448 Dr. C. A. Sadler on Homogeneous 



Radiation from an ordinary X-ray bulb falling upon the 

 radiator R 2 excited a beam of homogeneous secondary X rays 

 whose absorption coefficient by aluminium was known. This 

 homogeneous beam passed through the apertures A and B in 

 the ionization chamber I, and fell upon the radiator R 3 , which 

 served as a source of corpuscular radiation. 



The aperture B was covered by a thin sheet of aluminium 

 which also gave off corpuscular radiation usually ot small 

 intensity. This latter radiation could be allowed for, and 

 thus the corpuscular radiation from the radiator R 3 deter- 

 mined. 



Let us consider the case when corpuscular radiation is 

 emitted in a normal direction by such a plate as R 3 , upon 

 which a uniform parallel beam of homogeneous Rontgen 

 radiation falls perpendicularly. The Rontgen radiation 

 passing across the space of the ionization-chamber between 

 the aperture B and the radiator R 3 is absorbed to a certain 

 extent by the air. If the distance between B and R 3 be 

 made 1 cm., then if I be the energy passing per second 

 through unit area of the window B whose area is S, the 

 absorption per second of the energy of the secondary beam 

 by the air of the ionization-chamber is given by 



j (l_,-x 2 )S, 



where X 2 is the absorption coefficient in air of the secondary 

 beam. 



Jt was pointed out in a previous paper* that the ionization 

 produced in a given volume of air traversed by a beam of 

 homogeneous Rontgen radiation is approximately propor- 

 tional to the absorption of that radiation by the air through 

 which it passes. We may, therefore, write for the ionization 

 in this case 



d (i_£-x 2 )S, 



where C is a constant. 



Turning now to the corpuscular radiation excited by the 

 secondary Rontgen beam, we may define the quantity k such 

 that the fraction of the energy of the incident secondary 

 Rontgen beam passing normally through a thin layer of the 

 radiator R 3 of thickness 8x which is transformed into energy 

 of corpuscular radiation is k 8%. 



If 1' be a measure of the energy of the secondary Rontgen 

 beam passing normally per second through unit area of the 

 tertiary radiator at a depth x below the surface,, the fraction 



