Dr Brewster on a New Analysis of Solar Light. 197 



Art. II. — On a New Analysis of Solar Light, indicating 

 three Primary Colours, forming Coincident Spectra of equal 

 length. By David Brewster, LL.D. F.R.SS. Lond. and 

 Ed. ( Abridged Jrom the Transactions qf the Royal Society 

 of Edinburgh. Read 21st March 1831.) 



In submitting to the scientific world a new analysis of light, 

 I am fully aware of the difficulties which I have to encounter. 

 Even in physical science it is an arduous task to unsettle long 

 established and deeply-rooted opinions, and that task becomes 

 Herculean when these opinions are entrenched in national feeling 

 and associated with immortal names. There are cases indeed, 

 where the simple exhibition of new truths is sufficient to dispel 

 errors the most deeply cherished, and the most venerable from 

 their antiquity ; but it is otherwise with doctrines which depend 

 on a chain of reasoning where every step in the inductive pro- 

 cess is not rigorously demonstrative ; and of this we require 

 no other proof than is to be found in the history of Newton's 

 optical discoveries, and particularly in the opposition which 

 they experienced from such distinguished men as Dr Hooke 

 and Mr Huygens. 



In the investigations which I am about to explain, the in- 

 strument employed is the absorbent action which different 

 bodies exercise on different rays of white light- This principle, 

 which science had hitherto scarcely recognized, was brought in- 

 to notice bv the recent discoveries respecting polarization and 

 double refraction, and was, I believe, first employed as an in- 

 strument of analysis, in a paper printed in the Edinburgh 

 Transactions, (vol. ix. p 433.) In the experiments there de- 

 scribed, I examined Dr Wollaston's spectrum of four colours, 

 viz. red, green, blue, and violet, by means of a purplish-blue 

 glass. This glass absorbed the blue rays, which, when mixed 

 with the yellow, made green, and the yellow rays, which, when 

 mixed with the red, made orange; and by insulating the yd- 

 loxo and red, it thus effected a perfect analysis of the compound 

 green and \\w compound orange. From this experiment I drew 

 the conclusion that yellow light lias an independent existence in 

 the spat mm; and that the pnsm is incapable of decomposing 



