380 PROFESSOR FORBES ON THE COLOURS OF THE ATMOSPHERE. 



mentum of the red ray (deduced, I presume, from the Newtonian theory of re- 

 fraction), as the explanation of its greater transmissibility, and the reflection of 

 the blue, attributing the colours of sunset to the former, those of a pure atmo- 

 sphere to the latter. It would have been more correct, however, simply to as- 

 sume the blueness of the atmosphere for reflected, and its redness for transmitted 

 light, since we see in differently coloured media, that the assumed prerogative of 

 the red ray does not hold, being absorbed by a green or blue glass, whilst the 

 other rays persevere. 



Humboldt gives no positive opinion upon the colours of the atmosphere, or 

 of water.* 



It is singTilar that I have been unable to discover in Dr Young's various 

 writings very positive notices of his opinion on this subject, though it is pro- 

 bable that he coincided in general with the view last stated, f He seems to have 

 leaned strongly to Newton's theory of the colour of bodies, though he was not 

 insensible to its difficulties. 



Sir John Leslie very explicitly adopts the theory of air reflecting blue light, 

 and transmitting orange, as a full and adequate solution of the colour of a pure 

 sky, and also of the tints of yellow, orange, red, and crimson, which characterize 

 the sun's light when near the horizon. ^ The important observation of Sir D. 

 Brewster, || that the blue light of the sky is polarized, and therefore has under- 

 gone reflection, is conclusive on that point, although the cause of the peculiari- 

 ties of the plane of polarization in different regions of the sky is" not easily ex- 

 plained. § 



Sir John Herschel coincides with Newton in considering the colour of the 

 sky as the blue of the first order, and as one of the most satisfactory applica- 

 tions of the Newtonian theory.^ 



But the author who, of all others I have met with, supports Bouguer's 

 theory of the colour of the sky with greatest fulness and ingenuity, is Brandes, 

 in the article Abendrothe (evening redness), in Gehler's Physikalisches Wor- 

 terbuch.** He maintains the colour of the sun, and surrounding clouds, at 

 sunset and sunrise, to be due soMt/ to the colour of pure air, — a doctrine 

 which he supports by many striking arguments. The presence of vapours, he 

 observes, is always indicated by a dull white, mixed with the azure of the 



* See his Relation Historique, 8vo, ii. 116, &c. 



f See his Nat. Phil. ii. 321. Compare pages 637, 638, 646, on Newton's Theory of the Colour 

 of Bodies. 



J Encyclopaedia Britannica, art. Meteorology. The same theory is maintained in the article 

 Physical Geography by Dr Traill, just published. 



II On New Philosophical Instruments, p. 349. 



§ Peclet, Traite de Physique, ii. 307. Brussels edit. ; Herschel on Light, art. 858, and Quetelet's 

 Supplement to the French translation. 



t Essay on Light, art. 1143. ** Vol. i. p. 4. &g. 1825. 



