The Intensity of Reflexion of X- Rays by Rock-Salt. 329 



planes at right angles to the crystal face reflected the 

 radiation, the reflexion being observed in the usual way. 

 On redetermining the absorption-coefficient it was found 

 to have increased by about 15 per cent. This effect is 

 discussed in Darwin's paper referred to above, and will 

 reduce the intensity of the reflected beam. Its efr'ect will be 

 smaller for higher orders of reflexion, since the increase in 

 absorption is due to multiple reflexion within the crystal 

 interfering with the primary beam, and reflexion is so much 

 weaker in the higher orders. 



The effect will not be taken into account in the calcu- 

 lations, since it is not obvious what allowance should be 

 made for it. It is to be remembered, however, that the 

 reflexions from (100) and (110) must be diminished by the 

 increase in the absorption -coefficient. 



It may possibly be the case that the effect of grinding a 

 cleavage face, which increases so greatly the intensity of 

 reflexion, is due to the fact that grinding breaks the crystal 

 up into a number of small homogeneous crystals oriented 

 in slightly different directions, so that absorption at the 

 reflecting angle plays a less important part in diminishing 

 the intensity of reflexion. 



The Comparison of the Theoretical and Observed Results. 



16. The formula for the reflecting - power of a face 

 states that 



Eo> W ¥2 e \ 1+ cos 2 20 ^_ B , u2 , 



I ~~ 2a sin 28 



.2, A 



jLfx sin lo m"cr 



Since all the quantities have been measured except F, we 

 can calculate the absolute value of F for a range of values 

 of 8. 



If the effect of the electrons in the chlorine atoms be 

 represented by Fci, and of those in the sodium atom by Fx a , 

 then for reflecting-powers corresponding to points on the 

 upper curve of fig. 2 we have 



F = F C i + F Na ; 



for those corresponding to points in the lower curve 



F = F C1 — F Na . 

 From the formula 



,-,_ /Eft) m c 2 2/J / sin 26 +?sin2fl 



where N is the number of molecules of NaCl in unit volume 

 of the crvstal. 



