeness and 
complexity of the notation and analytical processes 
to English readers saturated with Maxwell’s notation 
and his more intuitive procedure. One might perhaps 
guess that both of them were worked up gradually, as 
seems to have been Prof. Lorentz’s custom, out of 
professorial lectures: for they include digests of 
previous papers. The main feature in both is the 
expansion of the Maxwell theory on the basis of mobile 
elementary ions, regarded simply as coherent volume 
distributions of electricity, as the sources of the field. 
That point of view had already been clearly expressed 
in the paper of 1878-80 on refraction - equivalents 
and incidentally on the explanation of dispersion, but 
was then developed more in terms of attractions at a 
distance after Helmholtz. As regards the dynamical 
side, both memoirs proceed through the principle of 
d’Alembert in a form which makes it to some extent 
a’ substitute for minimal Action. Looking through 
them in the light of to-day the second, which appeared 
early in 1895 and referred largely to optical phenomena, ~ 
seems much the more striking. Thus he recognises 
that the Maxwell stress for free space does not balance 
when the state of the system is not steady, unless a 
quantity which Poincaré afterwards described as a 
distribution of a momentum connected with the stress 
is taken into account: this was the beginning of the 
stress-energy-momentum tensor. The correction in the 
Fresnel convection-coefficient for transparent media is 
obtained, arising from dispersion, which in recent years 
Zeeman has fully verified. All kinds of optical convec- 
{ tive phenomena are closely considered. 
