The Intensity of Reflexion of X- Rays by Rock- Salt. 315 



to make certain that this was the case, the crystal was 

 turned slightly between each reading, in order to ensure 

 that the halting-places did not occur at exactly the same 

 angles. 



7. The faces used in this experiment were prepared by 

 grinding, and were of sufficient area to intercept the whole 

 of the incident beam of rays. In general, faces were 

 prepared 3 or 4 centimetres in length and breadth. The 

 perfection of the crystal structure may be judged from 

 the range of angles at which reflexion takes place. In most 

 cases the greater part of the effect was observed to take 

 place within less than a degree of arc as the crystal was 

 turned, the faces being prepared from large blocks of rock- 

 salt which were very little distorted. 



The face should be cut so that it is as nearly parallel to 

 the planes of the crystal structure as possible. If this is not 

 the case, there will be a difference in the intensities of 

 reflexion when the crystal is mounted facing right and left 

 on the spectrometer table. This effect is described and 

 explained in the paper by W. H. Bragg referred to above 

 (Phil. Mag. loc. cit. p. 888). When the crystal face is not 

 parallel to the planes of the structure, the incident and 

 reflected beams do not make equal angles with the face of 

 the crystal. If the glancing-angle of incidence is less than 

 that of reflexion, the rays suffer less absorption in the crystal 

 than when the reverse is the case. The smaller the angle 

 of incidence, the greater is this effect. 



As an example of this effect, the Hollowing table gives a 

 comparison of the 1st-, 2nd-, and 3rd-order reflexions from 

 a natural face (llO) of a ruby crystal (A1 2 3 ). The crystal 

 had the form of a six-sided prism bounded by the faces (110), 

 but had so developed that this prism tapered towards one 

 end. The angle between the face of the prism and the 

 crystal planes could be measured by comparing the angles 

 at which reflexion took place on the right-hand and left-hand 

 sides. In this case it was 1° 49'. It will be observed what 

 a large effect is produced by the small deviation from truth 

 in the orientation of the face- 



Comparison of (110), (220), (330) A1 2 3 . 



Face 110 220 350 



Eight-hand side 100 61'0 41*5 



Left-hand side 52 38'9 39*5 



