216 
SCIENTIFIC LITERATURE. 
Vorlesungen tiber die Elektromagnetische Theorie 
des Lichts, von H. VON HELMHOLTZ. Heraus- 
gegeben von ARTHUR KONIG und CARL 
RunGe. Hamburg and Leipzig, Verlag von 
Leopold Voss. 1897. 
Since the experiments of Hertz proved the 
existence of electro-magnetic waves propagated 
through dielectrics, the attention of an increas- 
ing number of physicists has been turned to the 
careful study of Maxwell’s ‘ Electro-magnetic 
Theory of Light,’ and the belief has become 
practically universal that this theory, in its 
general outlines at least, corresponds closely to 
physical facts. Several hundred important 
papers on matters more or less nearly related 
to the theory have been published during the last 
ten years, and the results of investigation have 
been made fairly accessible to students through 
the books of Boltzmann, Drude, Hertz, Poin- 
caré, J. J. Thomson and others. When, how- 
ever, a great university teacher, who has had 
much to do with the creating of a new branch 
of science, writes a systematic treatise on the 
subject for the use of his pupils, the event must 
always be of interest to the scientific world, 
and this is especially true when the subject is 
so important and in some respects still so ab- 
struse as the Electro-magnetic Theory of Light 
is. The lectures, which are now published 
under the editorship of Professors Konig and 
Runge, were delivered substantially in their 
present form by Helmholtz in the Winter Sem- 
ester of 1892-93. An accurate stenographic re- 
port of the words of the lecturer was made by 
Dr. Borchardt, and-this report, with slight edi- 
torial changes, made in part by Helmholtz him- 
self, and with some additions, made by his 
directions, to the chapters on Geometrical Op- 
tics, is reproduced in a beautifully printed royal 
octavo volume of about 370 pages. 
The lectures begin with a short account of the 
Newtonian and the Huyghenian Theories of 
Light and of the objections to each of them. <A 
discussion of the conditions which the propaga- 
tion of plane longitudinal and transverse waves 
through elastic media presupposes, makes clear 
the necessity of ascribing to the ether the elastic 
properties of a solid, if it is to transmit trans- 
verse vibrations mechanically, and leads nat- 
SCIENCE. 
(N.S. Vou. VI. No. 136. 
urally to a preliminary presentation of Max- 
well’s theory based on Faraday’s conceptions 
of magnetic and dielectric polarizations. After 
this introduction, a long chapter is devoted to 
a very simple and clearly written but very 
complete discussion of electro-magnetic oscilla- 
tions, intelligible to any person who already 
has a fair knowledge of the meaning of polari- 
zation and of the differences between ‘real,’ 
‘apparent’ and ‘induced’ magnetic and 
electric densities. This discussion calls atten- 
tion anew to the fact that the nomenclature and 
the notation of the subject are in a very un-. 
satisfactory state. Helmholtz himself some- 
times defined inductivity so as to make that of 
the ether 1 and sometimes so as to make it 47, 
and the editors of these lectures were obliged 
to change the notation in some places so as to 
make the whole book consistent. The subject 
will be needlessly difficult for students so long 
as different writers give the name ‘polarization,’ 
without any modifying clause, to three very 
different quantities. 
In transforming Maxwell’s equations for elec- 
tro-magnetic fields, Helmholtz treats the princi- 
ples first elucidated in his own great paper on 
Vortex Motion, published in 1858, simply as 
analytical devices useful in integrating differ- 
ential equations of a certain form. Neither 
here nor elsewhere in the book does the lecturer 
make any reference to his own contributions to 
the subject. In the third chapter the properties 
of spherical waves are studied in detail, and 
Huyghens’s Principle is put into a very satisfac- 
tory shape by the help of an extended form of 
Green’s Theorem in which the time and the 
space coordinates appear as independent vari- 
ables. This makes it possible to treat Diffraction, 
Interference and Geometrical Optics in the next 
two chapters very much as they are treated in 
older books on the Undulatory Theory of Light. 
The final chapter is devoted mainly to Polari- 
zation, Absorption and Dispersion, and is especi- 
ally interesting since it gives the author’s the- 
ory of Dispersion in its latest form. This theory 
assumes that every molecule of matter is made 
up of two ions, one charged positively and the 
other negatively. The amounts of these two 
charges in any molecule are very large and nu- 
merically equal, and each depends only upon 
