aan 
NOVEMBER 16, 1916] 
together instead of distributed between the strati- 
graphical chapters. 
Among the excellent features of this part is an 
explanation of the binomial nomenclature, which 
students are often expected to understand without 
any such help; but though the first edition of the 
“Systema Natura” was published in 1735, that 
is not accepted as the date of the establishment 
by Linneus of the binomial system. The relega- 
tion of the technical names of the fossils illustrated 
in the text to an appendix is symptomatic of the 
present state of paleontological nomenclature; 
only general names are given in the legends of the 
figures. ‘The use of popular names has the draw- 
back that they vary so much locally, and English 
students are not likely to know what are meant 
by Sowbugss or Pillbugs (p. 605). The author 
accepts as undoubted the identification of some 
impressions in pre-Cambrian rocks of Brittany as 
radiolaria, though this conclusion is rejected by 
some who have examined the material. There is 
less evidence for the author’s view that Eozoon 
is a caleareous alga than for its original reference 
to the Foraminifera, and it seems rather a “bull ” 
to refer to some of these plants as fresh-water 
seaweeds, 
In the historical geology the chief departures 
from the usual classification are the adoption of 
two additional systems; of these the Ozarkian 
System occurs between the Cambrian and the 
Ordovician, but as the Beekmantown beds are 
excluded its value appears doubtful. If the 
Beekmantown and allied faunas be included in 
the Ozarkian System a much stronger case could 
be made out for it. The Lower Cretaceous is 
raised: to a system, the Comanchian, which in- 
cludes from the Wealden to Albian inclusive. 
Prof. Schuchert’s account of the historical geo- 
graphy is illustrated by admirable maps of geo- 
graphical distribution; it gives a most useful 
summary of the stratigraphy of North America, 
and its up-to-date account of the principles of 
stratigraphy could be read with advantage by 
all British teachers of geology; but as the his- 
torical geology of the British area is incomplete 
and less accurate, this part of the volume will be 
of less value to British students than Prof. 
Pirsson’s section. c 
The most serious drawback to this valuable 
text-book is the absence of references. 
Je Wee Ge 
RADIOGRAPHIC METHODS. 
Localization by X-rays and Stereoscopy. By Sir 
J. Mackenzie Davidson. Pp. xi+72+xxvi 
plates. (London: H. K. Lewis and Co., Ltd., 
1916.) Price 7s. 6d. net. 
re this book the author describes in detail 
several of the methods which have been 
devised for the accurate localisation of foreign 
objects in the human body. 
The first two chapters deal with the experimental 
‘conditions which should be observed in order that 
NO. 2455, VOL. 98] 
NATURE 
207 
good radiographs may be obtained, and with some 
simple yet excellent experiments illustrating the 
radiographic advantage of a good focus-point on 
the anti-kathode. The undesirable effects of 
secondary radiation receive mention; they are re- 
sponsible for a good many of the defects which 
occur in radiographs, and constitute a danger 
(second only to the primary rays) to the operator, 
which the author does well to insist upon. 
The main facts of X-ray stereoscopy are de- 
scribed and illustrated. By transposition of two 
stereoscopic photographs the point of view of the 
observer becomes reversed; some explanation of 
this would be of great assistance to the beginner. 
The advantages of stereoscopy to the surgeon 
who is to remove the foreign object are obvious, 
and we agree with the author that some successful 
method of rendering stereoscopic images upon a 
fluorescent screen would be a great advance ‘on 
present procedure, There are, however, many ex- 
perimental difficulties to be overcome before this 
can be effected. 
The author has done much to elaborate a precise 
means of localising foreign objects in the human 
body, and all the details of the “cross-thread” 
method are entered into. In cases where a foreign 
body is lodged in the eye or the orbit, precise 
localisation is absolutely essential; the chapter 
devoted to such cases is perhaps the best in the 
book. 
The author directs attention to the misleading 
nature of-a single X-ray photograph, but describes 
a method by which the depth of a foreign body 
below the surface may be obtained by means of a 
single X-ray exposure; this method entails the 
use of two sets of cross-wires which are placed 
at a known vertical distance apart. An oblique 
ray from the anti-kathode casts a shadow of the 
foreign body and of the lower set of cross-wires 
with respect to the other set of cross-wires, which 
is in contact with the photographic plate; simple 
measurements from the single photograph give the 
vertical depth of the foreign body below any point 
previously selected on the surface. 
The book closes with a series of twenty-one 
stereoscopic illustrations on plates, which will 
repay careful study; it is worth noting that stereo- 
scopic vision may be very considerably improved 
by practice. 
Much of the wreckage of human life occa- 
sioned by the war would be past repair were 
it not for the extra vision vouchsafed to the 
surgeon by X-rays; whether in dealing with pro- 
jectiles which have entered the body or in the 
damage to the structures, bony and otherwise, 
methods have been evolved which enable the sur- 
geon to know exactly where to look and almost 
as surely what to find. The attitude of the 
author is that too much information cannot be 
given to the surgeon thus engaged, and it is safe 
to say that the methods of: localisation described, 
together with a stereoscopic picture of the 
foreign. body giving its relation tothe neighbour- 
ing anatomical parts, inspire confidence. 
