
al e:. 
June 16, 1923] 
* 
NATURE 
805 

followed by an explanation of terms and elementary 
principles. Next there are chapters dealing with 
various aspects of photometry, leading to a discussion 
on calculations of illumination. Finally we have, in 
the second half of the book, a general discussion 
of practical lighting problems. Consideration of the 
human eye forms a natural starting-point, and in 
general a logical sequence of subjects is adopted. One 
may question, however, whether the separation of 
heterochromatic photometry and the flicker - photo- 
meter in the penultimate chapter is desirable. Surely 
this might more fitly have been included in the earlier 
section of the book dealing with photometry in general ? 
Mr. Walsh’s experience at the National Physical | 
Laboratory has stood him in good stead in dealing 
with this phase of the subject. The hints on laboratory 
practice are sound, and there is a useful description | 
of the chief forms of illumination-photometers. It is 
interesting to note that, with proper precautions, an 
accuracy of 2-3 per cent. is considered possible with 
this class of instrument. The chapter on industrial 
lighting contains a useful survey of the work of the 
Home Office Departmental Committee on Lighting in 
Factories and Workshops, and the contents of various 
American codes. One is glad to note the inclusion 
of a chapter on daylight illumination, which is now 
being studied in a more scientific manner than in 
the past. ? 
The final chapters on colour and light-projection 
include a variety of special applications of light such 
‘as motor-car headlights, searchlights, flood-lighting, 
and artificial daylight. (In the calculation of flood- 
lighting on p. 189 a slip appears to have been made.) 
The book is concluded by a series of definitions of the 
chief photometric quantities, a bibliography, and an 
adequate index. 
L’Arc électrique. Par Maurice Leblanc fils. (Recueil 
des Conférences-Rapports de Documentation sur la 
Physique. Vol. 3, 1 Série, Conférences 7, 8. 
Edité par la Société Journal de Physique.) Pp. 131. 
(Paris : Les Presses Universitaires de France, 1922.) 
to francs. 
Tue first chapter of the work under notice contains 
the more important formule obtained by modern 
physicists in connexion with the electric arc. A 
drawback to the use of these formule is that it is 
difficult to find out where theory ends and empiricism 
begins. If we accept the formule we have to abandon 
the theory of dimensions as applied to equations. In 
the second chapter Mrs. Ayrton’s work is well described. 
It is stated that Blondel was the first to prove that_ 
there was no appreciable counter electromotive force 
in the arc. If E denotes the potential difference across 
the arc and I the current through it, then dE/dI is 
called the resistance of the arc, and it is pointed out 
that it is a negative quantity. A good description is 
given of magnetic and mercury vapour lamps. The 
phenomena shown by the so-called ‘“ non-arcing ” 
metals, such as phosphor-bronze, aluminium-bronze, 
aluminium and zinc, are attributed to a metallic oxide 
covering the electrodes with an insulating layer or 
to the vapour being difficult to ionise. It is pointed 
out that although the current and voltage vanish 
instantaneously in alternating current arcs, yet their 
NO. 2798, VOL. 111] 

_ against lightning. 
power factor is not unity. This is stated to be due to 
| the fact that they do not both follow the sine law. As 
a matter of fact, provided that the ordinate of the 
current wave is always proportional to the ordinate 
_ of the voltage wave, and they vanish instantaneously, 
the power factor would be unity whatever the shape 
of the voltage wave. The third chapter gives the 
technical applications of the electric arc to searchlights, 
| furnaces, electric welding, for producing high-frequency 
waves, and in electric “ safety valves” for protection 
A. R. 
By G. Wentworth, D. E. 
(Wentworth-Smith 
(London : Ginn 
Machine-Shop Mathematics. 
Smith, and H. D. Harper. 
Mathematical Series.) Pp. v +162. 
and Co., 1922.) 55. 6d. net. 
THE presentation, in most cases without proof, of 
a hundred and twenty formule in as many pages can 
scarcely lay claim to the title of mathematics, but this 
book forms nevertheless a clearly worded and practical 
introduction to machine-shop calculations. Its scope 
is confessedly limited to the needs of those who hope 
to become expert machinists with little or no mathe- 
matical grounding, and throughout the book more 
attention is paid to the explanation of technological 
details than to the development of method. The use 
of measuring instruments and calculations affecting 
cutting speeds, taper turning, screw cutting, indexing 
and gear cutting are clearly described, generally with 
the aid of excellent diagrams. The number of formule 
is large, and a judicious reduction in this respect would 
bring emphasis on to the more important without 
prejudice to the range of the book. Illustrative 
problems are worked out in the text, and examples 
throughout are numerous and well chosen. Their value 
to the self-dependent student would, however, be greatly 
enhanced if answers were given. Calculations are in 
most cases made to cover British as well as American 
practice, but reference tables at the end give American 
standards only. H. W. 5S. 
An Introduction to the Psychology of Religion. By 
R. H. Thouless. Pp. vi+286. (Cambridge: At 
the University Press, 1923.) 75. 6d. net. 
THERE is an undoubted movement of thought towards 
a restatement of religion and religious problems along 
the lines of recent psychological achievement. This 
has already taken the place, to a large extent, of the 
apologetic defence of religion on the plane of the 
sciences of Nature. Mr. Thouless writes for those 
who wish to make a study of the problems of religion 
from a psychological point of view without any prior 
knowledge of psychological terminology. The most 
interesting parts of his book are those in which he 
relates religion to certain of the instinctive tendencies 
of man, while defending it from the charge of being no 
more than a subjective experience of gregariousness 
or sex ; and his chapter on the phenomena of mysticism, 
for which he goes in the main to the mystics of the 
Roman Catholic Church. He treats this subject with 
great insight and exactness, and interprets it sym- 
pathetically in the light of recent psychoanalytical 
theory. The book, while avowedly “ popular,” will 
be of interest to students both of religion and of 
psychology. 
