The Intensity of Reflexion of X- Rays by Rock-Salt. 319 



(100): (200). The values for (100) and (111) are, how- 

 ever, difficult to measure accurately, since the nature of the 

 crystal face has so great an effect on the intensity, and 

 the figures given here must be regarded as approximate. 



10- 



9- 



Fig. 2. 



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 -24 

 -22 

 20 

 18 

 16 

 14- 



For the other faces, it is believed that the average error 

 does not exceed 2 per cent., except in the case of the very 

 small intensities of high order. The smallest intensity 

 measured is that denoted by (500), and is little more than 

 one thousandth of the (100) reflexion. 



