48 Mr Bragg, The Diffraction of 
The planes can now be classified by the corresponding values 
of p, g, 7, § aS parameters. 
The direction cosines of a plane p q 7 s will be 
rq ps ERs 
vps? + gr? + pr Vp? + Pr + pr an D'S) Qe sepals 
If these are called / m n the direction cosines of the reflected 
beam are 
2in, 2mn, 2n?—1, 
f 
Fic. 1. 
and the position of the interference maximum on the photographic 
plate can be found in terms of these quantities. 
The corresponding wave-length is 2dcos@ where d is the 
perpendicular distance between successive planes. Now @ is the 
angle of incidence, therefore cos @=nabove. It is easier to find 
the intercepts which successive planes cut off on the z axis, than 
their perpendicular distance apart. Calling these intercepts /, then 
XV = 2d cos 06 =2.1 cos @. cos 0 = 2In?. 
