DECEMBER 18, 1913] 
the partial pressure of each component taken 
separately.” So many of my correspondents, including 
Lord Rayleigh, have questioned the grounds of this 
statement, which is the crux of the whole problem, 
that it may be of interest to explain my reasons more 
fully. 
Arguing on the analogy of a gas, it is evident that 
the vibrations of radiation must be regarded as 
adiabatic, and cannot satisfy pu=constant, unless 
the value of the index y (the ratio of the total energy 
E+ pv to the intrinsic energy E) is equal to unity, 
which is impossible. We conclude either that the 
analogy is false, or that, if the vibrations are adiabatic, 
the ratio may have different values for different fre. 
quencies. Since the index y is equal to (E+ pv)/E, it 
is obvious that, to be consistent, we must have 
b=(y—-1)E/v, which agrees with Lord Rayleigh’s 
result for monatomic gases, and may be true generally 
4 for the pressure of adiabatic vibrations. 
According to my theory, radiation consists of the 
: vibration of equal elementary units (Faraday tubes 
. associated with ionic pairs) each possessing the same 
angular momentum, but having intrinsic energy pro- 
portional to the frequency » and independent of the 
temperature T. The pressure is assumed to be equally 
divided between the molecular units according to the 
gas law pu=RT, because this gives the simplest 
_ possible explanation of the exponential term e-*™ in 
_ the radiation formula as a direct consequence of 
= ©Carnot’s Principle, and because equipartition of 
pressure is the most universal condition of equilibrium 
; in physics. 
4 It follows that the ratio of the pressure to the energy 
_ density, denoted by y—1, must be of the form T/ dv, 
which is different for different frequencies at the 
| Same temperature, but gives the mean value 1/3 for 
full radiation. The possibility of having different 
_ values for this ratio is explained by the fact that the 
_ vibrations are adiabatic, and the correction thus intro- 
be duced into the theory of radiation is in this respect 
analogous to that introduced by Laplace into the 
Newtonian theory of the propagation of sound. 
The assumption here made admits of a fairly simple 
experimental test, such as the following. Divide the 
radiation from a source, such as an arc light, into 
two parts of different frequencies. Compare the total 
energies and pressures by suitable means. The ratio 
of the pressure to the energy should be the same for 
each part on Maxwell’s theory. On my theory, the 
part of lower frequency should have the higher pres- 
sure in a determinate ratio. I hoped to be able to 
_ try this crucial test before publishing even an outline 
of my theory, but the rapid extension of the Imperial 
College in recent years has left me insufficient leisure 
for so exacting an experiment, though it might not 
present serious difficulty to an expert in the measure- 
ment of radiation pressure. 
There are many other points in so brief a sketch 
which may require further elucidation, but these must 
be postponed. In the meantime I hope I have suc- 
_ ceeded in demonstrating at least the possibility, if not 
the probability, of the fundamental assumption of my 
theory. H. L. Carrenpar. 
Imperial College of Science, S.W., December 10. 
Scattering in the Gase of Regular Reflection from a 
Transparent Grating: an Analogy to the Reflection 
of X-Rays from Crystals. 
: - 1. The Phenomenon.—No doubt the following 
phenomenon has been noticed before, but I have seen 
, no description of it. If a vertical sheet of white light 
L from a collimator is reflected from the two faces 
_ of a plateglass grating, having about 10,000 or more 
lines to the inch, g being the ruled face, the two 
NO. 2303, VOL. 92] 
NATURE 
451 
beams b and y going to the opaque mirror N are 
respectively vividly blue and brownish-yellow. In 
other words, more blue light is regularly reflected 
from the ruled surface than is transmitted, and more 
reddish light transmitted than is reflected. Since the 
plate grating is not 
quite plane parallel, two 
of the four he: b* and MN 74 
y', are seen in the same 
colours in the telescope. 
This is a great conveni- 
ence in adjusting the g 
displacement _interfero- 
meter, where the spectra 
from b alone are 
wanted, and the y ray 
may be screened off at 
N, while the other y! 
has no spectrum. 
The transmitted rays, t, after reflection show very 
little difference, the one reflected at g being perhaps 
slightly yellowish as compared with the other. 
The spectra from b and y, if compared one above 
the other, are practically identical. The difference is 
not sufficiently marked to be discerned by the eye. 
Multiple reflection from the two faces gave no further 
results. 
Finally, to be coloured blue, the beam must be 
reflected from the air side and not from the glass 
side, where but little appreciable effect is produced. 
If the grating is turned 180°, both the b and y rays 
are nearly white, while the ¢ rays now correspond to 
the b and y rays, and are vividly coloured. 
Outside the ruled surface and with any ordinary 
unruled plate of glass, all images are, of course, 
white. I mention this merely since one might sup- 
pose the absorption or colour of the glass to have 
something to do with the experiment. The film grat- 
ing, where sharp reflection takes place from the glass 
and not appreciably from the film, does not show 
the phenomenon. 
2. Explanation.—Scattering is usually and perhaps 
essentially associated with diffuse reflection. The 
present phenomenon, however, is strictly regular re- 
flection—i.e. there is a wave front, for the blue and 
yellow slit images are absolutely sharp in the tele- 
scope. This is the interesting feature of the pheno- 
menon, which associates it at once with the recent 
famous discovery of Friedrich, Knipping, and Laue 
relative to the reflection of X-rays from the mole- 
cules of crystals, and it is for this reason that I 
direct attention to it. 
In case of the grating the sources of scattered light 
waves are not only identical as to phase, but these 
sources are at the same time equidistant. Hence 
collectively they must determine a wave front of 
somewhat inferior intensity, but otherwise identical 
with the wave front of normally reflected or diffracted 
light—i.e. the wave fronts of regularly reflected and 
scattered light are superposed. 
Moreover, if the grating is turned in azimuth even 
as much as 45° on either side of the impinging beam 
(after which: the many reflections and diffractions 
seriously overlap), the blue and brown colorations are 
distinctly intensified. This also is in accordance with 
anticipations ; for the number of lines which are com- 
prehended within the lateral extent s of the narrow 
beam L, as the angle of incidence i is varied, in- 
creases as s sec 1; whereas the lateral extent of the 
reflected beam is no larger than that of the imping- 
ing beam. Hence there should be increased intensity 
of scattered light in the ratio of sec i, or increasing 
markedly with i from 1 for i=o°, to © for 50a vie 
other words, the scattering lines of the grating are 
virtually more densely disseminated when i increases. 
