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



reflected was measured in the ordinary way. In this case, 

 since the rays are homogeneous, no allowance for general 

 radiation need be made. 



The amount of energy reflected is proportional to the 

 intensity of the incident beam, and inversely proportional 



to the angular velocity of rotation. The quantity -j- is 



therefore a constant characteristic of any one face and 

 order, where 



E = Total amount of energy reflected when the crystal is 

 rotated with angular velocity a> radians per second. 



I = Total amount of energy passing into ionization- 

 chamber when the incident beam enters it for 

 one second. 



TT 

 This constant -y- w T ill be defined as the " Reflecting 



Power " of the crystal face for the wave-length X. 



The chamber was placed so as to receive the whole of the 

 incident beam (i. e. that reflected from Cj), and the effect 

 measured when the rays entered the chamber for a known 

 time. It w T as then turned so as to receive the beam reflected 

 from C 2 w T hen the second crystal was mounted on the spectro- 

 meter table and turned with a known angular velocity. 



This was done for the face (100) of NaCl. The reflexions 

 from other faces are so much weaker that it was not con- 

 venient to compare them directly with the incident beam. 

 As a check, an absolute measurement was made of the 

 reflecting power for (222), which was found to be in agree- 

 ment with that calculated by a comparison with (100). 



A series of measurements gave for the constant 



^- =0-000612 for NaCl (100)*. 



Since the reflecting powers of the other faces have been 

 determined in terms of that from the face (100), their 

 absolute reflecting powers may now be calculated. These 

 values are given in the fifth column of Table I. 



[The reflecting power of a face can only be defined satis- 

 factorily in this way. It may be of interest, however, to 

 give approximately the proportion of homogeneous radiation 

 reflected when the crystal face is set at the most favourable 



* More recent determinations of this yalue have shown that the 

 figure 0-000012 is too high. The value varies somewhat with the nature 

 of the crystal face, and a better mean value is -00055. 



