The Intensity of Reflexion of X- Rays by Rock- Salt. 317 



Comparison of 1st, 2nd, and 3rd orders : — 



(100). (200). (300). 



Crystal D (after grinding) 100 18-4 41' 



Crystal A (cleavage face) 50'8 181 4*7 



These comparisons were made for reflexion on the right- 

 hand side only, and must be regarded as approximate. 



In this case, as in the case of the error due to inaccurate 

 grinding, the effect is much greater for the first-order 

 reflexion. On account of this effect, intensity measure- 

 ments of the (100) reflexion are doubtful, and this provides 

 an additional reason for using the reflexion (200) as 

 standard. 



The difference in intensity for the two positions of the 

 D cleavage face indicates that the reason for the imperfect 

 reflexion may be due to a distortion of a freshly-cleaved 

 surface. A cleaved surface has a rippled appearance, the 

 ripples being parallel to the line on which the knife-edge 

 was pressed in cleaving the crystal. The range of angles, 

 however, over which the crystal reflects is no greater for a 

 cleavage surface than for a ground surface, so that it would 

 not appear that the imperfect reflexion is due to small 

 variations in orientation of the face. Measurements made 

 at various stages in the grinding down of a crystal face 

 indicate that trie effect is deep seated, and a depth of a 

 millimetre at least was removed from the face D (100) 

 before it was used in obtaining the results given in this 

 paper. 



9. Certain precautions must be taken in order to ensure 

 an' accurate result. 



The crystal must be swept through a range of angles 

 sufficiently great to ensure that all the facets add their 

 share to the total effect. A range of three degrees is 

 generally used. To check whether this was sufficient, 

 a larger range was used in certain cases. This increased 

 the amount of general radiation, but when this was sub- 

 tracted, the intensity due to the homogeneous rays was 

 found to be the same as when the smaller range was used. 



The crystal planes must be parallel to the axis of the 

 instrument. The crystal is mounted on a table which can 

 be rocked about a horizontal axis parallel to the faces, and 

 the intensity of a high-order spectrum is measured for 

 various tilts of the face. The crystal is fixed at that setting 

 which gives a maximum effect. Unless this precaution is 

 taken, the beam on reflexion may be thrown upwards or 

 downwards, and not be completely received by the ionization- 

 chamber. 



