May 13, 1886] 
NATURE 27 
ACOUSTICS, LIGHT, AND HEAT 
Acoustics, Light, and Heat. By William Lees, M.A., 
Lecturer on Natural Philosophy, the Heriot Watt 
College, and Lecturer on Mathematics and Experi- 
mental Physics, Free Church Training College, Edin- 
burgh. New and Enlarged Edition. (London and 
Glasgow: Wm. Collins, Sons, and Co.) 
THIS is one out of many of the text-books which 
have been called into existence by the “May” 
Examinations of the Science and Art Department. Being 
written especially to meet the requirements of the student 
who wishes to pass these examinations, it is only brought 
up to the standard given in the directory of the Depart- 
ment, and may therefore for this purpose be useful. The 
fact that a new and enlarged edition is now appearing is 
certainly evidence that this is the case. To make it 
more serviceable, the questions of all the May Examina- 
tions in Subject VIII. from 1872 to 1885 are given. 
Though the simple and numerous diagrams and the 
generally clear nature of the text give it a certain value 
as a text-book, it is by no means so free from faults and 
ambiguities as might be expected in a new edition. 
It may be well to refer especially to a few places where 
alterations suggest themselves. 
Figs. 19 and 20 show the contrast between a musical 
sound anda noise. Though it is explained that “noises 
are due to irregular vibration or a confused mixture of 
musical sounds which produce aérial waves of great com- 
plexity and wanting in periodicity,” no explanation is 
offered of a peculiarity in the “ curve of anoise” (Fig. 20), 
which in three places is actually made to slope back- 
wards. 
Some of the figures in optics are rather wanting in pre- 
cision. Thus in Fig. 73, which shows a real image 
formed by a concave mirror, a pair of slightly diverging 
rays are made to cross between the mirror and the princi- 
pal focus. Again, Fig. 77 showsa caustic on the surface 
of milk in a glass with its cusp reaching close to the 
centre. Fig. 103 shows the action of a refracting plate on 
a beam of light by the turning and approximation of 
successive wave-fronts. Those two wave-fronts which 
obliquely cut the surface are shown straight and partly 
swung round, as if they were rigid lines meeting with 
resistance at one end. It would surely have been better 
to have bent the line at the point of intersection, leaving 
all the wave-fronts and parts of a front outside the 
medium parallel to one another, and also all inside 
parallel to one another, but it is possible that wave-fronts, 
strictly speaking, are not intended. 
The explanation of so important a thing as the 
achromatic lens can hardly be considered satisfactory. 
Owing to its brevity it is possible to give this in full. 
“This defect in a lens [the defect of chromatic aberra- 
tion] is obviated by the combination of a double convex 
. lens of crow glass, with a convexo-concave of /n¢ glass 
(Fig. 122). The effect of the second lens is to re-blend 
the coloured rays which the first has produced, and at 
the same time such an amount of refraction is preserved 
as to bring the light toa focus.” As nowhere is it directly 
pointed out that for the same degree of refraction flint glass 
produces more dispersion than crown, it is not difficult to 
imagine that a student might fail to form any very clear 
idea of the principle of the achromatic lens, nor is he 
likely to be materially helped by the figure (122), which 
certainly does not represent either the section or any 
other view of any achromatic lens that was ever made. 
If it were not for the section lines it would be a good 
perspective drawing of ashort cylinder ; the ellipse which 
appears to be the end of such a cylinder is really meant 
to show the crown lens in section, and the figure of 
uniform thickness by its side, as thick everywhere as the 
ellipse is in the middle, which seems to be the side 
of the cylinder, is meant for the section of the flint lens. 
Simplicity in a diagram is a thing to be desired, but 
there is more than simplicity here. 
Very little is said about spectrum analysis; and its 
application to the measurement of the motion of the 
heavenly bodies in the line of sight is not even men- 
tioned. 
The general weakness of the optical part is to a 
certain extent compensated for by the chapters on polar- 
isation, which have much to recommend them. There is 
here, however, a paragraph which requires explanation. 
“Now it is found that whatever quantity of polarised 
light there is for any incidence other than the polarising 
angle in the reflected beam, there is always the same 
quantity in the refracted beam. At the polarising angle, 
however, the refracted beam exhibits traces of polarisa- 
tion.” What is meant by this distinction is by no means 
clear. : 
Heat is more precisely and clearly treated than light, 
but here the general excellence is marred by an example 
to illustrate expansion in which the working out of the 
result shows that the obvious meaning of the ques- 
tion is not intended. What any one would under- 
stand by the words, “ Find the length of a rod of brass 
which would expand equally with a rod of steel 3 feet 
long under a change of temperature of 10° C.,” is evidently 
—Find what length of brass will increase in length by 
the same amount that a 3-foot rod of steel does for a 
change of 10° C. But what is found in the working of 
the answer is the length of a piece of brass which will 
expand so as to be as dong as a piece of steel 3 feet long 
when each is vazsed 10° C. 
Sufficient has been said to show that this book is not 
as clear and accurate in either the text or the figures as 
might be expected in a new edition. 
OUR BOOK SHELF 
Cholera Curable. By John Chapman, M.D. 
Churchill, 1885.) 
Dr. CHAPMAN has had the opportunity of testing, in the 
Hépital de la Charité in Paris, his method of the so- 
called neuro-dynamic treatment in Asiatic cholera, and 
his demonstration of the success of this treatment 
constitutes, we take it, the cardinal motive for the 
production of this book, although a good many other, 
mostly theoretical, considerations are brought into the 
discussion. 
The symptoms of Asiatic cholera are explained by a 
number of assumptions on the action of the spinal cord 
and the sympathetic nervous system, but as to which we 
look in vain for experimental proof. The theories con- 
cerning the etiology and causation of cholera are fully 
treated, and then Dr. Chapman promises to furnish us 
with a complete solution of these problems in a discovery 
made by him as to the cause of cholera. When, how- 
ever, we come to analyse what he really has discovered, 
(London : 
