Experimental Philosophy. [Lecture 14. 



appear bluish white by the reflected light, but 

 orange by that which is transmitted through it. 



The blue colour of the sky may be accounted 

 for upon this principle. The atmosphere may 

 be considered as a semipellucid medium, which 

 is loaded with small and light particles of va- 

 pour ; and these particles may be compared with 

 the particles of arsenic, which are mingled in the 

 glass above mentioned. If the air is very heavily 

 charged with these vapours, therefore, a large 

 proportion of the light will be reflected, and 

 that dusky whiteness appears which distinguishes 

 mists and fogs ; but in a clear state of the atmo- 

 sphere only the weaker and more refrangible rays, 

 such as the blue, violet, &c. are reflected, and 

 hence proceeds the blue colour of the sky. 



On the same principle depends the green 

 colour of the sea. It is a mixed mass, charged 

 with heterogeneous particles. All the more re- 

 frangible rays, therefore, are reflected, while the 

 stronger rays, the red, orange, &c. are trans- 

 mitted. Thus Dr. Halley, in a diving-bell, sunk 

 many fathoms deep in the sea, observed, that 

 when he extended his hand out of the bell into 

 the water, the upper part of it was red, and the 

 lower part a blueish green. The redness was 

 occasioned by the strong red rays, which in 

 their progress through the mass of water were 

 intercepted and reflected by his hand ; while, 

 on the contrary, the heterogenous particles dis- 

 persed through the water reflected only the re- 



