[KING] NOTE ON THE COSINE LAW OF RADIATION 93 
2. The Cosine Law of Emission. 
We do not have to look far for an immediate application of (3). 
Radiations from incandescent bodies emanate in practically all cases 
from a volume distribution of vibrating elements (atoms or molecules). 
Fie. 2. 
Thus, if we have an incandescent body bounded by a surface_S, 
radiations from the vibrating elements in an element of volume dv in 
the interior are practically all absorbed before reaching the surface at 
A and hence an observer at P. If « be the coefficient of absorption of 
the incandescent body to its own radiations, the intensity at P (situated 
outside S in a medium of co-efficient of absorption 4) due to a single 
vibrating element at S is, from (3) 
Se Goh), 
é é 
s 
T° 
where hk — Als and + — Ps. 
If there are N vibrating elements per unit of volume, the intensity 
at P due to radiations from an element of volume dv is 
AT, — x (r=r;) Hy 
aie INis-e e = 
1? 
where y——— A and 7, = PP: 
If dv refer to an element of volume cut out by the intersection of 
spheres of radii r and r + dr and centre at the vertex P of a small cone 
of solid angle w, we have dv = r” w dr. 
The total intensity at P per unit cross-section normal to PA due 
to contributions from all the elements in the solid angle is 
