SECT. XIX. CONSTITUTION OF LIGHT. 159 



SECTION XIX. 



Constitution of Light according to Sir Isaac Newton Absorption of Light 

 Colours of Bodies Constitution of Light according to Sir David 

 Brewster New Colours Fraunhoffer's Dark Lines Dispersion of 

 Light The Achromatic Telescope Homogeneous Light Accidental 

 and Complementary Colours M. Plateau's Experiments and Theory 

 of Accidental Colours. 



IT is impossible thus to trace the path of a sunbeam through our 

 atmosphere without feeling a desire to know its nature, by what 

 power it traverses the immensity of space, and the various modi- 

 fications it undergoes at the surfaces and in the interior of ter- 

 restrial substances. 



Sir Isaac Newton proved the compound nature of white light, 

 as emitted from the sun, by passing a sunbeam through a glass 

 prism (N. 195), which, separating the rays by refraction, formed 

 a spectrum or oblong image of the sun, consisting of seven colours, 

 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 coloured 

 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 colour the same re- 

 frangibility ever belongs, and to the same refrangibility the same 

 colour. Since the discovery of absorbent media, however, 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 contrary, the clearest crystal, the 

 purest air or water, stops or absorbs its rays when transmitted, 

 and gradually extinguishes 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 



