The Intensity of Reflexion of X-Rays by Rock-Salt. 7 

 Substituting e = 2'718, /n= r 16*30, we obtain 

 Q (100) = 16-16 xlO" 3 . 



From the value of Q ( ioo) the absolute value of (Fci4- F Na ) 

 can be calculated, from equation (1). 

 This yields Foi + Fniv = 2 o-10 



at the glancing-angle 6° 17', corresponding to reflexion 

 from (100). In other words, when the rays are diffracted 

 through an angle of 12° 34', the total effect of the 28 electrons 

 in a pair, of sodium and chlorine atoms is reduced, by 

 interference, to an effect 20*10 times that due to a single 

 electron. 



This value for -y may be compared with that obtained 



by reflexion from the face (100) of rock-salt. Formulae (3) 

 and (4) state that 



-~r- = ~ for reflexion at a face. 



1 ZfJb 



(-Y-) = — for reflexion through a plate. 

 I /max. eft fe ' 



The experimental value for the reflexion in the first case is 

 ^ = 5-41 xlO" 4 *. 



If the effective coefficient of absorption //, were the same 



for the crystal face and for the plate, the value of -y- for 

 reflexion through the plate would be 



5*41 xl0" 4 x-= 3*99 xlO" 4 . 

 e 



The experimental value, as stated above, is 3*65 X 10~ 4 . 

 As will be seen, the value of //< for a crystal plate depends 

 on the way in which the plate has been prepared, so that too 

 much stress must not be laid on the numerical agreement; 

 but this comparison serves to show that jul is approximately 

 the same in both cases, and that the absolute values for the 

 reflecting power of a small crystal element got by these 

 different methods are in agreement with each other. 



* This redetermined absolute value is lower than that given in the 

 former paper ((3-12 X 10 -4 ). 



