JuLy 30, 1914] 
The Horticultural Notebook. Compiled by J. C. 
Newsham, Third edition, thoroughly revised. 
Pp. xx +418. (London: Crosby Lockwood and 
Son, 1914.) Price 4s. 6d. net. 
Tue fact that this work of reference has reached 
its third edition, and that its price has been re- 
duced, proves that its usefulness is now generally 
recognised. It is, indeed, a book of convenient 
size and shape, which anyone whose interests are 
largely bound up in horticulture will find useful 
to have on his writing-table. As everyone knows 
who follows this pursuit, minor problems are crop- 
ping up almost every day of one’s life. The 
strength of an insecticide or a manure, some 
simple way of ascertaining the height of a tree 
without climbing it, the right dimensions of a lawn 
tennis court, how to make a grafting wax: these 
are samples of the kind of question for which 
those concerned with gardens are constantly need- 
ing an answer. This the “Horticultural Note- 
Book ” sets out to supply, and we do not find that 
it often fails. 
Although the serious student will need some- 
thing more detailed than is here furnished, the 
book is not devoid of scientific teaching. A 
synopsis of the natural orders of plants, for in- 
stance, is concisely and conveniently arranged and 
helpful in “running down” a plant. It is not, 
however, in this direction (which suggests too 
much a shilling encyclopedia) that the value of 
the book consists, so much as in the collection of 
garden recipes and rules, and in much tabulated 
information. The ancient and remarkably per- 
sistent error that the plane tree of the streets is 
the American Platanus occidentalis is once more 
repeated here (p. 368), although it has ‘several 
times been pointed out in these columns that the 
tree is really the Old World P. acerifolia. 
Ornamental Lathework for Amateurs. By 
C. H. C. Pp. 121+xii plates. (London: Per- 
cival Marshall and Co., n.d.) Price 3s. 6d. net. 
PLAIN turning is carried out in an ordinary lathe 
by revolving the work and operating on it by tools 
held in the hand or in a slide-rest. In ornamental 
turning, an object already subjected to plain turn- 
ing processes is ornamented by further operations 
carried out on it by cutters which are made to 
revolve independently of the lathe mandrel. Orna- 
mental turning is an exceedingly beautiful art, and 
the object of the little book before us is to awaken 
the interest of those who have adopted turning as 
a hobby, and to show how simply an ordinary 
turning lathe may be modified so as to be capable 
of producing beautiful examples of ornamental 
turning. While many examples are given and 
illustrated by photographs, it is not the author’s 
intention that these should be used as designs to 
be worked out, but rather to stimulate the worker 
to devise new designs and methods for himself. 
Drawings of many useful types of tools are given, 
mostly of a simple character. The book can be 
recommended as a useful introduction to any 
amateur turner who has not yet taken up this 
fascinating branch of his art. 
NO. 2335, VOL. 93] 
NATURE 
| 
I 
LETTERS. TO THE EDITOR. 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications. ] 
Rayleigh’s Law of Extinction and the Quantum 
Hypothesis. 
Tue bearing of Rayleigh’s law of gaseous extinction 
on some of the fundamental aspects of radiation theory 
does not seem to have been sufficiently emphasised in 
recent reports and publications on modern molecular 
physics. The coefficient of attenuation « of radiation 
of wave-length A travelling through a gas containing 
n, molecules per unit volume was given by Rayleigh ' 
so long ago as 1871 in the form x=$2°(u?,—1)?A-*/no, 
uo being the refractive index of the gas. It is of 
importance to notice that the law in question is one 
of the most fundamental results of molecular dynamics, 
its final expression being an invariant with respect to 
the theories of the zther or of the molecule employed,’ 
while in its derivation there is no need to draw on 
resources outside classical dynamics and continuous 
energy-flow. From the point of view of elementary 
electromagnetic theory, the above expression for k is 
very easily derived along lines suggested in a problem 
set in part ii. of the ‘‘ Mathematical Tripos ”*; use is 
made of the conventional electrical doublet set into 
forced vibrations by a train of electromagnetic waves; 
by making use of the radiation formula for accelerated 
charges and Poynting’s theorem, the flow of energy 
from the doublet is easily calculated in terms of the 
amplitude of vibration; the oscillations of the doublet 
contribute a term to Maxwell’s displacement current, 
enabling the amplitude to be expressed in terms of the 
refractive index of the gas; by considering the deple- 
tion of energy from the original beam as a result of 
this scattering, and eliminating the amplitude, the 
above expression for x is easily obtained. In a recent 
paper, Natanson * has subjected the derivation of Ray- 
leigh’s law to minute criticism on the grounds of the 
classical electromagnetic theory, allowing for a damp- 
ing term arising from the mechanical reaction due to 
radiation, and taking into special consideration the 
summation of the aggregate radiation from the 
random distribution of doublets which are supposed to 
constitute the molecules of the gas; the final result is 
a vindication of the above expression for the coefficient 
of attenuation to a very high order of accuracy. It 
may be noticed in passing that the same electro- 
magnetic system forms the basis of Planck’s® theory 
of ‘tblack-body” radiation, the interpretation of ex- 
periment in this case, however, necessitating the hypo- 
thesis of discontinucus energy-flow, or the emission 
of energy by ‘‘ quanta.” 
For an adequate experimental verification of Ray- 
leigh’s law recourse must be had to observations on 
the extinction of solar radiation of different wave- 
lengths by the earth’s atmosphere. The importance 
of the observations of the Smithsonian Astrophysical 
Observatory on atmospheric transmission recently car- 
ried out by Abbot and Fowle® in connection with 
their determinations of the solar constant at Mount 
Wilson, in furnishing material for a study of mole- 
1 Rayleigh, PAz. Mag., xli., pp. 107, 274, 447 (1871); ‘‘ Collected Works,” 
i., pp. 87, 104, 518. 
2 Schuster, ‘‘ Theory of Optics,” 2nd ed. (1909), p- 325- 
3 Mathematical Tripos, Part ii., lune 2, 1906. 
4 Natanson, Bull. Luter. de lV Acadéniie des Sciences de Cracovie 
January 5, ror4. 
5-Planck, ‘Theory of Heat. Radiation” (Trans. by Masius, Blakiston’s, 
Philadelphia, 1914). part iv., chap. ili. p. 165. 
6 Annals of the Smithsonian Astrophysical Observatory. Washington > 
| vol. ii. (1908) ; vol. iii. (1913). 
