208 Sir D. Brewster on the Connexion between 



possibly appear within my present limits, on which account I 

 desire to reserve it to a separate communication. 



[To be continued.] 



XXXVII. On the Connexion between the Phenomena of the 

 Absorption of Light, and the Colours of thin Plates. By 

 Sir David Brewster, K.H., LL.D., F.R.S* 

 ^INCE the phenomena of the absorption of light by co- 

 loured 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 un- 

 founded. Mr. Delavalf was the first person who brought an 

 extensive series of experiments to bear upon this subject. Dr. 

 Thomas Young J 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 considered 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 pha2- 

 nomena of absorption, to which he considers the great ma- 

 jority 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 ex- 

 hibited by the juices of plants. In a paper " On the Colours 

 of Natural Bodies f ," 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 

 commentators assert, is a colour of no order whatever, and 

 having in its composition no relation at all to the colours of 

 thin plates. 



* From the Philosophical Transactions, 1837, p. 245. 

 •f Manchester Memoirs, vol. ii. p. 131. 



t Elements of Nat. Phil. vol. i. p. 469, 481 ; and vol. ii. p. 638. 

 § Philosophical Magazine, Dec. 1833, S. 3, vol. iii. p. 401. See also his 

 Treatise on Light, Encyc. Metrop., p. 580, 581. 

 || Life of Newton, chap. vii. 

 If Edinb. Trans., vol. xii. [Also Phil. Mag., Third Series, vol. viii. p. 468.] 



