172 McCueLtLanp—On Secondary Radiation. 
The first experiment shows a decided maximum when 6 = 45°; the other two 
experiments can be interpreted as showing a maximum when the law of reflexion 
is fulfilled with the secondary radiation! falling off unequally on the two sides of 
the maximum position. 
The numbers indicate that, superposed on an effect of the nature of reflexion, 
we have other effects ; the law of reflexion being fulfilled, we get more secondary 
radiation the more we approach the normal, which is to be expected, as then the 
radiation coming from some depth in the plate has a smaller thickness to 
penetrate. . 
The general effect is, therefore, a radiation in all directions from the part of 
the plate struck by the primary rays with a decided maximum in one direction, 
due probably to a sort of reflexion of the impinging particles. Many points 
described later show that the whole effect is not due to a simple scattering of the 
primary rays, but that it consists of a true secondary radiation from the plate. 
It may also be stated here that experiments with a magnetic field, to be 
described later, show that the secondary rays we are now dealing with consist of 
negatively charged particles. These travel through several centimetres of air, 
and also penetrate the tinfoil covering the end of the ionisation tube 7’ before 
they produce the effect by which they are detected; there may be also a more 
easily absorbed radiation from the plate which has not been studied in the 
present paper. 
Secondary Radiation not merely a Surface Effect. 
The secondary rays are not produced merely at the surface of the plate struck 
by the primary rays; it is easy to show that they come from all parts of a layer 
of considerable depth, the depth depending on the substance of which the plate is 
composed. 
This point was tested by using, for the plate in fig. 1, a single sheet of 
tinfoil (about -013 mm. thick) in the first place, then two sheets, and so on. The 
secondary radiation was as follows :— 
Sheets of Foil. Secondary Radiation. 
1 : ; : ; 13 
2 : : : 24 
4 : : : d 44°5 
8 : : : : 60:5 
16 : ; 3°5 
32 j 6 : ; OU 
Fig. 2 is plotted from these numbers. 
