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XV. On the Connexion between the Phenomena of the Absorption of Light, and the 
Colours of thin Plates. By Sir David Brewster, K. II. LL.D. F.R.S. 
Received May 9, — Read May 11, 1837. 
Since the phenomena of the absorption of light by coloured media began to be 
studied with attention, various philosophers have regarded them as inexplicable by 
the theory of the colours of thin plates, and have consequently regarded Sir Isaac 
Newton’s theory of the colours of natural bodies as either defective in generality, or 
altogether unfounded. Mr. Delaval* was the first person who brought an extensive 
series of experiments to bear upon this subject. Dr. Thomas Young 'f- considered it 
“ impossible to suppose the production of natural colours perfectly identical with 
those of thin plates,” unless the refractive density of the particles of colouring bodies 
was at least twenty or thirty times as great as that of glass or water, which he con- 
sidered as “ difficult to believe with respect to any of their arrangements constituting 
the diversities of material substances.” Sir John Herschel has expressed a still 
more decided opinion upon this subject. He regards “ the speculations of Newton 
on the colours of natural bodies” as only “a premature generalization,” and “limited 
to a comparatively narrow range ; while the phenomena of absorption, to which he 
considers the great majority of natural colours as referable, have always appeared to 
him to constitute a branch of photology sui generis ^.” 
The general opinion advanced by these three philosophers I have long entertained § ; 
and with the view of supporting them I have analysed a great variety of colours which 
are exhibited by the juices of plants. In a paper “ On the Colours of Natural Bodies ||,” 
I have shown that the green colour of plants, the most prevalent of all the colours of 
natural bodies, in place of being a green of the third order, as Newton and his com- 
mentators assert, is a colour of no order whatever, and having in its composition no 
relation at all to the colours of thin plates. 
In arriving at these conclusions, however, and drawing a distinct line between the 
phenomena of absorption and those of thin plates, two classes of facts are compared 
under very different circumstances. In the one case philosophers have studied in 
cumulo the result of the successive actions of an infinite number of the colorific parti- 
cles upon the intromitted light, whereas in the other case they have observed only the 
* Manchester Memoirs, vol. ii. p. 131. f Ed. Nat. Phil. vol. i. p. 469, 4S1. and vol. ii. p. 63S. 
| London and Edinburgh Philosophical Magazine, December 1833, vol. iii. p. 401. See also his Treatise 
on Light, Encyc. Metrop. p. 580, 581. 
§ Life of Newton, chap. vii. || Edinburgh Transactions, vol. xii. 
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