5A Mr Bragg, The Diffraction of © 
It is quite probable that the qualitative explanation put forward 
here to account for the intensities of the spots is not the right one, 
other explanations being possible. For instance, one might substi- 
tute for the factor termed ‘effective density’ above, one which 
expressed the fact that, other things being equal, spots nearer the 
centre of the pattern were more intense than those farther out. 
This, together with the right curve for the distribution of energy 
in the spectrum of the incident radiation, could be made to account 
for the intensities quite reasonably. This does not vitiate the 
conclusion that the spots in the pattern represent a series which 
is complete, and characteristic of a cubical crystalline arrangement. 
The other arrangements of cubical point systems cannot, as far as 
I can see, give such a complete series. The other possible arrange- 
ments have for elements of their pattern (1) a cube with a molecule 
or atom at each corner, the arrangement which Laue pictured, or 
(2) a cube with a molecule at each corner and one at the centre. 
Neither arrangement will fit the system of planes given above. It 
is only the third point system, the element of whose pattern has a 
molecule at each corner and one at the centre of each cube face, 
which will lend itself to the system of planes found to represent 
spots in the photograph. 
This last system, seeing that it forms an arrangement of the 
closest possible packing, is according to the results of Pope and 
Barlow the most probable one for the cubic form of zinc sulphide. 
In one of the photographs taken by Messrs Friedrich and 
Knipping the crystal was so oriented that the direction of the 
incident radiation made equal angles with the three rectangular 
axes of the crystal. In this case a figure is obtained in which the 
pattern is a repetition of the spots contained in a sector of angle 
= Regarding the spots as reflections of the incident beam in 
planes as before, these planes can be found almost as easily as 
those which reflect the spots in the square pattern, and indeed in 
many cases the planes are identical. I will not give the calcula- 
tions here, but one point is of especial interest. A photograph 
was taken of the crystal oriented so that the pattern obtained 
was perfectly symmetrical. The crystal was then tilted through 
3 about a line perpendicular to the incident beam and to one of 
the cubical axes. This distorted the pattern considerably, but 
corresponding spots in the two patterns are easily to be recognised. 
The points which I wish to consider especially are the following. 
In the first place, the spots in the distorted pattern are all 
displaced exactly as would be expected if they were reflections in 
planes fixed in the crystal. For instance, when the reflecting 
plane contains the line, about which the crystal was tilted 
through 3°, it can be ascertained that the movement of the spot 
