The Intensity of Reflexion of X-Rays by Rock-Salt. 337 



cases. F may become zero for certain values of 6. F', de- 

 pending on a random arrangement of electrons in eacli 

 individual atom, will have no zero values. 



18. The model of the atom which agrees most closely with 

 the observed values of F for the large values of 6 would be 

 one of type 3, in which, however, all the figures given above 

 for the distances of the electrons from the centre of the atom 

 must be reduced in the ratio 3:2. Assuming this type of 

 atom, it. is possible to calculate from formula (6) the re- 

 flecting powers for any plane of the rock-salt crystal. The 

 figures so calculated are shown by the continuous curves in 

 fig. 6, and can be compared with those actually obtained 

 (represented by points on the dotted curve). It is to be 

 emphasized again that the comparison is not relative, but an 

 absolute comparison of the reflecting powers actually observed 

 and those calculated from the formula. 



Summary. 



The absolute values of the reflecting power for different 

 faces of rock-salt have been measured. The reflecting power 

 has been determined for eighteen glancing-angles over a 

 range between 5° 30' and 30° 0'. 



The values obtained have been compared with those 

 calculated from the theoretical formulae for reflexion deduced 

 by Darwin and Compton, and it has been shown that they 

 afford strong confirmation of the accuracy of these formula?. 



The greatest care has been taken to make the measur§- 

 ments as accurate as possible, in order that they may serve 

 as a basis for an analysis of the arrangement of the electrons 

 in the atom. Possible arrangements are discussed. 



In order to confirm the results, the effect of temperature 

 on the intensity of reflexion must be more fully determined. 

 The authors intend to make a series of determinations at 

 liquid-air temperature,, in order to be able to extrapolate to 

 the values at absolute zero. It is hoped to extend the 

 measurements over a wider range of angles at low 

 temperatures. 



It is further intended to repeat the experiments with KC1 

 in order to check the formula in this case. Sylvine affords 

 a simpler case for investigation than rock-salt, since the ions 

 of potassium and chlorine will, in all probability, have a very 

 similar structure. 



The authors wish to acknowledge very gratefully the kind 

 assistance given them by Dr. W. D. Coolidge, of the General 

 Electric Company, Schenectady, to whom they are indebted 

 for the gift of the Coolidge tube with which the investigations 

 were carried out. 



