MeCriELLtanp— On Secondary Radiation. 173 
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 
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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. 
Analysing the Primary Radiation (8 and y rays) with respect to its power of 
producing Secondary Rays. 
The primary rays used in the above experiments were a pencil of 6 and y 
rays. It was necessary to investigate what part of the secondary rays was due 
to the more penetrating part of the primary, and what was 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 7, 
travelling parallel to its axis; and, as before, sheets of tinfoil were placed in front 
