92 THE ROYAL SOCIETY OF CANADA 
where s is the intensity across unit area at unit distance when there is 
no absorption («k==0), in which case (2) simply expresses the law of 
inverse squares. 
Thiel 1h 
Suppose s to be in a uniform medium whose co-efficient of absorp- 
tion is x, while the intensity is measured in a medium of co-efficient A, 
the two media being separated by a surface cutting sS at a point P such 
that sP —R. Then provided we neglect effects of refraction and 
reflection at the bounding surface, a condition which is justifiable in 
the application to be considered, we may consider the path of the rays 
forming the pencil of small solid angle w to be unaltered in crossing 
the boundary. Under these conditions the intensity at P in the medium 
Kis given by 
while the intensity in the medium X is given by 
i= 5 Bt NE 
2 
The constant of integration C is determined by equating intensi- 
ties at the boundary r — R, which we may do if we neglect losses due 
to refraction and reflection. 
We find 
—x«R dR 
CRE e 
so that 
— KR —vdX (r — R) 
‘€ (3) 
