46 Mr Bragg, The Diffraction of 
Regard the incident light as being composed of a number of 
independent pulses, much as Schuster does in his treatment of 
the action of an ordinary line grating. When a pulse falls on a 
plane it is reflected. If it falls on a number of particles scattered 
over’a plane which are capable of acting as centres of disturbance 
when struck by the incident pulse, the secondary waves from them 
will build up a wave front, exactly as if part of the pulse had been 
reflected from the plane, as in Huygen’s construction for a re- 
flected wave. 
The atoms composing the crystal may be arranged in a great 
many ways in systems of parallel planes, the simplest bemg the 
cleavage planes of the crystal. I propose to regard each inter- 
ference maximum as due to the reflection of the pulses in the 
incident beam in one of these systems. Consider the crystal as 
divided up in this way into a set of parallel planes. A minute 
fraction of the energy of a pulse traversing the crystal will be 
reflected from each plane in succession, and the corresponding 
interference maximum will be produced by a train of reflected 
pulses. The pulses in the train follow each other at intervals of 
2d cos 6, where @ is the angle of incidence of the primary rays on 
the plane, d is the shortest distance between successive identical 
planes in the crystal. Considered thus, the crystal actually 
‘manufactures’ light of definite wave-lengths, much as, according 
to Schuster, a diffraction grating does. The difference in this case 
lies in the extremely short length of the waves. Each incident 
pulse produces a train of pulses and this train is resolvable into a 
series of wave-lengths d, * x etc. where X = 2d cos 0. 
Though to regard the incident radiation as a series of pulses 
is equivalent to assuming that all wave-lengths are present in its 
spectrum, it is probable that the energy of the spectrum will be 
greater for certain wave-lengths than for others. If the curve 
representing the distribution of energy in the spectrum rises to a 
maximum for a definite ) and falls off on either side, the pulses 
may be supposed to have a certain average ‘ breadth’ of the order 
of this wave-length. Thus it is to be expected that the intensity 
of the spot produced by a tzain of waves from a set of planes in 
the crystal will depend on the value of the wave-length, viz. 2d cos @. 
When 2d cos @ is too small the successive pulses in the train are 
so close that they begin to neutralize each other and when again 
2d cos @ is too large the pulses follow each other at large intervals 
and the train contains little energy. Thus the intensity of a spot 
depends on the energy in the spectrum of the incident radiation 
characteristic of the corresponding wave-length. 
Another factor may influence the intensity of the spots. 
Consider a beam of unit cross-section falling on the crystal. The 
