The Intensity of Reflexion of X-Rays by Rock-Salt. 321 



reflexion has been measured. In order to compare directly 

 the energy in the incident and reflected homogeneous beams, 

 it is necessary to obtain a homogeneous beam by reflexion 

 from a crystal face, and observe the total amount of radiation 

 reflected by a second crystal, turning with coustant angular 

 velocity, on which this homogeneous beam is allowed to fall. 

 Fig. 3 shows the arrangement of the apparatus to effect 

 this. The rays from the anticathode were reflected by the 



Fig. 3. 



crystal (\ so as to pass after reflexion through the collimator 

 slits of the spectrometer. The incident beam was not limited 

 by slits, but by being forced to pass on reflexion around the 

 lead wedge W *, the edge of which was pressed against 

 the crystal face. The position of the bulb and the orientation 

 of the crystal face were adjusted until the reflected beam 

 passed truly through the axis of the spectrometer. This 

 beam fell on the second crystal 2 , which was rotated with 

 uniform velocity co, and the total amount of radiation 



* Cp. Seeman, Phys. Zeit. xv. p. 795 (1914). 



