Permanent Colours of Opake Bodies. 237 



greatly exceeds the denfity of air, and thereby 

 approaches much nearer to that of the colouring 

 matter itftrlf. 



When diftant objeifls are feen, through a 

 confiderable traft of fuch rare air as constitutes a 

 blue fky, thofe objeds afTume the blue colour of 

 the air. For they mull be confidered as re- 

 flexive bodies, acting on the incident light, in 

 the fame manner as the refieflive particles, which 

 are difFufed through the air. So that the re- 

 fleded light paffcs back, from them, mixed with 

 the blue rays, which are reflected by the minute 

 extraneous particles of the atmofphere. 



If a denfer atmofphere, or a longer trad of ir, 

 intervene between the objeds and the eye, they 

 become fainter and Icfs diftind, and appear 

 whiter; as the rays refleded from them are 

 mixed with the whiter hue, which numerous 

 particles communicate to the air. 



By the interpofition of a ftiil greater quantity 

 of the refledive particles of the air, the objeds 

 difappear. For the weak refleded light of 

 diftant objeds, cannot, like the ftrong light of 

 luminous bodies, penetrate femipellucid fub- 

 flances, fo as to tranfmit through them even the 

 lefs refrangible rays. 



Thefc, and all the phenomena of the atmo- 

 fphere, may be imitated, and explained, by 

 artificial femipellucid fublianceSj fuch as have 

 been here defer i bed. 



Sir 



