178 THE CONSTITUTION OF" LIGHT. [SECT. xix. 



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 Complemen- 

 tary 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 modifications it undergoes at the surfaces and in the 

 interior of terrestrial 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. 190), which, separating the rays by re- 

 fraction, 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, find- 

 ing that it was no longer capable of decomposition by re- 

 fraction, concluded that white light consists of seven kinds of 

 homogeneous light, and that to the same colour the same 

 refrangibility ever belongs, and to the same refrangibility the 

 same colour. Since the discovery 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 contrary, the clearest 



