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



The corresponding figures for the face (100) of NaCl are 

 given below : — 



(100). (200). (300). 



Right-hand side 100 213 



5-08* 

 Left-hand side 116-6 21'8 



The error in the orientation of the (100) face was in this 

 case too small to be measured with accuracy ; it was 

 less than 30 minutes of arc. 



Since the effect of inaccurate grinding of the face is so 

 much less for the second order than for the first order, the 

 intensity of reflexion from (200) NaCl was taken as 

 standard, and all other intensities compared with it. On 

 account of the difficulty of grinding the face accurately, 

 there was generally a difference in the intensities on the 

 two sides. It can be proved, however, that the mean of 

 the intensities on the two sides can be taken as the right 

 value without making an appreciable error so long as the 

 difference in the value does not exceed about 25 per cent. 

 The intensities were in all cases measured on both sides 

 and the mean taken. In most cases the difference between 

 them was small ; for the higher orders it did not exceed 

 5 per cent. 



8. It is necessary to use faces which have been ground. 

 The strength of the reflexion is very different, especially for 

 small giancing-angles, when a cleavage surface is compared 

 with one which has been ground. As an example, a com- 

 parison is given below of the reflexion from a very perfect 

 cleavage face of a rock-salt crystal (A) with a similar 

 cleavage face on a crystal (D), which was afterwards ground 

 until a layer 1 millimetre thick had been removed. 



The intensity of reflexion from crystal (D) was measured 

 with two orientations of the face. In the first, the crystal 

 was set so that the edge, on which the knife was pressed in 

 cleaving the crystal, was horizontal. In the second position, 

 the edge was vertical and therefore at right angles to the 

 horizontal beam of X-rays. 



Face. Intensity. 



A (100). (Cleavage face)' 50'8 



D (100) before grinding : — 



First position 254 



Second position 129 



D (100) after grinding 100 



Hence D (100), after grinding, reflected eight times as 

 well as in the second position before grinding. 



* The difference between right-hand and left-hand sides was less 

 than the error of determination. 



