Reflexion of X Rays by Crystals. 889 



We may now consider some experimental results in the 

 case o£ rocksalt. 



The anticathode A (see fig. 1) was so placed that the rays 

 left it at a grazing angle in order to pass through the slit 

 at B. The slit was half a millimetre wide, and was placed 

 as close to the crystal as possible. The distance from A to B 

 was 17 cm., from B to C 3 cm., from C to D 12 cm. 

 Although the distances AC and CD were not equal, the 

 focussing was sufficiently good. The crystal was turned by 

 hand, five minutes at a time, one movement for each beat of 

 the clock. About three degrees of total movement was 

 sufficient to take in all the reflexion the crystal could give. 

 The rays used were those constituting the principal line of 

 rhodium. To increase the ionization current the chamber 

 was filled with methyl bromide. 



The measurements were very consistent, consecutive- 

 observations differing by not more than one or two per 

 cent. The results are given in the following table, in which 

 the intensity of the first order reflexion is put equal to 100 

 and the rest are given as percentages. 



1st order. 2nd order. 3rd order. 4th order. 

 Face (100) 100 18-7 6-23 



Face (110) 41 7*05 



Face (111) 16-5 24'4 3*1 4-2 



The results for the first two faces show the rapid decline of 

 intensity with increasing order of reflexion which has already 

 been referred to. The planes parallel to the 111 face contain, 

 alternately, sodium atoms only, and chlorine atoms only : as 

 has already been mentioned, the spectra of even orders are 

 therefore much stronger than those of odd orders. 



A new experimental fact appears in this, that the intensities 

 of the different faces are comparable with each other. We 

 plot the intensity of each order against the ratio of the sine 

 of its glancing angle to the sine of the glancing angle of the 

 first order of the 100 face. We then find that they all lie 

 nearly on one curve (fig. 4, lower curve), no matter what 

 face thev belong to, excepting, however, the first and third 

 spectra of the 111 face. These last, however, are peculiar, 

 and differ from all the others in that they arise from a 

 difference in atomic weights. In the case of sylvine, where 

 potassium has replaced sodium and the weights of potassium 

 and chlorine are nearly equal, these two spectra do not 

 appear. 



