30 Professor Forbes on the Colours of the Atmosphere. 



The late Mr. Harvey of Plymouth, gives a minute analysis 

 of the colours of the clouds*, which he considers only expli- 

 cable on the theory of absorption, which office he assigns to 

 the particles of the clouds themselves, though he admits that 

 these often transmit pure white light. He is even ready to 

 believe that the sun has sometimes been observed blue or 

 green, an observation which I think M. Arago has rightly 

 considered as an optical deception arising from the contrasted 

 colour of an intensely red sky, such as that which occurred 

 in many parts of the world on the occasion of the dry fog of 

 1831t. 



Brandes's theory of the evening red is especially applicable 

 to the rich purple hue thrown over Mont Blanc and the higher 

 Alpsf after the sun has set to the plains, and that kind of 

 redness is usually observed in cloudless skies, not like the 

 gorgeous colouring of our northern sunsets, to which I parti- 

 cularly referred in my former paper. In a communication 

 read to the British Association in 1837, M. De la Rive ac- 

 counts ingeniously for a repetition of this phaenomenon which 

 is sometimes observed 10 or 15 minutes after the first disap- 

 peared. This he plausibly attributes to a total reflection im- 

 dergone by the rays of light in the rarer regions of the atmo- 

 sphere when in a state of great humidity and transparency || . 

 Probably upon the principle of multiplied reflections, the 

 cases of preternaturally protracted twilights may be explained, 

 such as those recorded by Kamtz §. 



It is now time that we endeavour to sum up briefly the 

 evidence we have collected. 



If we exclude the theory of Leonardo da Vinci and Giithe, 

 attributing the colour of the sky to a mixture of light and 

 shade ; and that of Muncke, which would make it a mere 

 optical deception, we shall find the chief principles which have 

 been maintained, reduced to three. 



(1.) That the colour of the sky is that reflected by pure air, 

 and that all the tints it displays are modifications of the re- 



from the interior, and it then appears green (Coniptes Rendus, 23d July 

 1838.). Most authors have admitted the intrinsically blue or green colour 

 of pure water, as Newton (Optics, b. i., part ii., prop, x.), Mariotte (already 

 quoted), and Euler : Humboldt seems doubtful (Voyage, 8vo, ii. 133.). 



* Encyc. Metropolitana, art. Meteorology, p. 163, &c. 



t Annuaire 1832, p. 248. Whilst this Paper is passing through the press, 

 I have seen a notice by M. Babinet {Comptes Jiendus, 25th Feb. 1839), on 

 the subject of the blue colour of the sun, which he considers as real, and 

 endeavours to explain by the theory of mixed plates. [See on this subject 

 the Miscellaneous articles in the present Number. — Edit.] 



I Germ. " GUihen der Alpen." 



II Seventh Report of British Association. Transactions of Sections, p. 10. 

 6 Lehrbuch der Meteorologie, iii. 58, 



