672 Mr. A. 8. Eve on the Secondary Radiation 
minute. The secondary radiation divided by the density, 
and the coefticients of absorption of the secondary radiations 
by aluminium and brass, are also stated. 
The results are chown graphically in fig. 3, where the 
Fig. 3, 
Secondary Aadtiation 
° 2 ¢ s im) z-) ‘2 (4 
Densiry 
densities are denoted by abscissze and the secondary radiation 
by ordinaies. 
Some experiments were made in order to ascertain the 
depth from which secondary radiation was effective at the 
surface. In fig. 4 are shown the secondary radiations due to 
successive layers of paper. The radiation was nearly pro- 
portional to the number of sheets until 10 or 12 sheets of 
foolscap (each *06 mm. thick) were used. After that the 
rate of increase of secondary radiation fell away rapidly, and 
36 sheets gave the same result asa much larger number, 
Good radiators, such as brick or lead, causing strongly 
penetrating secondary radiation, could be detected behind 
