42 McCrennanp anp Hackert—The Absorption of B Radium Rays by Matter. 
Mernop or EXPERIMENT. 
A lead screen ZL is so arranged as to give a well-defined pencil of 8 rays 
coming from the radium R. Although not shown in the diagram, the opening 
in the screen is arranged so that the issuing pencil of rays consists largely 
of primary 8 rays, and not of secondary rays from the edges and sides of 
the aperture. 7 is an ionisation vessel of the usual type, consisting of an 
Electrometer 
Electromefer 
> 
Cells 
Fia. 1. 
outer brass tube joined to small storage cells and with an insulated brass 
rod along the axis of the tube joined to an electrometer the rate of charge of 
which measures the ionisation produced by the rays. Plates of the substance 
under examination are placed at P, and observations are made with different 
thicknesses of each substance so as to be able to calculate \ from the observed 
