500 
NATURE 
[JANUARY I, I914 
Such, in brief, is the “burden” of this erudite 
but eminently readable book. It is a fine text- 
book of open-air history, an attempt to write his- 
tory “writ large on the face of the country i 
(p. vii). The available documents are read and 
expounded in situ, so to speak. Archeology, 
traditions, and-folklore “assign their true value 
to records which have hitherto been loosely read” 
(p. viii). Accounts of the Scandinavian invasions 
of western England are read, very properly, in 
the light of northern antiquities. The first 
Danish invaders allied themselves with the 
“bottled-up”” “One and Alls,” and we learn 
much about peaceful Danish settlements on the 
coasts of the Severn. Two archeological maps, 
many plans and diagrams of camps, and a copious 
index mark the thoroughness and finish which 
characterise the whole work. JOHN GRIFFITH. 
The British Journal Photographic Almanac, 1914. 
Edited by G. E. Brown. Pp. 1496. (London: 
Henry Greenwood and Co.) Price 1s. net. 
To photographers the approaching end of the 
year and the beginning of a new one is always 
heralded by the announcement of the publication 
of this almost indispensable year-book, which is 
so familiar to them and a natural fixture in their 
studios. The copious material contained between 
the two covers and the useful facts embodied in 
it has made it a book of reference difficult to part 
with. The issue for this year follows mainly the 
lines of its predecessors, but new features of course 
have been inserted. These, to state them briefly, 
comprise a glossary of photographic terms, which, 
no doubt, will be helpful to many a beginner in 
the subject of photography. 
Lists are given of the German, French, and 
Italian equivalents for the chief appliances and 
operations, and these should be most serviceable 
to those who study foreign photographic journals 
and books, but have no technical dictionary 
at their elbow. The beginner is also favourably 
treated with an excellent series of reproductions 
of negatives incorrectly exposed and developed, 
which should show him more than words can 
express what he must avoid; the accompanying 
text will also prove of service. The epitome of 
progress, novelties in apparatus, formula for the 
principal photographic processes, miscellaneous 
information, tables, &c., are as full and complete 
as ever, and the great number of advertisements 
are a valuable feature of the volume. 
Hazeli’s Annual for 1914. Edited by T. A. 
Ingram. Pp. exiiit592. (London: Hazell, 
Watson and Viney, Lid., 1914.) Price 3. 6d. net. 
In addition to its revision up to November 25 
last, this twenty-ninth issue of “ Hazell’s Annual” 
contains a section entitled “Occurrences during 
Printing.” It justifies its claim to give the most 
recent information on the topics of the day. A 
section running to some forty pages is headed 
“The March of Science,” and provides a summary 
of progress made in the world of science during 
1913. Amn jaindex containing 10,000 references 
makes it easy for the reader to find his way about 
the volume. 
NO. 2305, VOL. 92] 
LETTERS TO THE EDITOR. 
(The Editor does not hold himself responsible for 
opinions expressed by his cotrespondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other purt of Nature. No notice is 
taken of anonymous communications. ] 
The Pressure of Radiation. 
Witn reference to my letter on this subject im 
Nature of December 18, the majority of my corre- 
spondents complain that, although I may have indi- 
cated the possibility of my own view, I have not 
shown why the simpler ielation, that the pressure is 
always one-third of the energy density, is untenable. 
As I cannot reply to each individually, I shall be glad 
if you will allow me space to rectify this omission. 
It is generally admitted that the total heat re uired 
for the emission (or evolved in the absorption) of unit 
volume of radiation is the sum of the intrinsic energy 
density, E/v, and the external work ~. By Carnot’s 
principle, this must be equal to T(dp/dT). Whence, 
if E/v=3p, we obtain immediately the fourth power 
law for full radiation in the usual manner. It would 
appear, however, by similar reasoning, if E/v is equal 
to 3p for each separate frequency, that radiation of 
constant frequency should also increase with tempera- 
ture according to the fourth power law, which is cer- 
tainly not the case. Either Carnot’s principle does 
not apply, or E/v is not equal to 3p for each separate 
frequency. I have chosen the latter alternative. 
It has been shown by Lord Rayleigh, Lorentz, 
Larmor, Jeans, and others that the electromagnetic 
equations (from which E/v=3p was first de uced} 
lead inexorably to Rayleigh’s formula, 
,=8rRA-4T/N, 
without the exponential term, for the partition of 
energy in full radiation per unit range of wave- 
length ?. This result appears to be true in the limit 
for long waves and high temperatures, but is other- 
wise so hopelessly at variance with experiment as to 
suggest that something may have been overlooked in 
the application or interpretation of the equations. 
Some of my correspondents point out that Nichols 
and Hull have already shown by experiment that the 
pressure of a beam of light is equal to the energy 
density irrespective of wave-length. According to my 
theory, the mechanical effect which they measured 
should be equal to the total energy density, E/u+p, 
as deduced from their energy measurements. Their 
result is in perfect agreement with my theory, but it 
is not quite such a simple matter (and may even prove 
to be impossible) to measure p separately from E/v, 
which is the experiment that I proposed to attack. 
H. L. CaLrenDar. 
Imperial College of Science, S.W., 
December 27, 1913. 
Atomic Models and X-Ray Spectra. 
Mr. H. G. K. Mosexey has published in the Decem- 
| ber issue of the Philosophical Magazine a very in- 
teresting paper describing his measurements of the 
wave-lengths of the characteristic X-ray lines of 
various metals. He has succeeded in calculating the 
wave-lengths of one-half of the lines he observed, 
assuming Bohr’s atom and supposing the positive 
charge on the nucleus to correspond to the place of 
the element in the periodic table as suggested by 
| van den Broek. He concludes that the agreement 
between calculated and observed wave-lengths strongly 
supports the views of Rutherford and of Bohr. 
It appears to me that Moseley’s research really only 
supports the views of Rutherford and of van den 
| Broek. As I propose to show in detail in a paper to 
