Short Electromagnetic Waves by a Crystal. al 
1, 1, 3, 1 and 1, 1, 3, 3 would reflect the beam so as to miss the 
photographic plate. 1, 1, 3, 5 and 1, 1, 3, 7 are considered. 
1, 1, 3, 9 has already been considered as 1, 1, 1, 3, and 1, 1, 3, 
11 gives a value for the wave-length outside the ‘visible’ range. 
In fig. 3, Plate II, is given a photograph of the interference 
pattern which Laue obtained. In fig. 4, Plate II, the key to the 
pattern has been drawn, showing in what planes the spots are to 
be considered as reflected. 
TABLE II. 
Planes for which L.c.M. of p and r=2, l=a, X= 2an’. 
p q r s . Jutensity hy he hs 
iL eae 3 ° Qi ee? 
1 i 2 8 9 * 2 8 2 
I 1 2 12 19 Invisible 2 12 2 
2 4 2 0 2-5 Invisible 4 0 2 
2 4 2 a 4:5 ° 4 4 2 
2 zt 2 8 10:5 eo? 4 8 2 
il 5) 2 0 5 % 0 2 
1 5) 2 4 7 * 6 4 2 
ee 2). 8 | 18 2! 6 | 2 
2 8 2 0 8:5 e 8 0 2 
2 2 4 10:5 eo 8 4 2 
1 yd a 0 13 Invisible | 10 0 2 
Consider a reflecting plane which passes through the atom at 
the origin and a neighbouring atom, let us suppose the atom whose 
coordinates are a, 0, a. As the plane is turned about the line 
through these two points the reflected beam traces out a circular 
cone, which has for axis the line joining the two points and for 
one of its generators the incident beam. This cone cuts the 
photographic plate in an ellipse. If the atom through which 
the plane passes is in the #z plane as above, the ellipse touches 
the y axis on the photographic plate at the origin. Now take 
a plane passing through the origin and a point 0, a, 3a. The 
4—2 
