94 THE ROYAL SOCIETY OF CANADA 
A To 
—ÀT, —x(r—,) 
L == Nowe Ae dr, 
1 
or 
: —x(r, — 7) )... (4) 
c= Nso ein fie K 
K 
In an actual case, A is the co-efficient of absorption for air which 
is very small, so that we may take e ms to be unity. The expo- 
—k(r,—r steals : 
nential term e (rs y represents the fraction of radiation which 
can penetrate the entire thickness AB. In solid incandescent bodies 
whose thickness is large compared to a wave-length this fraction is 
negligible. 
Thus the intensity at P is given very approximately by 
7. e. the intensity per unit cross-section of the radiation contained in a 
small solid angle w viewed in any direction PA is proportional to the 
solid angle w. This is equivalent to stating that the whole surface S 
appears uniformly bright. Under these conditions we may replace 
the volume-distribution of radiating elements by a distribution of 
intensity over the surface S. If dS be an element of surface at A cut 
out by the cone w and ¢ the angle between PA and the normal at 4, 
we have 
so that (5) takes the form 
Ns dS cos¢ 
2 
K 7 
i.e. if Io be the normal intensity at unit distance contributed by the 
element dS we have the familiar form of the Cosine Law 
