﻿Cosine Law of Radiation. 239 



the boundary. Under these conditions the intensity at P in 

 the medium ic is given by 



I- a — KT 



r" 



while the intensity in the medium \ is given by 



T C A 



The constant of integration C is determined by equating 

 intensities at the boundary r = R, which we may do i£ we 

 neglect losses due to refraction and reflexion. 



We find 



so that 



(3) 



§ 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. 



Fin-. 2. 



Thus, if we have an incandescent body bounded by a sur- 

 face 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 thence an observer 

 at P. If k be the coefficient of absorption of the incan- 

 descent body to its own radiations, the intensity at P (situated 

 outside S in a medium of coefficient of absorption \) due to 

 a single vibrating element at s is, from (3), 



,S ' -kK -A(,— io 



where R = A* and r = Ps. 



If there are N vibrating elements per unit of volume, t ho 



