——; — = °&«xOF ss. 
DECEMBER II, 1913] 
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.] 
The Structure of the Atom. 
In a letter to this journal last week, Mr. Soddy has 
discussed the bearing of my theory of the nucleus 
atom on radio-active phenomena, and seems to be 
under the impression that I hold the view that the 
nucleus must consist entirely of positive electricity. 
As a matter of fact, I have not discussed in any 
detail the question of the constitution of the nucleus 
beyond the statement that it must have a resultant 
positive charge. There appears to me no doubt that 
the a particle does arise from the nucleus, and I have 
thought for some time that the evidence points to the 
conclusion that the 8 particle has asimilarorigin. This 
point has been discussed in some detail in a recent paper 
by Bohr (Phil. Mag., September, 1913). The strongest 
evidence in support of this view is, to my mind, (1) 
that the 8 ray, like the « ray, transformations are 
independent of physical and chemical conditions, and 
(2) that the energy emitted in the form of 6 and 
y rays by the transformation of an atom of radium 
C is much greater than could be expected to be 
stored up in the external electronic system. At the 
same time, I think it very likely that a considerable 
fraction of the 6 rays which are expelled from radio- 
active substances arise from the external electrons. 
This, however, is probably a secondary effect result- 
ing from the primary expulsion of a 6 particle from 
the nucleus. 3 
The original suggestion of van der Broek that 
the charge on the nucleus is equal to the atomic 
number and not to half the atomic weight seems to 
me very promising. This idea has already been used 
by Bohr in his theory of the constitution of atoms. 
The strongest and most convincing evidence in sup- 
port of this hypothesis will be found in a paper by 
Moseley in The Philosophical Magazine of this 
month. He there shows that the frequency of the 
X radiations from a number of elements can be 
simply explained if the number of unit charges on 
the nucleus is equal to the atomic number. It would 
appear that the charge on the nucleus is the funda- 
mental constant which determines the physical and 
chemical properties of the atom, while the atomic 
weight, although it approximately follows the order 
of the nucleus charge, is probably a complicated 
function of the latter depending on the detailed 
structure of the nucleus. E. RutHERFORD. 
Manchester, December 6, 1913. 
The Reflection of X-Rays. 
In view of the great interest of Prof. Bragg’s and 
Messrs. Moseley and Darwin’s researches on the dis- 
tribution of the intensity of the primary radiation from 
X-ray tubes, it may be of interest to describe an 
alternate method which I have found very convenient 
(Comptes rendus, November 17, 1913). 
As we know, the wave-length of the reflected ray 
is defined by the equation nA=2dsin 6, where n is a 
whole number, d the distance of two parallel planes, 
and @ the glancing angle. If one mounts a crystal 
with one face in the axis of an instrument that turns 
slowly and regularly, such as, for instance, a register- 
ing barometer, the angle changes gradually and con- 
tinuously. 
NO, 2302, VOL. 92] 
NATURE 
423 
| If, therefore, one lets a pencil of X-rays, emerging 
from a slit, be reflected from this face on to a photo- 
graphic plate, one finds the true spectrum of the 
X-rays on the plate, supposing intensity of the primary 
beam to have remained constant. (This can be tested 
by. moving another plate slowly before the primary 
beam during the exposure.) 
| The spectra thus obtained are exactly analogous to 
| those obtained with a diffraction grating, and remind 
one strongly of the usual visual spectra containing 
continuous parts, bands, and lines. 
So far I have only identified the doublet, 11° 17’ and 
11° 38', described by Messrs. Moseley and Darwin. 
The spectra contain also a number of bright lines 
about two octaves shorter than these, and the continuous 
spectrum is contained within about the same limits. 
These numbers may be used in the interpretations of 
diffraction Roéntgen patterns, as they were obtained 
with tubes of the same hardness as those used for 
producing these latter. 
The arrangement described above enables us to dis- 
tinguish easily the spectra of different orders, as the 
interposition of an absorbing layer cuts out the soft 
rays, but does not weaken appreciably the hard rays 
of the second and higher orders. 
| It is convenient also for absorption experiments; 
thus a piece of platinum foil of o-2 mm. thickness 
showed transparent bands. The exact measurements 
will be published shortly, as well as the result of 
some experiments I am engaged upon at present upon 
the effect of changing the temperature of the crystal. 
Maurice DE BRoGLir. 
29, Rue Chateaubriand, Paris, December 1. 
As W. L. Bragg first showed, when a beam of soft 
X-rays is incident on a cleavage plane of mica, a 
well-defined proportion of the beam suffers a reflection 
strictly in accordance with optical laws. In addition 
to this generally reflected beam, Bragg has shown 
that for certain angles of incidence, there occurs a 
kind of selective reflection due to reinforcement be- 
tween beams incident at these angles on successive 
parallel layers of atoms. 
Experiments I am completing seem to show that a 
generally reflected beam of rays on incidence at a 
second crystal surface again suffers optical reflection ; 
but the degree of reflection is dependent on the 
orientation of this second reflector relative to the first. 
The method is a photographic one. The second 
reflector is mounted on a suitably adapted goniometer, 
and the photographic plate is mounted immediately 
behind the crystal. The beam is a pencil 1-5 mm. 
in diameter. When the two reflectors are parallel 
the impression on the plate, due to the two reflections, 
is clear. But as the second reflector is rotated about 
an axis given by the reflected beam from the first and 
fixed reflector, the optically reflected radiation from the 
second reflector—other conditions remaining constant— 
diminishes very appreciably. As the angle between 
the reflectors is increased from 0° to go°, the impres- 
sion recorded on the photographic plate diminishes in 
intensity. For an angle of 20° it is still clear; for 
angles in the neighbourhood of 50° it is not always 
detectable; and for an angle of 90° it is very rarely 
detectable in the first stages of developing, and is 
then so faint that it never appears on the finished 
print. 
These results, then, would show that the generally 
reflected beam of X-rays is appreciably polarised in a 
way exactly analogous to that of ordinary light. 
Owing to the rapidity with which the intensity of 
the generally reflected beam falls off with the angle 
of incidence of the primary beam, it has not. been 
| possible to work with any definiteness with angles of 
