JuLty 16, 1914] 
NATURE 
503 
esting to find there some account of intensity and 
audibility measurements and also of experiments 
to test the theories of vowel sounds. 
The value of the book is increased greatly by 
an excellent collection of examples. 
(2) This work on the Brownian movements is 
the outcome of a dissertation of the author, which 
was an account of a new method of attacking the 
theory of the subject. This method in itself seems 
to open a very promising field, for it can be applied 
to various branches of physics. Its application to 
Brownian movements consists in putting down the 
equation of motion of a particle in the form 
m du/dt= —wu+F, 
where w is given by Stokes’s formula, w=67 (a, 
and F is a force which alters in direction in an 
irregular manner due to collisions with the mole- 
cules of the liquid. The mean velocity of a particle 
is calculated after n collisions, and it is shown how 
the influence of the initial motion diminishes in 
importance as n increases. Then the mean dis- 
tance that a particle gets from its starting point in 
time t is calculated, and this comes to be exactly 
the same as that calculated by Einstein, whose 
formula has been experimentally verified. 
An example of the application of this method 
is to calculate the energy of a magnetic needle at 
the centre of the coil of a tangent galvanometer 
due to a succession of small impulses of current 
in the coil. Some other applications are given, 
one of which has been worked out by Prof. H. A. 
Lorentz. 
The work includes an account of the history of 
the development of our knowledge of the Brownian 
movements. The methods and results of the most 
important experimental researches on the subject 
are given and the theories of Einstein, Smolu- 
chowski, and others discussed. The work of 
Millikan and others on the Brownian movements 
in gases is given a prominent place. 
The new method of treating this interesting 
subject will be found instructive. Also from the 
point of view of a general treatment it can be 
recommended to all seeking a connected account 
of work on Brownian movements. 
' (3) Under the heading Photo-Electricity, is 
usually understood the emission of electricity from 
a metal surface when light falls on it, and the 
present volume is the first to be published which 
is devoted almost entirely to that subject. Dr. 
Allen, however, also includes in his book certain 
other subjects that are allied to the main one, such 
as fluorescence and phosphorescence, photo-chemi- 
cal actions, and photography. Other relations 
between electricity and light such as the alteration 
NO2ae3, VOL: 93 | 
of the resistance of selenium by light are not 
discussed. This is quite easy to understand, 
for the author had quite a large task without 
that. 
Anyone who wishes to obtain a good account of 
the photo-electric effect ought to read this book. 
The subject is treated historically so far as pos- 
sible, and a very clear account of the principal 
experimental work on the subject is given. The 
whole subject is so vast that the author is to be 
congratulated for having collected such a mass of 
results as he has done. There is a clear account 
of the methods for measuring the photo-electric 
current and the velocities of the electrons. The 
chief results for metals and solids and fluids gen- 
erally are given. An _ exceedingly interesting 
chapter on the effect for gases comes about the 
middle of the book, and the importance of this in 
general physics is indicated, such as, for instance, 
the ionisation of the upper atmosphere. 
Perhaps the most difficult task for the author 
was to give an account of the theories which have 
been advanced, and to decide on one as the most 
probable. In the present state of the subject Dr. 
Allen has taken the wisest course in deciding that 
the selective effect points to a resonance between 
the light and the electrons in the molecules, and 
indicating that the normal effect is most probably 
due to the same cause. 
Readers will find the chapter on fluorescence and 
phosphorescence very interesting. A very clear 
account of Stark’s and Lenard’s views on these 
subjects are given. So also will praotical photo- 
eraphers be interested in the chapter on photo- 
graphy. But the chief importance of the book is 
its value to the physicist who has not time to read 
through all the literature on photo-electricity and 
wishes to get a connected account of it. 
(4) This is the first of three volumes which give 
in book-form the substance of a course of lectures 
on general physics at the University of Lille, 
1911-13. The present volume is devoted chiefly to 
electricity and magnetism, which is treated under 
the headings Electrostatics, Magnetism, Current 
Electricity, and Electrons and Ions. In addition 
there are chapters on Gravitation and the Sym- 
metry of Systems. Every part is treated so as to 
introduce the newest results. New work like the 
diffraction of X-rays by crystals, Barkla’s work on 
X-rays, C. T. R. Wilson’s photographs of the 
paths of single ions, and the magneton theory 
follow so logically each in its place, that one does 
not find it strange to see these newest develop- 
ments in a general text-book. Many students will 
find the book of value because of the very clear 
account given of the most modern work in physics. 
