174 



The secondary incidence radiation increased in amount 

 as the thickness of the radiator was increased. In fig. vi. 

 is shown the relation between the mass of the radiator and 



the amount of the secondary incidence y radiation for C, 

 Al, Zn, and Pb. 



C and Al give maximum values of nearly the same 

 amount, viz., 300; Zn not quite so much, viz., 270; while 

 Pb gave only 100. Comparing these with the corrected 

 values for the emergence radiation from Pb and Zn respect- 

 ively, which were 680 and 1400 approximately, it is 

 seen that the lack of symmetry between the effects 

 on the two sides of a plate is very marked — the effect on the 

 emergence side being from about 4'5 to 6'5 times that on 

 the incidence side. The quantities on the incidence side are 

 consequently so small that they become difficult to measure 

 with any great accuracy, especially when an attempt is made 

 to determine the quality of the radiation. 



To measure the quality of the incidence radiation the 

 top of the ionization chamber was covered with a lead sheet 

 about 1 cm. thick, from the centre of which was cut a cir- 

 cular hole 17*5 cm. diameter. One of the larger domes was 

 then inverted and placed so as to intercept the radiation from 

 the radiator, which was placed in the large circular hole just 

 mentioned. A thickness of radiator sufficient to give the 

 maximum effect was used in all cases. There appeared to 

 be very little difference in the quality of the radiation re- 



