341 
subjected to Radium Rays. 
passed upwards into a cylindrical ionization chamber B , which 
was carefully screened from the action of the primary rays by 
thick lead blocks S, S. The current through the ionization 
chamber, was measured in the usual way, with a Wilson electro- 
scope. 
The radium was contained in a small depression in a brass 
cylinder, and was covered with a thin sheet of mica. This 
cylinder fitted into a brass tube T, the end of which was also 
covered with mica, and which was securely fastened in position 
on the bench. The block W, on which the exposed plate A was 
mounted, also moved between wooden guides, screwed down in 
position, on the bench. The radium and the plate could thus be 
removed and the plate exposed to the radium at a short distance ; 
they could then be replaced again in exactly the same positions, 
for the purpose of measuring the secondary radiation. It 
was convenient to measure the radiation from A with the 
radium some distance away from the plate in order to allow 
sufficient space for the efficient shielding of the ionization 
chamber and electroscope, from the direct action of the primary 
rays. 
The guides worked so well that the readings could always be 
recovered to well within °/ 0 if the radium and plate were both 
removed, and subsequently replaced. 
VOL. XIV. PT. IV. 
23 
