6 Prof. W. L. Bragg and Messrs. James and Bosanquet : 



appropriate to rays passing through the Ci-ystal at the 

 reflecting-angle, since all rays entering the ionization- 

 chamber have passed through at this angle. 



If a number of plates of different thickness are taken, 

 the intensity of reflexion measured in each case, and its 

 value plotted against t , the points should lie on a curve 

 such as that in fig. 3. The circles in this figure are the 



Fig. 3. 



■uh 





•12 16 



t in cms. 



experimental determinations of the intensity of reflexion 

 (100) for a number of plates varying in thickness from 

 0*2 to 2' 5 mm., and a curve of the form y = ate~^ has 

 been drawn so as to conform as closely as possible to the 

 absolute experimental values. 



The constants of this curve give the values of Q and yu,. 

 a /jL " can be measured by noting the value of t at which 



the curve has its maximum, since at this point t sec 6 = -. 



The maximum occurs at £ = 0'610 mm. ; whence a = 16 - 30. 

 The value measured directly is /x=10'7, so that the increase 

 in fju at the reflecting angle is 52 per cent. 

 The value of Q is given by the equation 



(5?) =3 (6) 



V 1 /max. efl 



The mean of our experimental determinations gave 

 (- T -) =3-65xl0- 4 . 



^ -*- /max. 



