350 Mr. C. A. Sadler on 



it, and upon the secondary homogeneous radiation that 

 excites it. 



Using the same form of apparatus as that previously used 

 for the determination of the absorption coefficients, the 

 method of experimenting was as follows. 



A given metal to be used as a tertiary radiator was placed 

 in the position R 2 with its plane perpendicular to the axis of 

 the sliding tube. The aperture B was covered with aluminium 

 sheet (*003 cm.), and through this passed a portion of the 

 secondary homogeneous beam from E^ exciting corpuscular 

 radiation from the inner face of the aluminium window and 

 from the face of the tertiary radiator upon which the beam 

 fell. (In all cases the tertiary radiator was sufficiently thick 

 to completely absorb the secondary homogeneous beam.) A 

 measure of the total ionization in the space between the 

 aluminium window and the tertiary radiator when the sur- 

 faces were separated by a distance of I cm. and 2 era. 

 respectively was determined by observing the deflexion, on 

 the electroscope E 2 for a constant deflexion of 20 divisions on 

 the electroscope E 3 . 



Let the ratio of the deflexion on the electroscope E 3 to 

 that on the electroscope E 2 be d\ and d 2 in these two cases 

 respectively. Then d 2 —di is a measure of the ionization 

 produced by the exciting Rontgen-ray beam and the tertiary 

 Rontgen radiations in a layer of air 1 cm. thick. The ioni- 

 zation due to the tertiary radiation was determined in- 

 dependently. Let its value be denoted by a. Then d 2 —di—a 

 is a measure of the ionization produced in a layer of air 1 cm. 

 thick by the secondary beam alone. With the same notation 

 d\ — (do — d 1 )=2di — d 2 is a measure of the total ionization 

 due to the corpuscular radiation from the aluminium window 

 and the tertiary radiator. We thus obtain in the expression 



2^i - d 2 = jj 

 d 2 —di — a 



the ratio of the total ionization due to the corpuscular 

 radiation, to the ionization produced in a layer of air 1 cm. 

 thick by the secondary exciting beam. 



If the radiator R 2 be made of aluminium, and if the 

 intensity of the corpuscular radiation does not depend upon 

 whether corpuscles are produced by the Rontgen rays at inci- 

 dence or on emergence, then one half of the value of the 

 expression R in this case may be taken as the correction to be 

 subtracted from the values of R obtained with other metals 

 as tertiary radiators. Other investigators had made this 



