from a Plate exposed to Rays from Radium. 177 



lead, except through the conical opening. The shape of the 

 block was so arranged, with a corner sawed off at a, and a 

 prolongation added at b, that no secondary radiation from 

 the sides of the conical opening, nor from the absorbing 

 plates A, nor from the surfaces at that end of the block, 



t> sailn. 



could enter the ionization-cell I, as the direction of the dotted 

 line ad will show. A channel c was cut in the protuberance 

 b to hold the ends of the plates A, which were used as 

 absorbing layers to cut down the primary radiation. A 

 conical frustum of lead C (shown in position in the diagram) 

 could be inserted in the conical opening when still further 

 reduction of the beam was required. The height of the 

 frustum used was 1*18 cm. The axis of the block made an 

 angle of 34° with the horizontal. 



The intensity of the secondary radiation was measured by 

 allowing the rays to enter an ionization-cell I which was 

 connected with a Wilson tilted-electroscope. This cell was 

 a brass cylinder of 24*2 cm. and 7*2 cm. diameter, having 

 a central wire insulated by sulphur with an earthed guard- 

 ring. The mouth of the cell was covered by thinnest 

 aluminium-leaf. The central wire was connected to the 

 gold-leaf of the electroscope, which was situated about a 

 metre distant, and the connecting wire was surrounded by 

 an earthed metallic conductor extending from the earthed 

 guard-ring to the box of the electroscope. The cell I and 



Phil. Mag. S. 6. Vol. 14. No. 79. July 1907. N 



