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Proceedings of the Royal Society of Edinburgh. 



183^, Dec. 3. — Sir Thomas Makdougall Brisbane, 

 President, in the Chair. At this Meeting the following com- 

 munications were read : — 



1. On the Colours of Natural Bodies. By Sir David 

 Brewster, V. P. R. S. Ed. 



The only Theory of the Colours of Natural Bodies that has met 

 with reception in modern times, is that of Sir Isaac Newton, who 

 considers them as identical with those of their plates, and as vary- 

 ing with the size of the ultimate particles of the body. 



Although this theory, ingenious as it is, be liable to many great 

 objections, and be not capable of explaining the phenomena, even 

 if its postulates be admitted ; yet the author of the present paper 

 does not assail it with any arguments of this kind. He has, on 

 the contrary, attacked it in its stronghold, and has endeavoured to 

 bring it to the test of direct experiment. 



Sir Isaac Newton considers the green colour of plants (the most 

 general colour which nature presents to us) as a green of the third 

 order of periodical colours, and has also given us the exact compo- 

 sition of this particular colour. 



In order to determine the composition of the green colour of 

 plants, the author dissolved their colouring matter in alcohol ; and 

 having analyzed it by a fine prism, he found it to have, in every 

 case, the same composition. The portions of the spectrum, how- 

 ever, which entered into its compound tint, were totally different 

 from its theoretical composition, as assigned by Sir Isaac Newton ; 

 and had no relation whatever to the colour of their plates. The 

 green colouring matter exercised an arbitrary specific action upon 

 different parts of the spectrum, and its green colour was owing to 

 its having absorbed a certain number of rays, which, when sub- 

 tracted from the white light, left the colour under consideration. 



In order to render this result more general, the author examined 

 an immense number of coloured solutions, obtained from plants and 

 artificial salts, and a great variety of coloured solids, either formed 

 by art, or obtained in nature ; and in all these cases, he found no 

 indication whatever of periodical colours. The colours were inva- 

 riably produced by the absorption of certain definite rays taken ar- 

 bitrarily and unequally from different parts of the spectrum ; and, 



