The Intensity of 'Reflexion .'of X-Rays by Rock-Salt. 5 



be too small. This was pointed out in the former paper, 

 but no allowance could be' made for this effect, as the 

 increase in ft had not been measured. This increase has 



Crista/ Setting. 



now been determined by a direct method, and in this paper 

 the necessary corrections have been made to the values 

 of Q, and so of F. 



4. The increase in /jl cannot be determined from a curve 

 such as that of fig. 2. The beam tailing on the crystal 

 is not absolutely parallel ; therefore it is not possible to 

 set the crystal so that all the rays of the beam pass through 

 at that angle for which absorption is a maximum. The 

 apparent increase in //. indicated by the curve sets a lower 

 limit to the effect, which may be considerably greater. 



When reflexion takes place inside a crystal plate, as 

 in fig. 1, the strength of the reflected beam is given by 



equation (4), 



where t. = t sec 9. 

 In this equation, fi is 



E 



O) 



= Q^- 



■/** 



the effective absorption-coefficient 



