The Intensity of Reflexion of X-Rays by Rock-Salt. 311 



Comparison of the Intensity of Reflexion by different Races. 



2. The method employed is fundamentally the same as 

 that described by W. H. Bragg. Homogeneous rays are 

 emitted from the focal spot on the target, and are limited to 

 a narrow beam by a slit termed the bulb-slit. The beam 

 falls on the crystal, and the reflected beam is received by an 

 ionization-chamber through a second slit. If the chamber- 

 slit and the bulb-slit are equidistant from the axis of the 

 instrument with which the crystal face coincides, the 

 chamber-slit can be set so that it receives all rays of any 

 particular wave-length reflected by the crystal face, although 

 the reflexion may take place at various points on the face 

 owing to irregularity of the crystal structure. This focussing 

 effect has been described by W. H. Bragg and one of the 

 authors *. 



The chamber may therefore be placed so as to receive all 

 reflected homogeneous rays of any required wave-length — 

 for instance, those corresponding to the K a -line of Rhodium. 

 If this is done, and the crystal is slowly rotated about the 

 axis of the spectrometer, very little effect is observed until 

 the crystal planes come into the position where the equation 

 for reflexion is satisfied. The ionization then rapidly rises 

 to a maximum, and falls away again as the crystal passes 

 beyond this position. The more perfect the crystal, the 

 narrower the range over which reflexion takes place. No 

 •crystals are perfect ; in all cases the rays are reflected by a 

 number of facets making a small angle with each other. As 

 the crystal turns, these come, one alter the other, into the 

 correct position for reflexion, the reflected beam from each 

 falling on the chamber-slit in virtue of the focussing effect 

 described above. 



The intensity of the reflexion cannot be measured by the 

 effect observed when the crystal is set at the position which 

 gives the most intense reflected beam, for the strength of this 

 beam is dependent on the degree of perfection of the crystal 

 face. It is measured by sweeping the crystal with uniform 

 angular velocity through the entire range over which it 

 reflects, and by observing the total ionization produced in 

 the chamber during this process. In this way, every part 

 of the pencil of homogeneous rays will fall at some time on 

 a portion of crystal which reflects it, and will contribute its 

 share to the whole effect. Experiments made with different 

 crystals show that the intensity, measured in this way. is 

 not dependent on the degree of perfection of the crystal. 



* 'X-llays and Crystal Structure,' p. 31. 

 Y2 



