154 CONSTITUTION OF LIGHT. SECT. XIX. 



the rays by refraction, formed a spectrum or oblong 

 image of the sun, consisting of seven colors, red, orange, 

 yellow, green, blue, indigo, and violet ; of which the 

 red is the least refrangible and the violet the most. But 

 when he reunited these seven rays by means of a lens, 

 the compound beam became pure white as before. He 

 insulated each colored ray ; and finding that it was no 

 longer capable of decomposition by refraction, concluded 

 that white light consists of seven kinds of homogeneous 

 light, and that to the same color the same refrangibility 

 ever belongs, and to the same refrangibility the same 

 color. Since the discoveiy of absorbent media, how- 

 ever, it appears that this is not the constitution of the 

 solar spectrum. 



We know of no substance that is either perfectly 

 opaque or perfectly transparent. Even gold may be 

 beaten so thin as to be pervious to light. On the con- 

 trary, the clearest crystal, the purest air or water, stops 

 or absorbs its rays when transmitted, and gradually ex- 

 tinguishes them as they penetrate 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 

 irregularhy-efleeted in all directions by the imperfec- 

 tions 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 sub- 

 stances, after decomposing the white light which falls 

 upon them, reflect some colors and absorb the rest. A 

 violet reflects the violet rays alone, and absorbs the 

 others. Scarlet cloth absorbs almost all the colors ex- 

 cept red. Yellow cloth reflects the yellow rays most 

 abundantly, and blue cloth those that are blue. Con- 

 sequently color 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 par- 

 ticles of the silk acquire the property of reflecting the 

 red rays most abundantly and of absorbing the others. 



