TKANSPARENT AND OPAQUE BODIES. 



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. 



A very thin plate of glass is almost perfectly transparent, a 

 thicker is less so, and according as the thickness is increased the 

 transparency will be diminished. The distinctness with which 

 objects are seen through the air diminishes as their distance in- 

 creases, because more or less of the light transmitted from them 

 is absorbed in its progress through the atmosphere. This is the 

 case with the sun, moon, and other celestial objects, which when 

 seen near the horizon are more dim, however clear the atmosphere 

 may be, than when seen in the zenith. In the former case, the 

 light transmitted from them passes through a greater mass of 

 atmosphere, and more of it is absorbed. According to Bouguer, 

 sea- water at about the depth of 700 feet would lose all its trans- 

 parency, and the atmosphere would be impervious to the sun's 

 light if it had a depth of 700 miles. 



The transparency of the same substance varies according to the 

 density of its structure, the transparency generally increasing 

 with the density. Thus, charcoal is opaque, but if the same char- 

 coal be converted into a diamond, which it may be, without any 

 change of the matter of which it is composed, it will become 

 transparent. 



Bodies are said to be imperfectly transparent, or semi-transpa- 

 rent, when light passes through them so imperfectly, that the 

 forms and colours of the objects behind them cannot be distin- 

 guished. Ground glass, paper, and thin tissues in general, foggy 

 air, the clouds, horn, and various species of shell, such as tortoise- 

 shell, are examples of this. 



The degrees of this imperfect transparency are infinitely various, 

 some substances, such as horn, being so nearly transparent as to 

 render the form of a luminous object behind it indistinctly visible. 

 Porous bodies, which are imperfectly transparent, usually have 

 their transparency increased by filling their pores with some trans- 

 parent liquid. Thus paper, which is imperfectly transparent, is 

 rendered much more transparent by saturating it with oil, or by 

 wetting it with any liquid. The variety of opal called hydrophane 

 is white and opaque when dry, but when saturated with water it 

 becomes transparent. Ground glass is rendered more transparent 

 by pouring oil upon it. Two plates of ground glass placed one upon 



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