612 
NATURE 
[AUGUST 5, 1915 

—investigations which have had important con- 
sequences.in many directions. Not only did the 
Cavendish Laboratory take a prominent part in 
the earlier discoveries, but many of the important 
later contributions in this subject have been made 
either by the students in that laboratory or by 
those who had come under its influence. In this 
review, however, we must not omit to mention 
the earlier contributions of Perrin and Sagnac, 
and the important theoretical and experimental 
investigations of Langevin, who for a time was 
an advanced student in the Cavendish laboratory. 
The present volume is a very lucid and admir- 
able exposition of our knowledge on this subject. 
The various theories which have been proposed 
and the experimental data are analysed with 
critical skill, and there are numerous original 
calculations added by the author himself. The 
mathematical theory of the subject is worked out 
with considerable detail, but is controlled at all 
points by careful consideration of the experi- 
mental data. The theory of the relation between 
current and voltage for ionised gases is mainly 
confined to the case of uniform ‘ionisation, which, 
as is well known, presents many mathematical 
difficulties. No mention is made of the interest- 
ing cases that arise when the ionisation is non- 
uniform, or mainly confined to one electrode. 
Such cases often arise in experimental work, and 
it is important to direct the attention of the 
student to the marked differences in the current- 
voltage curves which are exhibited under different 
conditions of distribution of the ionisation. While 
the author does not profess to discuss the whole 
of the large number of papers that have been 
published in this field, a clear account is given of 
all the more important investigations, and great 
care has been taken in the numerous references 
to literature. It is probable that many of those 
who have followed closely the rise of this sub- 
ject will not in all cases agree with the relative 
importance tacitly assigned to practically inde- 
pendent investigations in this field; but this, after 
all, is largely a matter of personal opinion. We 
can strongly recommend the present work as a 
sound and valuable contribution to our know- 
ledge of the development of a very interesting and 
important branch of modern physics. 
R. 

THE PRINCIPLE OF RELATIVITY. 
The Principle of Relativity. By E. Cunningham. 
Pp. xiv+221. (Cambridge: At the University 
Press, 1914.) Price gs. net. 
VEN to the present day, the existence of 
the Principle of Relativity, which arose as 
a branch of physical theory, is somewhat trouble- 
NO. 2388, VOL. 95] 

some to the individual physicist, owing to the 
efforts of his philosophical friends to obtain a 
precise account of its origin, treated from an his- 
torical point of view, which can emphasise the 
exact movement of thought leading to its 
adoption. The literature of the subject has been 
scattered in various periodicals, and so loaded 
with mathematical symbols that its appeal has 
been to a very limited audience, in spite of the 
real simplicity of all the ideas concerned. ‘Three 
recent volumes will serve to remove this diffi- 
culty very effectually, and two of them possess 
the advantage of proceeding from entirely different 
points of view. The first, by Dr. Robb, has 
already been noticed in these columns, and the 
second is the volume by Mr. Cunningham now 
under review, which is designed to fill the gap 
specially indicated in our first sentence. The 
third, by Dr. Silberstein, already noticed also, is 
similar to Mr. Cunningham’s, but is based on a 
different mathematical treatment. 
Mr. Cunningham’s intention throughout has 
been to consider those aspects of the Principle 
which have a direct bearing on the practical ques- 
tions of physics, without more recourse to mathe- 
matics than is absolutely essential for a really 
valuable consideration of these questions, and it 
may be said at once that the simplicity of the 
treatment is remarkable. The matter is put in a 
very interesting way throughout, and even to the 
non-mathematician the volume is not difficult to 
read. The first part gives a very lucid outline 
of the manner in which the Principle grew out of 
electrical theory, and one feature which must be 
recorded with satisfaction is the accuracy of the 
account, which does not share a tendency, noted 
in the case of several writers, to bestow too much 
of the scientific credit for the Principle on those 
who have worked out the details, at the expense of 
the real originators, one of whom belongs to this 
country. The space devoted to any problem is 
strictly commensurate with its importance. 
The author makes it clear in this section that 
there is a place for the Principle as a hypothesis 
supplementary to electrical theory, and on account 
of the necessary limits of that theory, independent 
of it. He avoids, at the same time, excursions 
into questions with metaphysical bearing. 
In the second part of the work an excellent 
account of Minkowski’s two representations is 
given. The four-dimensional world of Minkowski 
is by far the most interesting development of the 
Principle, and, unfortunately, hitherto the most in- 
accessible in this country. This section is un- 
doubtedly the most valuable part of the book for 
the general reader, and although it is essentially 
mathematical, the author has minimised the diffi- 
