Iv] NATURE OF THE RADIATIONS 99 
If screens of various thickness are placed on the plate, it is 
observed that the plate is not appreciably affected within a certain 

Big. 22) 
distance from the active matter, and that this distance increases 
with the thickness of the screen. This distance is obviously equal 
to twice the radius of curvature of the path of the rays, which are 
just able to produce an impression through the screen. 
These experiments show very clearly that the most deviable 
rays are the most readily absorbed by matter. By observations of 
this kind Becquerel has determined approximately the inferior 
limit of the value of HA for rays which are transmitted through 
different thicknesses of matter. 
The results are given in the table below: 



: Inferior limit 
Substance WIVES |e ere) an 
in mms. : 
transmitted rays 
Black paper ... 0:065 650 
Aluminium ... 0-010 350 
ss 0:100 1000 
F 0°200 1480 
Mica s.. -. 0°025 520 
Glass ... ah 0:155 1130 
Platinum oe 0:030 1310 
Copper oe 0-085 1740 
Lead ... =a 0:130 2610 





If = is a constant for all the rays, the value of HR is propor- 
tional to the velocity of the rays, and it follows from the table that 
the velocity of the rays which just produce an effect on the plate 
through "13 mms. of lead is about 7 times that of the rays which 
(—2 
