SCRFACK /.AVERS OF CRVSTAFS 



147 



rotated l)ecause tlic misoricMiled parlick's will lie hniut^'lit into llic rclli'i tinj^ 

 position as the main plate is turned away from it. Ilie farther the main 

 l)lale is turned from the reflecting position, the weaker will he the reflected 

 radiation because less f|uart/, will he misoriented to this an^de. In this 

 respect the double crystal spectrometer technifjue is similar to ('. J. Da- 

 visson's photographic technique, but is measurinjf much smaller angular rota- 

 tion and higher reflection-intensity. A curve of the variation of reflection- 

 intensity with angular rotation of the B crystal plate for two diff'erently 

 linished crystal plates measured by Davis and Stempel'- is shown in Fig. 9 



.06 



.05 



.04 



W .03 



Z 



.02 



__ ICELAND SPAR 

 CLEAVAGE SURFACE 

 UNTOUCHED 



, ^ ICELAND SPAR 



CLEAVAGE SURFACE 

 POLISHED 



60 30 30 60 



ROCKING ANGLE OF CRYSTAL B IN SECONDS 



DAVIS AND STEMPEL 

 PHYS. REV 17(1921) 



Fig. 9. — Double crystal spectrometer rocking curves 



The higher reflection-intensity at the "zero angle" and the ver>- rai)id de- 

 crease of intensity as the plate is turned away from this position show 

 that the untouched surface is less disturbed. The lower reflection-intensity 

 at the "zero angle" and the less rapid decrease of intensity as the plate is 

 turned away from this position show the polished surface to be more dis- 

 turbed. The width of such "rocking" curves at half-ma.\imum (as .\ -x' and 

 Y-Y' in Fig. 9) has often been taken as a measure of perfection of the reflec- 



" Davis, B. and Stempel, W. M., "\n F.xi)crinienlal Study of the Rellection of X-Rays 

 from Calcite," Pliys. Rev. 17 (1921) |)p. 608-623. 



