692 Mr. C. G. Barkla on Secondary 



The arrangement was then as follows : — 



A bnlb and induction-coil were placed inside a lead-covered 

 box M (fig. 3) in one side of which was a rectangular aper- 

 ture A through which a beam of Eontgen rays passed. 

 Immediately outside this aperture were lead shutters S l5 so 



Fig-. 3. 



Earth 



Earth 



that the width could be varied as desired. Two large lead 

 shutters S 2 were placed at a distance of 17 cms. from this 

 aperture and parallel to the side of the box, so that the width 

 of a second aperture B was adjustable. 



The gas chamber described above was placed with its end 

 window immediately behind the second screen, with its end 

 face against the two shutters and in such a position that a 

 beam of X-rays passed through the parchment window and 

 the opposite end of the box while the sides were perfectly 

 protected. The electroscope L, which was placed with its thin 

 face opposite and parallel to the side window F, was protected 

 by lead from radiation which might otherwise have entered 

 it through its other faces. The thin face was protected from 

 radiation from all directions except through the window of 

 the gas chamber. 



The primary beam did not fall upon this window, so that 

 no primary radiation was received by the electroscope. 



Again, the two sources of secondary radiation from solids 



