I 



Professor Forbes on the Colours of the Atmosphere. 425 



quoted. The writer of ihe article Optics in the 4th edition of 

 the Encyclopedia Britannica, which was revised by Professor 

 Robison, gives, as an opinion which he considers new, that of 

 BougLier and Melvill, with very little modification or addition. 

 He assumes the greater momentum of the red ray (deduced, 

 I presume, from the Newtonian theory of refraction), 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 atmosphere to the latter. It would have been 

 more correct, however, simply to assume 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 singular 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 probable that he coincided in ge- 

 neral 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 re- 

 flecting blue light, and transmitting orange, as a full and ade- 

 quate 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 obser- 

 vation of Sir D. Brewster |], that the blue light of the sky is 

 polarized, and therefore has undergone reflection, is conclusive 

 on that point, although the cause of the peculiarities of the 

 plane of polarization in different regions of the sky is not easily 

 explained §. 



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 applications of the Newtonian theory f. 



But the author who, of all others 1 have met with, supports 



Bouguer's theory of the colour of the sky with greatest full- 



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



t See his Nat. Phil. ii. 321 . Compare pages 637, 638, 646, on Newton's 

 Theory of the Colour of Bodies. 



X Encyclopedia Britannica, art. Meteorology. The same theory is main- 

 tained in'thc article rhijs'ual Geography hy Dr. Traill, just published. 



II On New Philosophical Instruments, p. 34!). 



$ Pcclet, Traili- dc Physique, ii. 307. Brussels edit. ; Ilcrschel on Light, 

 art. 8.58, and (iuetclet's Supplement to the French translation, 



II Essay on Light, art. 1143. 



