Mr Bragg, Diffraction of Short Electromagnetic Waves, etc. 43 
The Diffraction of Short Hlectromagnetic Waves by a Crystal. 
By W. L. Braae, B.A. Trinity College. (Communicated by 
Professor Sir J. J. Thomson.) 
[Read 11 November 1912. ] 
[PLATE TT.] 
Herren Friedrich, Knipping, and Laue have lately published 
a paper entitled ‘Interference Phenomena with Roéntgen Rays*,’ 
the experiments which form the subject of the paper being carried 
out in the following way. A very narrow pencil of rays from an 
X-ray bulb is isolated by a series of lead screens pierced with fine 
holes. In the path of this beam is set a small slip of crystal, 
and a photographic plate is placed a few centimetres behind the 
erystal at right angles to the beam. When the plate is developed, 
there appears on it, as well as the intense spot caused by the 
undeviated X-rays, a series of fainter spots forming an intricate 
geometrical pattern. By moving the photographic plate back- 
wards or forwards it can be seen that these spots are formed by 
rectilinear pencils spreading in all directions from the crystal, 
some of them making an angle of over 45° with the direction 
of the incident radiation. 
When the crystal is a specimen of cubical zinc blende, and one 
of its three principal cubic axes is set parallel to the incident 
beam, the pattern of spots is symmetrical about the two re- 
maining axes. This pattern is shown in Plate II. Laue’s 
theory of the formation of this pattern is as follows. He con- 
siders the molecules of the crystal to form a three-dimensional 
grating, each molecule being capable of emitting secondary 
vibrations when struck by incident electromagnetic waves from 
the X-ray bulb. He places the molecules in the simplest possible 
of the three cubical point systems, that is, molecules arranged in 
space in a pattern whose element is a little cube of side ‘a, with 
a molecule at each corner. He takes coordinate axes whose 
origin is at a point in the crystal and which are parallel to the 
sides of the cubes. The incident waves are propagated in a 
direction parallel to the z axis, and on account of the narrowness of 
the beam the wave surfaces may be taken to be parallel to the zy 
plane. The spots are considered to be interference maxima of the 
waves scattered by the orderly arrangement of molecules in the 
erystal. In order to get an interference maximum in the direction 
* Sitzwngsberichte der Kéniglich Bayerischen Akademie der Wissenschaften. 
June 1912. 
