COMMON THINGS COLOUR. 



the other are very imperfectly transparent; but if the space between 

 them be filled with oil, and their external surfaces be rubbed with 

 the same liquid, they will be rendered nearly transparent. 



7. Bodies, however opaque, lose their perfect opacity when 

 reduced to the form of extremely attenuated laminae. Gold, one of 

 the most dense of metals, is, in a state of ordinary thickness, per- 

 fectly opaque ; but if it be reduced to the form of leaf-gold by the 

 process of the gold-beater, and attached to a plate of glass, light 

 will pass partially through it, and to an eye placed behind it, it 

 will appear of a greenish colour. Other metals, when equally 

 attenuated, show the same imperfect opacity. 



8. "When rays of light encounter the surface of an opaque body, 

 they are arrested in their progress, such surfaces not being pene- 

 trable by them. A certain part of them, more or less according 

 to the quality of the surface and the nature of the body, is ab- 

 sorbed, and the remaining part is driven back into the medium 

 from which the rays proceed. This recoil of the rays from the 

 surface on which they strike is called reflection, and the light thus 

 returning into the same medium from which it had arrived, is said 

 to be reflected. 



The manner in which the light is reflected from such a surface 

 varies according as the surface is polished or unpolished, and ac- 

 cording to the degree to which it is polished. 



If light fall upon a uniformly rough surface of an opaque body, 

 each point of such surface becomes the focus of a pencil of reflected 

 light, the rays of such pencil diverging equally in all directions 

 from such focus. 



The pencils which thus radiate from the various points are those 

 which render the surface visible. If the light were not thus re- 

 flected indifferently in all directions from each point of the surface, 

 the surface would not be visible, as it is from whatever point it 

 may be viewed. 



The light which is thus reflected from the various points upon 

 the surface of any opaque body, has the colour which is commonly 

 imputed to the body. The conditions, however, which determine 

 the colour of bodies will be fully explained hereafter. For the 

 present, it will be sufficient to establish the fact, that each point 

 of the surface of an opaque body which is illuminated is an inde- 

 pendent focus from which light radiates, having the colour proper 

 to such point, by which light each such point is rendered visible. 



9. This mode of reflection, by which the forms and qualities of 

 all external objects are rendered manifest to sight, has been 

 generally denominated, though not as it should seem with strict 

 propriety, the irregular reflection of light. 



There is, nevertheless, nothing irregular in the character of the 

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