Particles from Thorium Active Deposit. 



577 



mounted on a common supporting base in such a way that 

 the axis of the microscope passed through the centre of the 

 active surface and the hole in the brass plate (see fig. 1). 



Fkr. 1. 



^ gzzzzzzzzz 



*5i 



Zinc >SuLert.' as <Scaee*f 



As<so*&t/v<$ tSCGEGW. 



Hole /•/ mm. £?/as*t. 



Acr/ve. <Sukfac£. 



OS rrr. rrr. £7/A M. 

 ((J QAM TO iScAl-£ ^) 



The air-equivalents of the absorbing screens used in the 

 investigation were determined first of all by weighing 

 measured areas and deducing the air-values from data 



