the Absorption of Rontgen Rayi 



743 



radiation on the primary, and that the primary was itself 

 unaltered. 



j- As an example, Ag radiation, after the scattered rays had 

 been sifted out, was absorbed to the extent of 14' 5 per cent, 

 by a sheet of Al '0208 cm. in thickness ; but after trans- 

 mission through Fe — 51 per cent, being absorbed — the 

 transmitted radiation was absorbed by 2l"7 per cent, by 

 the same sheet of Al. It thus appeared much softer. It 

 was seen, however, that this emergent radiation still con- 

 sisted of an untransformed Ag radiation with a more easily 

 absorbed radiation superposed. For when the transmitted 

 radiation was again passed through a sheet of Al '0208 cm. 

 thick, the easily absorbed Fe radiation was absorbed com- 

 pletely and the transmitted radiation was again practically 

 pure Ag radiation, being absorbable to the extent of 14'4 per 

 cent, by the same sheet of aluminium. 



In order to ascertain if the constitution of the emergent 

 beam in such a case could be quantitatively accounted for, 

 the following simple experiment was made : — 



A thin sheet of Fe was placed in the path of the homo- 

 geneous radiation from Cu, so that the angle of incidence 

 of the central ray was about 20° (fig. 1). 



Fin-. 1. 



An electroscope capable of being rotated about a vertical 

 axis through the centre of the Fe sheet B (set in a lead screen) 

 was placed successively in the two positions D x and D 2 . In 

 position D x it received the direct beam after passing through 

 the thin Fe sheet, and a portion of the secondary radiation 

 emerging from the iron sheet, the central ray being inclined 

 at an angle of about 20° to the normal ; in position D 2 it 

 received only the secondary radiation which emerged from 

 the sheet at the same angle on the other side of the normal. 



The two secondary beams then suffered approximately the 

 same absorption on emerging from the iron plate. 



The ratio of the ionizations in the electroscope in positions 



