Juty 9, 1914] 
medium can be reconciled with discontinuous trans- 
ferences of radiation energy. Some solution there 
must be to this problem. The second important thing 
is that the new methods will surely help us on the 
way to find that solution. We can now examine 
X-rays as critically as we have been able to study 
light by means of the spectrometer. The wave-length 
ot the X-ray has emerged as a measurable quantity. 
The complete range of electromagnetic radiations 
now lies before us. At one end are the long waves 
of wireless telegraphy, in the middle are, first, the 
waves of the infra-red detected by their heating 
effects, then the light waves and then the short waves 
of the ultra-violet. At the other end are the extremely 
short waves that belong to X-radiation. In the com- 
parative study of the properties of radiation over this 
very wide range we must surely find the answer to 
the greatest question of modern physics. 
So much for the general question. Let us now 
consider the procedure of the new investigations, and 
afterwards one or two applications to special lines of 
inquiry. 
The experiment due to Laue and his collaborators 
Friedrich and Knipping has already been described 
in this lecture-room, and is now well known. A fine 
pencil of X-rays passes through a thin crystal slip 
and impresses itself on a photographic plate. Round 
the central spot are found a large number of other 
spots, arranged in a symmetrical fashion, their 
arrangment clearly depending on the crystal struc- 
ture. Laue had anticipated some such effect as the 
result of diffraction by the atoms of the crystal. His 
mathematical analysis is too complicated to describe 
now, and indeed it is not in any circumstances easy 
to handle. It will be better to pass on at once to a 
very simple method of apprehending the effect which 
was put forward soon after the publication of Laue’s 
first results. I must run the risk of seeming to be 
partial if I point out the importance of this advance, 
which was made by my son, W. L. Bragg. All the 
recent investigations of X-ray spectra and the exam- 
ination of crystal structure and of molecular motions 
which have been carried out since then have been 
rendered possible by the easy grasp of the subject 
which resulted from the simpler conception. 
Let us imagine that a succession of waves con- 
stituting X-radiation falls upon a plane containing 
atoms, and that each atom is the cause of a secondary 
wavelet. In a well-known manner, the secondary 
wavelets link themselves together and form a reflected 
wave. Just so a sound wave may be reflected by a 
row of palings, and very short sound waves by the 
fibres of a sheet of muslin. : 
Suppose a second plane of atoms to lie behind the 
first and to be parallel to it. The primary wave, 
weakened somewhat by passing through the first 
plane, is again partially reflected by the second. When 
the two reflected pencils join it will be of great im- 
portance whether they fit crest to crest and hollow 
to hollow, or whether they tend to destroy each other’s 
effect. If more reflecting planes are supposed, the 
importance of a good fit becomes greater and greater. 
If the number is very large, then, as happens in many 
parallel cases in optics, the reflected waves practically 
annul each other unless the fit is perfect. : 
It is easily seen that the question of fit depends on 
how much distance a wave reflected at one plane 
loses in comparison with the wave which was reflected 
at the preceding plane; the fit will be perfect if the 
loss amounts to one, two, three, or more wave-lengths 
exactly. In its turn the distance lost depends on the 
spacing of the planes—that is to say, the distance 
from plane to plane—on the wave-length and on the 
angle at which the rays meet the set of planes. 
NOw2g32, VOL. 93| 
NATURE 
495 
The question is formally not a new one. Many 
years ago Lord Rayleigh discussed it in this room, 
illustrating his point by aid of a set of muslin sheets 
stretched on parallel frames. The short sound waves 
of a high-pitched bird-call were reflected from the set 
of frames and affected a sensitive flame; and he 
showed how the spacing of the planes must be care- 
fully adjusted to the proper value in relation to the 
length of wave and the angle of incidence. Rayleigh 
used the illustration to explain the beautiful colours 
of chlorate of potash crystals. He ascribed them to 
the reflection of light by a series of parallel and regu- 
larly spaced twinning planes within the crystal, the 
distance between successive planes bearing roughly the 
same proportion to the length of the reflected wave 
of light as the distance between the muslin sheets _ 
to the length of the wave of sound. fies 
Our present phenomenon is exactly the same thing 
on a minute scale; thousands of times smaller than 
in the case of light, and many millions of times 
smaller than in the case of sound. 
By the kindness of Prof. R. W. Wood I am able 
to show you some fine examples of the chlorate of 
potash crystals. If white light is allowed to fall 
upon one of them, the whole of it is not reflected. 
Only that part is reflected which has a definite wave- 
length or something very near to it, and the reflected 
ray is therefore highly coloured. The wave-length is 
defined by the relation already referred to. If the 
angle of incidence is altered, the wave-length which 
can be reflected is altered, and so the colour changes. 
It is not difficult to see the analogy between these 
cases and the reflection of X-rays by a crystal. Sup- 
pose, for example, that a pencil of homogeneous 
X-ravs meets the cube face of such a crystal as 
rocksalt. The atoms of the crystal can be taken to 
be arranged in planes parallel to that face, and regu- 
larly spaced. If the rays meet the face at the proper 
angle, and only at the proper angle, there is a 
reflected pencil. It is to be remembered that the re- 
flection is caused by the joint action of a series of 
planes, which in this case are parallel to the face; it 
is not a reflection by the face itself. The face need not 
even be cut truly; it may be unpolished or deliberately 
roughened. The reflection takes place in the body of 
the crystal and the condition of the surface is of little 
account. 
The allotment of the atoms to a series of planes 
parallel to the surface is not, of course, the only one 
possible. For example, in the case of a cubic crystal, 
parallel planes containing all the atoms of the crystal 
may also be drawn perpendicular to a face diagonal 
of the cube, or to a cube diagonal, or in many other 
ways. We may cut the crystal so as to show a face 
parallel to any series and then place the crystal so 
that reflection occurs, but the angle of incidence will 
be different in each case since the spacings are 
different. It is not necessary to cut the crystal except 
for convenience. If wave-length, spacing, and angle 
between ray and plane are rightly adjusted to each 
other, reflection will take place independently of any 
arrangement of faces. 
This is the ‘reflection’? method of explaining the 
Laue photograph. W. L. Bragg showed in the first 
place that it was legitimate, and in the second that 
it was able to explain the positions of all the spots 
which Laue found upon his photographs. | The 
different spots are simply reflections in the different 
series of planes which can be drawn through the 
atoms of the crystal. The simpler conception led at 
once to a simpler procedure. It led to the construc- 
tion of the X-ray spectrometer, which resembles an 
ordinary spectrometer in general form, except that the 
grating or prism is replaced by a crystal and the 
