AUGUST 21, 1902 | 
streams, introduced by passage through a plate of quartz, 
only the distance travelled in the second zone is to be 
taken into account. It may at once be conceded that 
Jamin’s results do not afford a very striking confirmation 
of Airy’s theory, which may in great measure be at- 
tributed to the experimental methods that he employed ; 
but M. Quesneville in his criticism does not appear to 
have sufficiently recognised the distinction between the 
phase-difference of the streams on emergence into air 
and that of the rectangular plane polarised components 
of the resultant elliptically polarised train of waves. 
In the second section we have the author’s own ex- 
perimental investigations, that were made with double 
prisms of quartz cut in different directions with respect 
to the optic axis and arranged in the shape of rectangular 
parallelopipeds. © When the primitive polarisation is 
circular, M. Quesneville’s theory leads to the same final 
results as that of Airy, but a divergence occurs when the 
initial polarisation is plane. Consequently it is found in 
accordance with both theories that if the light traverse 
the first prism along the optic axis and its path in the 
second be inclined to this direction, the emergent pencils 
can be completely quenched by means of a quarter-wave 
plate and an analyser ; on the other hand, with a pair of 
prisms, such that the direction of propagation of the 
light was in the first perpendicular to the optic axis and 
in the second inclined to it at an angle varying from 5° 
to 9°, M. Quesneville was unable to obtain complete ex- 
tinction of the emergent streams either with an analyser 
alone or with a quarter-wave plate and analyser, what- 
ever might be their orientations. He thence deduced 
the inference that each pencil is formed of two elliptically 
polarised streams of opposite rotations that, having 
traversed the quartz with the same velocity, remain 
superposed on emergence, and on this experiment he 
relies for his proof of the four-fold refraction of quartz 
near the axis. It is noteworthy that this result was only 
obtained with small prisms, a fact for which a very in- 
adequate explanation is offered. 
The last section is devoted to a discussion of the rings 
preduced by plates of quartz in polarised light. Two 
instances must suffice to show the manner in which 
Airy’s formula are treated. On p. 280 it is argued 
that these would give the so-called quadratic curves 
even when the initial and final planes of polarisa- 
tion are parallel, the fact being overlooked that the term 
that introduces this form of the curves has a factor that 
is then equal to zero; and on p. 341 the result that in 
the same circumstances the circles in white light would | 
be black instead of coloured is deduced by equating to 
zero a sum of essentially positive terms. This section, 
however, contains several points of interest, the most im- 
portant being the question whether or no circles exist in 
conjunction with the phenomenon known as Airy’s 
spirals. In the photographs published in the book these 
circles do not appear, though they are present in those 
given by other authors. A question therefore arises as to 
the accuracy of the plates employed by M. Quesneville, 
but if his contention be proved it must be recollected 
that Airy’s result, is confessedly only approximate, and 
it is possible that a more complete inyestigation would 
lead to a formula giving spiral curves alone. 
The book is wanting in the clearness of exposition that 
NO. 1712, VOL. 66] 
NATURE 
| 
387 
we are accustomed to expect from a French writer on a 
physical subject, and it is a matter for regret that the 
author, in his anxiety to make a strong case for his own 
views, should have permitted himself to repeatedly accuse 
Verdet and other physicists of ‘‘ prudently passing over 
in silence” facts that tell against the theory that he is 
attacking. 
ELEMENTARY PHYSICS. 
A Text-book of Physics, with Sections on the Applications 
of Physics to Physiology and Medicine. By R. A. 
Lehfeldt, D.Sc. Pp. 304; 112 figures. (London: 
Edward Arnold, 1902.) Price 6s. 
NRE LERELY new arrangement has been adopted 
in this book in the order in which subjects are 
presented to the student. The traditional order of the 
text-book of physics has been abandoned, in many cases 
with advantage, but often a student will be sadly at loss 
in consequence. He will find it hard to fathom the object 
of proving (p. 81) that the elasticity of a gas is equal to 
its pressure before he is familiar with the idea of the 
elasticity of liquids (p. 85) or of solids (p. 130), and he 
can derive little help in his efforts by being given for the 
time being the definition of “constant ratio of stress ‘to 
strain” when the immediate object is to prove that it is 
not constant, but equal to the variable Z. The study of 
gases alone would never have suggested attaching im- 
portance to the ratio of stress to strain. 
Again, to take another example, the subject order 
adopted involved introducing in connection with con- 
ductivity of heat (p. 72) the question of the anomalous 
expansion of water described later (p. 90). 
Likewise, in treating of electrolysis (p. 188), the term 
electrical charge is used without explanation. In the 
order adopted this does not appear until p. 249. 
Of course, it is not always possible to avoid such an- 
ticipations. Students often, however, in consequence 
fancy they do not understand the point in hand, when it is 
really the anticipation which is troubling them. 
In the order of «the chapters, “ Heat” comes before 
“Properties of Fluids,” and next comes “ Properties of 
Solids.” Sometimes the analogies adopted in conse- 
quence come quaintly to one accustomed to the old 
traditional order of things, and are apt to appear upside 
down, as the analogy taken from conductivity of heat to 
help the reader to grasp the idea of flow in liquids. 
The work contains a vast number of distinct things for 
its size. Scarcely any branch of physics is omitted, but 
it is a question whether there is not too much in the book 
and whether less matter more carefully arranged would 
not have better chance of sticking. We must avoid 
giving our students mental indigestion from overloading. 
There is an unpleasant feeling throughout of being 
rather rushed, and that nothing must be left out which 
the external examiner may perchance alight on in setting 
the paper. Perhaps the fault, if it be one, may not lie 
with the book. 
The attractions and repulsions of currents appear to 
be no further alluded to than in the statement de- 
scribing the Kelvin current balances, that “of the two 
fixed coils on the right, one attracts the movable coil 
