130 



THE ROYAL SOCIETY OF CANADA 



^A5 



THiaP^N£S5 OF ALUMIN/L'M 



Fig. 1 



primary beam through various thicknesses of aluminium and then, after 

 that, the percentage intensity of the rays passing through a sheet of 

 aluminium 1.63 mm. thick was found. In the figure this percentage 

 intensity has been plotted against the thickness of aluminium, through 

 which the primary beam had previously passed. 



The softening and consequent decrease in frequency of the 

 secondary X-rays was shown in the following manner (see Fig. 2). 



A 



M 



P 



R 



P 



\^ 



Fig. 2 



5 represents the source of X-rays, M the tin filter, R the radiator 

 which, unless otherwise stated, was a block of paraffin wax, and E the 

 ionization chamber of an X-ray spectroscope, the slits of Avhich had 

 been removed. The intensity of the secondary rays entering E was 

 measured by a Wilson electroscope. The chamber E was appropri- 

 ately screened so as to reduce the intensity of extraneous radiations 

 to a minimum. The same absorption plate P, usually of aluminium, 

 was alternately placed in positions A and B. Under these circum- 

 stances it can be shown that (except for a small correction), if the 

 primary and secondary rays are identical in quality or frequency, 



