320 Prof. W. L. Bragg and Messrs. James and Bosanquet : 



The figures obtained by W. H. Bragg are given below 

 for the sake of comparison : — 



Plane. Intensity. 



100 100 



200 18-7 



300 6-25 



110 41-0 



220 7-05 



222 24-4 



444 4-20 



11. In fig. 2 the square-root of the relative intensity has 

 been plotted against the cosecant of the glancing-angle. 

 By plotting the intensities in this way, the approximate 

 relation found by W. H. Bragg — that the intensity varies 

 inversely as sin 2 6 — is made evident. 



All the points lie on tw T o smooth curves, showing that 

 they form two groups within each of which the intensity is 

 a function of the glancing-angle alone. For instance, the 

 reflexions from the faces (511) and (333) occur at the same 

 angle, and the corresponding intensities 0"74 and 72 are 

 identical within the error of observation. 



The points which lie on the lower curve are those for 

 which all the indices are odd — the faces (111), (311), (331), 

 (333), (511], (711), (555). These reflexions are from planes 

 which contain alternately sodium and chlorine atoms. The 

 wave-train reflected from the planes containing sodium 

 atoms is 180° out of phase with that reflected by the plane 

 containing chlorine atoms. The other reflexions are either 

 from planes which contain both sodium and chlorine atoms 

 and are identical in their nature, or are reflexions of an even 

 order from planes containing sodium and chlorine atoms 

 alternately. In both cases the sodium and chlorine atoms 

 reflect wave-trains which are in phnse with each other. 



Since the square-root of the intensity has been plotted, 

 this may be taken as being proportional to the amplitude 

 of the reflected wave-trains. The upper curve, therefore, 

 represents the sum of the amplitudes due to sodium and 

 chlorine atoms, the lower, the difference of these two 

 amplitudes. 



Comparison of Incident and Reflected Beams. 



12. The rays from the bulb consist of heterogeneous 

 radiation of all wave-lengths over a certain range, super- 

 imposed on the homogeneous radiation whose intensity of 



