Symmetry shown by Secondary X-Rays. 857 



materials. But if a thin sheet of any substance was laid 

 over the hole at B, secondary X-rays spread out therefrom, 

 and some passed through the screen SS, and caused a 

 deflexion in the electrometer. The difference between the 



Fijy. 1. 



To 

 Electromek 



200 VoltS 



.4> in. 



Fig. 2. 



D^ 



D'. i ;D 



& 



wgl 



'D 



Fig. 3. 



-i^i&is 



deflexions (a) without and (/>) with the sheet at B was taken 

 as a measure of the emergence secondary X-ray radiation. 

 When the sheet was removed from B, and the same or a 

 similar sheet placed in the plane of the top of the screen so 

 as to be struck from below by the primary rays, then the 

 measure of the incidence secondary radiation was obtained 

 as the difference between the deflexions (a) without and (c) 

 with the sheet so placed. 



