COLOUR AND CHEMICAL STRUCTURE 



By S. C. BRADFORD, B.Sc. 

 The Science Museum, South Ke7isingto7i, London 



Sir I. Newton showed that white light is not a simple colour, 

 but can be split up by means of a prism into the spectral colours : 

 red, orange, yellow, green, blue, indigo and violet. White light 

 is therefore merely the colour of sunlight and probably owes 

 its apparently homogeneous character to the fact that it is 

 the average colour of the light that enters our eyes under normal 

 conditions. 



With the exception of luminous and fluorescent bodies, the 

 colours of the objects we see around us are not due to any power 

 which these possess of creating light of a particular colour, but 

 are produced by the absorption of certain constituents of 

 sunlight and the transmission or scattering of the rest. Absorp- 

 tion may either be distributed generally through the spectrum, 

 or it may be selective and confined to distinct regions. In the 

 case of selective absorption, the " absorption bands," as they 

 are termed, may occur in the visible spectrum, or in those 

 regions extending beyond the range of vision known as the 

 " infra-red " and " ultra-violet." Only those substances which 

 exert strong selective absorption within the limits of vision 

 appear to be coloured, but in considering the relations between 

 light absorption and chemical constitution it has been found 

 necessary to extend the study to the invisible regions of the 

 spectrum which affect a photographic plate. An apparatus 

 known as a spectograph invented by Sir William Hartley is 

 used for this purpose. It consists essentially of a spectroscope 

 attached to a camera with prisms and lenses of quartz instead 

 of glass which would absorb the ultra-violet rays. 



The first observations on the chemical structure of light- 

 absorbing bodies were naturally confined to visibly coloured 

 substances, and the earliest suggestion as to the cause of colour 

 in organic compounds was made by Graebe and Liebermann in 

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