56 Mr Bragg, The Diffraction of 
The atoms of a crystal may be arranged in ‘doubly infinite’ 
series of parallel rows, as well as in ‘singly infinite’ series of 
planes. The incident pulse falls on atom after atom in one of 
these rows, if the row is not parallel to the wave front, and 
secondary waves are emitted, one from each atom, at definite 
time intervals. Along any direction lying on a certain circular 
cone with the row of atoms as-axis, these secondary waves will 
be all in phase, one generator of the cone being, of course, parallel 
to the direction of the incident radiation. If the row of atoms 
makes a small angle with the direction, this cone with vertex at 
the crystal slip-may now be considered to cut the photographic 
plate in an almost circular ellipse passing through the big central 
iP 
Lead Screen 
Crysial 
Posilions of Pholographic Plate 
Cross sections of a of rays a |r. |. 
Fic. 2. 
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Cet OT 
spot. Drawing the ellipses which correspond to the most densely 
packed rows of the crystal, a spot is to be expected at the inter- 
section of two ellipses, for this means that pulses from a doubly 
infinite set of atoms are in that direction in agreement of phase. 
Thus it ought to be possible to arrange the spots in the photograph 
on these ellipses, in whatever way the crystal is oriented, and indeed 
they appear in all cases. They come out very strongly in the 
photographs taken with copper sulphate crystals. 
So far it has been assumed that the atoms of zinc and sulphur act 
in an identical manner with regard to the production of secondary 
waves, but this assumption is not necessary. What is brought 
