2 he Intensity of Reflexion of X-Rays by Rock-Salt. 333 



The values of: F for chlorine and sodium calculated for 

 these three types of atom model are plotted against sin in 

 tigs. 4 and 5, the curves corresponding to the first, second, 

 and third models being numbered I., II., and III. The 

 measured values of F are shown for comparison as a series 

 of small circles. 



Of the three models chosen, the third type is the only one 

 which gives diffraction curves of the same general shape as 

 those actually observed. Both of the other models yield 

 curves which have maxima and minima. The actual values 

 of F fall off more slowly than do any of the theoretical 

 curves, indicating that the distances of the electrons from 

 the centre of the atom have been taken to be greater than 

 their true value, both for sodium and chlorine. 



The curve for sodium becomes nearly horizontal at some 

 distance from the vertical axis, and the curve for chlorine 

 show T s a similar tendency. This must be ascribed to the fact 

 that the reflecting power for small glancing angles is 

 diminished by the increase of the absorption-coefficient, and 

 that our values for (100) and (110) are too small. We 

 know that this effect must exist, since measurement has 

 shown that the absorption-coefficient increases by 15 per cent, 

 in the neighbourhood of the angle corresponding to the (100) 

 reflexion, and the increase at the exact angle of reflexion 

 may be far greater than this. It is difficult to allow for this 

 effect. In tig. 4 the circles on the dotted curve have been 

 plotted so as to give that curve a more probable form passing 

 through a maximum at 10. The actual values for sin # = 0*1 

 and sin^ = 0'13 lie well below this dotted curve. In fig. 5 

 the two greatest values of F have been increased by the 

 same amounts as for the other curve. This increase is of 

 the order to be expected from the variation which was found 

 to exist in the coefficient of absorption /u, but its exact value 

 is, of course, merely conjectural. 



There is another striking feature of the curves for the 

 observed values of F. The curve for chlorine approaches 

 the axis more slowly than that of the sodium. This is just 

 the reverse of what would be expected from the relative 

 dimensions of the two atoms, since we would expect the 

 electrons to be on the whole at a greater distance from 

 the centre in chlorine than in sodium. Here, again, the 

 third type of atom chosen as a model gives results which 

 agree most closely with those actually observed. 



Although it is necessary to check the form of the curves 

 for F by measurements on other crystals before drawing any 



