SKCT. xix.] THE ABSORPTION OF LIGHT. 179 



crystal, the purest air or water, stops or absorbs its rays when 

 transmitted, and gradually extinguishes them as they pene- 

 trate to greater depths. On this account objects cannot be 

 seen at the bottom of very deep water, and many more stars 

 are visible to the naked eye from the tops of mountains than 

 from the valleys. The quantity of light that is incident on 

 any transparent substance is always greater than the sum of 

 the reflected and refracted rays. A small quantity is irre- 

 gularly reflected in all directions by the imperfections of the 

 polish by which we are enabled to see the surface ; but a 

 much greater portion is absorbed by the body. Bodies that 

 reflect all the rays appear white, those that absorb them all 

 seem black : but most substances, after decomposing the white 

 light which falls upon them, reflect some colours and absorb 

 the rest. A violet reflects the violet rays alone and absorbs 

 the others. Scarlet cloth absorbs almost all the colours except 

 red. Yellow cloth reflects the yellow rays most abundantly, 

 and blue cloth those that are blue. Consequently colour is 

 not a property of matter, but arises from the action of matter 

 upon light. Thus a white riband reflects all the rays, but, 

 when dyed red, the particles of the silk acquire the property 

 of reflecting the red rays most abundantly and of absorbing 

 the others. Upon this property of unequal absorption the 

 colours of transparent media depend. For they also receive 

 their colour from their power of stopping or absorbing some 

 of the colours of white light, and transmitting others. As for 

 example, black and red inks, though equally homogeneous, 

 absorb different kinds of rays ; and, when exposed to the sun, 

 they become heated in different degrees ; while pure water 

 seems to transmit all rays equally, and is not sensibly heated 

 by the passing light of the sun. The rich dark light trans- 

 mitted by a smalt-blue finger-glass is not a homogeneous co- 

 lour like the blue or indigo of the spectrum, but is a mixture 

 of all the colours of white light which the glass has not 

 absorbed. The colours absorbed are such as mixed with the 

 blue tint would form white light. When the spectrum of 



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