16 M‘Ciettanp—The Energy of Secondary Radiation. 
values of @ less than 25° or greater than 75°; there can, however, be little 
error introduced by extending the curve to the axes, as is done in the figure, 
especially as the numbers observed showed that the intensity of the radiation 
varied very closely as cos 6. 
The area of the portion of the surface of a sphere cut off by two coaxial cones 
of semi-angles 6 and @+d0 is proportional to sin@d0, Two curves (not 
shown in figure) were therefore plotted, one having for ordinates the ordinates of 
the given curve multiplied by sin@, and the other having for ordinates the 
ordinate CO (corresponding to 45° on the given curve) multiplied by sin 6, the 
abscissee in both cases being degrees from 0 to 90. 
The ratio of the areas enclosed by these two curves and the axis was ‘72. 
Finally, therefore, the total energy of the secondary radiation from the lead 
plate bears to that of the primary radiation absorbed by the plate the ratio 
68 x °72 
1 
= °49. 
The following table gives the value of this ratio (which we shall afterwards 
denote by p) in column IV. The numbers in column III. express, in an arbitrary 
scale, the secondary radiation from the different substances for the same amount 
of incident radiation absorbed ; the numbers are copied from a previous paper.* 
The numbers in column IY. are calculated from column III., and the value of 
p found for lead above, viz. °49. 
The quantity «, which is given in column V., is defined later in the paper. 
The quantities p and « are calculated in the table to the nearest half per cent. 
* Royal Dublin Soc. Trans., Vol. ix., Part i. 
