ProE. W. H. Bragg on the Intensity of 



important. Of course it is well to choose for experiment as 

 good a piece of crystal as is possible. 



Some of the crystal faces must be prepared, since natural 

 faces are limited in number. A prepared face should be cut 

 so as to be nearly true, say within half or a quarter of a 

 degree. Want of truth makes very little difference in the 

 determination of the angle of reflexion, but may affect an 

 intensity measurement seriously. This may be seen at once 

 from the figure. 



Fig. 3. 



If the prepared face A A' is not parallel to the crystal 

 planes, whose direction is shown by the dotted line, it will 

 make a considerable difference whether it is set as repre- 

 sented in (a) or as in (b). The primary pencil I A enters 

 the crystal at A and is reduced to a certain value when it 

 reaches B, B being any point on the path and A B the same 

 length in the two figures. In the one case, however, the 

 reflected pencil has to traverse a much smaller mass of 

 crystal before emerging than it has to do in the other, and 

 the reflexion appears correspondingly greater. This effect 

 is very marked in practice. For example, a (111) face of 

 rocksalt, which reflects the principal rhodium ray at a 

 glancing angle of 11°, was cut 5° out of truth. The in- 

 tensity of reflexion was then twice as great in the one 

 position as in the other. In the case of a prepared face 

 it is therefore well to measure the intensity effects for both 

 positions and to take the mean, in case the face has not been 

 cut quite truly. 



