20 M‘Crettanp—The Energy of Secondary Radiation. 
section, the exciting 8 rays falling normally on the plate. The numbers given 
above show that the secondary radiation in a direction inclined at an angle 0 
to the normal varies approximately as cos 0. 
In a previous paper it was shown that, in the general case, there is a maximum 
secondary radiation in the direction fulfilling the laws of reflection; but the 
secondary radiation falls off unequally on the two sides of the maximum, the 
less rapid decrease occurring on the side nearer the normal. 
In the section above, the conditions are simpler, as the incident radiation is 
normal to the plate. We get an explanation of the approximate cos@ law by 
considering that the radiation travelling from any element at a small vertical 
depth d@ in the plate to an outside point in a direction 6 from the normal has 
to pass through a thickness of the plate equal to 
d 
cos 8” 
and the intensity contributed by this element will therefore vary approximately 
as cos 6. 
The whole question of the relation between the intensity of radiation and 
the direction, with respect both to the normal and to the direction of the 
exciting radiation, might repay further consideration. 
‘THEORETICAL INVESTIGATION, 
Having determined in a previous section the ratio (p) of the total energy 
of the secondary radiation from a thick plate to that of the exciting radiation, 
we can calculate what per cent. of the energy absorbed by any small element 
of volume of the plate is set free again as energy of secondary radiation. 
B 
Let AB represent the plate, the exciting radiation falling normally on the 
face A. 
Let the energy of the incident radiation passing through unit area of the 
face A per unit time be f). 
