16 r Ihe Intensity of Reflexion of X-Rays by Rock-Salt. 



trend of the corrected curves is towards maxima in the 

 neighbourhood of these values, whereas the uncorrected 

 curves drop too quickly as they approach the y axis, 

 this being particularly noticeable in the curve for sodium. 

 The fact that the corrected curves tend towards these 

 maxima confirms the accuracy, both of the experimental 

 determinations and of the assumptions which have been 

 made in the calculations. 



Summary. 



The object of these experiments has been the deter- 

 mination of the amplitude of the wave diffracted in various 

 directions by the chlorine and sodium atoms when homo- 

 geneous X-rays of unit amplitude fall on these atoms. 

 This amplitude is expressed in terms of that diffracted 

 by a single electron, the results for chlorine and sodium 

 being shown in fig. b\ 



The following formulae for the intensity of reflexion have 

 been used : — 



E© Q 



I ~ 2fl 



for reflexion at a face ; 



^ = Q* o sec0 e - Wo8ecfl 



for reflexion at glancing angle 6 through a crystal plate of 

 thickness t cut perpendicular to the reflecting planes. 

 Q is given by the formula 



O- WX * F* '* 1 + cos' 2(9 in2e 



^~ sin 20 mV 2 sin 20 * 



Allowance has been made for the increase in the effective 

 coefficient of absorption jjl, in the case of strong reflexions, 

 due to the existence of the " extinction-coefficient." 



The amplitude of the wave diffracted by a pair of sodium 



e 2 

 and chlorine atoms is given by the factor F — 5 . The two 



atoms diffract waves which are in phase with each other 

 for all planes except those with wholly odd indices, when 

 the wa\ es are opposed in phase. By measuring the intensity 

 of reflexion for both types of plane, it has been possible 

 to assign values to the amplitude diffracted by the chlorine 

 and sodium atoms separately over a range of angles from 

 10° to 60°. 



