234 Prof. J. A. McClelland on 



Fig. 2 is plotted from these numbers. 



Fig. 2. 























































<2S 















' 























1 









































































1 





































1 



/ 



■ 

































/ 





































/ 































T/j/CKA/£SS OF T/A/FO/L P/.AT£ 



We should of course expect the secondary rays to come 

 from all depths reached by the primary, unless the penetrating 

 power of the secondary be less than that of the primary, in 

 which case the effective thickness of plate will be determined 

 by the penetrating power of the secondary. 



The penetrating power of the secondary rays will be 

 discussed later, but it may be pointed out here that the above 

 numbers indicate that the secondary rays must at least 

 approximate in penetrating power to the primary. The 

 second sheet of tinfoil contributes almost as much secondary 

 radiation as the first, and the rays from the second sheet 

 must pass through the first before reaching the testing-tube T. 

 The secondary rays must therefore suffer no very great loss 

 of intensity in passing through the sheet of foil. 



Analyzing the Primary Radiation (/3 and y rays) with 

 respect to its power of producing Secondary Rays. 



The primary rays used in the above experiments were a 

 pencil of /3 and y rays : it is necessary to investigate what 

 part of the secondary rays is due to the more penetrating part 

 of the primary and what is due to the less penetrating- 

 primary rays. 



This was done by making two separate experiments. In 

 the first the apparatus was used as in fig. 1, and successive 

 layers of tinfoil were placed in front of the lead screen so 

 that the primary rays passed through them, and the intensity 

 of the secondary rays was measured at each step. 



In the second experiment, the radium with the thick lead 

 screen was placed as in fig. 4, so that a pencil of primary 

 rays fell directly on the testing-tube T travelling parallel to 

 its axis, and as before sheets of tinfoil were placed in front 



