TRANSMISSION THROUGH AN ABSORBING MEDIUM. 211 



law of density in order that the intensity may vary as 

 some inverse power of the distance : in this case 



m 



so that the general equation becomes 



t n 

 Differentiating, we obtain 



since 



^■-*«- 



But 



<l>(t)=d, 



Hence we get the following equation : — 



so that d varies as the distance inversely. Since 



t_ m 



we get, by eliminating t, 



I«.g)V; 



this gives the relation between the density and the inten- 

 sity at any point. 



Note. — In several papers on colorimetry which I have 

 read before this Society, I have frequently had occasion to 

 refer to the hypothesis that as the length of an absorbing 



