358 Mr. W. L. Bragg on the 



(111) of fig. 2. Here we have two peaks close together in 

 the curve, representing the two lines in the spectrum of 

 palladium. The curve shows a decided first- and second- 

 order spectrum, and even perhaps a third, though this last i& 



Fig. 2. 





Copper 

 (100) 



* 18 50 



k 



38 









(no) 





c5 40' 







S4?"40' 



^ 



^ 



(m) 



A 



* 16 





32 W 



49* 



30' 



0° 10° 20° 



Angle of C^.iM8£/t 



60 



somewhat doubtful. It must be noted that the range of 

 angles, over which the ionization chamber may be set so as 

 to receive the reflected beam, is confined to 1°, though the 

 crystal structure is so imperfect. If the crystal were perfect 

 it would be scarcely smaller, and the reason for this is clear. 

 Although there are a number of settings for the crvstal which 

 enable a set of planes somewhere on its distorted face to 

 receive the incident X rays at a glancing angle of 8° and 

 so to reflect them, the reflected rays all converge and are 

 received by the chamber when set exactly at 16°*. There- 

 fore, when the chamber and crystal are moved simultaneously,, 

 the reflexion is only found when the chamber is in the neigh- 

 bourhood of 16°. 



Fig. 2 also show r s the curves for faces (110) and (100) of 

 copper. The angles at which the spectra are found are as 

 follows : — 



0<ioo)= 9° 24' sin 5> ( ioo) = '163. 

 # (1]() )=13 18' sin(9 (11 o) = *230. 

 (111) = 8°0' sin0 (111) = -139. 



Calculated angle 9° 13' 

 „ 13° 2' 

 „ 8° 0' 



sin#( 100 ):sin 6 { 



(110) 



sin 6 



(in) 



= 1:1-41: '85 = 1: V^-a/^. 



This relation between the angles of reflexion from th& 



* Cf. W. H. Bragg, Phil. Mag. May 1914, p. 



