LIGHT. 



called opaque ; and such bodies consequently render a luminary 

 invisible if interposed between it and the eye. 



Transparency and opacity exist in various bodies in different 

 degrees. Glass, air, and water are examples of very transparent 

 bodies. The metals, stone, earth, wood, &c. are examples of 

 opaque bodies. 



Correctly speaking, no body is perfectly transparent or per- 

 fectly opaque. 



There is no substance, however transparent, which does not 

 intercept some portion of light, however small. The light is 

 thus intercepted in two ways ; first, when the light falls upon the 

 surface of any body or medium, a portion of it is arrested, and 

 either absorbed upon the surface, or reflected back from it ; the 

 remainder passes through the body or medium, but in so passing 

 more or less of it is absorbed, and this increases according to the 

 extent of the medium through which the light passes. Analogy, 

 therefore, justifies the conclusion that there is no transparent 

 medium which, if sufficiently extensive, would not absorb all the 

 light which passes into it. 



208 



