as 
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January 6, 1923] 
NATURE 3 

by Maxwell, Boltzmann, Rayleigh, Gibbs, which 
originated this domain of knowledge and, though now 
beset with fundamental experimental difficulties, are 
still the ultimate foundation of our ideas. The articles 
“Maxwells Electromagnetische Theorie” (June 1903) 
and “ Elektronentheorie” (December 1903) in the 
Mathematical Encyclopedia are standard treatises. 
His doctor’s dissertation (1875) was a treatise (177 
pp.) on the reflection and refraction of light, which 
was abstracted at considerable length by E. Wiede- 
mann in his Beiblatter, vol. i., 1887. Proceeding 
from Helmholtz’s form of the Maxwell theory, it 
develops a hint contained in a footnote in Helmholtz’s 
first memoir, that the interfacial conditions of the 
electric theory are precisely those that lead naturally 
to Fresnel’s standard laws of reflection. Transmission 
in metals also comes under review, and the laws of 
reflection from their surfaces ; following up Maxwell’s 
remark that gold leaf is far more transparent for the 
rapid electric alternations in light than its steady 
electric resistance would lead one to expect. It is 
curious that Maxwell himself has nowhere indicated 
the application of his theory to the dynamically 
fundamental subject of reflection. In a letter of 1864 
to Stokes * in which he hints at his electric theory, 
then taking form, he remarks : “ I am trying to under- 
stand the conditions at a surface for reflection and 
refraction, but they may not be the same for the 
period of vibration of light and for experiments made 
at leisure.” 
Other early papers published in Dutch, and reported 
in the Beiblatter by long abstracts, include a dis- 
cussion of the propagation of sound according to the 
kinetic theory of gases (1880), and a note (1882) 
stimulated by a discussion of Korteweg, on formule 
for the interaction between two electrodynamic 
elements constructed after the manner of that of 
Ampere. 
The famous memoir in which he applied for the first 
time considerations relating to discrete molecules to 
electric propagation in material bodies, and_inci- 
dentally arrived at a rational refraction-equivalent 
(yz? — 1)/(u?+2)p for each substance, independent of 
its density, is abstracted by himself in Amnnalen der 
Physik, ix., 1880, pp. 641-684. Here again the version 
of Maxwell’s theory developed in the first of Helmholtz’s 
critical memoirs (1870) is followed, possibly as being 
more accessible outside England. Indeed the expression 
for the refraction-equivalent is largely independent of 
any particular theory of propagation in the molecular 
medium ; as is illustrated by the fact that his formula 
was identical with a result deduced ten years earlier in 
‘Danish on lines of elastic solid theory by his namesake 
* “Scientific Correspondence of Sir George Stokes,” vol. ii. p. 26 
NO. 2775, VOL. 111] 
L. Lorenz. The discussion of its range verified the 
rough substantial invariance of this expression even 
for change from the gaseous to the liquid state, and 
showed that it provides an additional atomic constant 
persisting through many types of chemical bonding 
of the atoms. This is now of course a large domain in 
physical chemistry. ; 
The contribution of a vibrating molecule to the 
radiation is treated, after the manner of the general 
Stokes-Kirchhoff equations, in close correspondence 
as it happens with the familiar later formulation of 
Hertz for a dipole vibrator emitting electric radiation. 
Extension to include optical dispersion is considered. 
The result, already known to the masters, is enforced 
that Cauchy’s statical theory which ascribed dispersion 
to a sensible value of the ratio of molecular distance to 
wave-length, is for actual matter entirely insufficient, 
unless as he remarks the laws of attraction are quite 
changed at molecular distances: but its effect is not 
absolutely null, and it is pointed out that cubic crystals, 
which are isotropic on Maxwell’s theory, should on 
this account exhibit a small secondary double refrac- 
tion of very symmetric type. Recently Prof. Lorentz 
has returned to this topic, and announced the detection 
of this quality, amidst others due perhaps to imper- 
fection of the crystal, in his laboratory at the Teyler 
Institute. The detailed investigations of Rayleigh 
(1892) on atomic obstacles arranged in lattices stop 
short of the approximation here required. Later 
both Lorentz and Rayleigh noted that a perfect crystal 
should not scatter at all the light passing through it. 
A static theory being thus inadequate, dispersion 
has to be ascribed to resonant vibration excited in the 
molecular structures. He works out as an example the 
very simplest ideal case, that of an electric charge e 
attracted to a massive nucleus by elastic force propor- 
tional to distance ; which is the identical illustration 
that served him nearly twenty years later to elucidate 
the Zeeman magnetic spectral effect and the polarisa- 
tion of the emitted radiation. The result of course 
also provides an illustration of the anomalous or 
selective refraction discovered by Kundt, which he 
does not then notice, restrained possibly by our ignor- 
ance which he remarks of the actual structure of 
molecules. Nowadays the argument for the Lorentz 
refraction-equivalent is made almost intuitive by 
correlating it with the equivalent (K-—1)/(K+2)p for 
the dielectric inductance K, usually ascribed to Mosotti 
and to Clausius. No demonstration could however be 
simpler than the one given even earlier by Maxwell in 
1873 for the cognate problem of the conductance of a 
medium filled with small spheres of different material : 
“Elec. and Mag.” 1., § 314. 
In 1884 Prof. Lorentz directed his attention to the 
