PROFESSOR FORBES ON THE COLOURS OF THE ATMOSPHERE. 385 



parent than it is in an opposite state, the distant mountains then appearing more 

 distinct, — a well known prognostic of rain, and the sun then sets without pro- 

 ducing colours ; in the fogs and vapours of the morning, the light of the sun is 

 white, but the red colour of the clouds at sunset is generally regarded as the fore- 

 runner of a fine day, because these colours are a proof of the dryness of the air, 

 which then contains nothing more than the particular disseminated vapours to 

 which it owes its opaline property." In this interesting passage we have, I am 

 persuaded, all that is known of the cause of atmospheric colours, with the single 

 want of the link which shall shew that watery vapour is sometimes capable of 

 absorbing all but red rays, and sometimes not.* 



The late Mr Harvey of Plymouth, gives a minute analysis of the colours of 

 the clouds,! which he considers only explicable on the theory of absorption, which 

 ofl&ce 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 1831. ± 



Brandes's theory of the evening red, is especially applicable to the rich purple 

 hue thrown over Mont Blanc and the higher Alps|| 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 particularly referred in 

 my former paper. In a communication read to the British Association in 1837, 

 M. DE LA Rive accounts ingeniously for a repetition of this phenomenon which is 

 sometimes observed 10 or 15 minutes after the first disappeared. This he plausi- 

 bly attributes to a total reflection undergone by the rays of light in the rarer 

 regions of the atmosphere when in a state of great humidity and transparency. § 



* Count Maistre explains the colour of the water by similar reasoning. He considers it blue for 

 reflected, and yellowish-orange for transmitted light, and the green colour of the sea and some lakes he 

 attributes to diffused particles which reflect a portion of the transmitted tint, and mingle with the blue. 

 This is well confirmed by Davy's Observations, (Salmonia, 3d edit. p. 317). Arago has very ingeni- 

 ously applied the same reasoning to the ocean, shewing that when calm it must be blue, but when ruffled, 

 the waves acting the part of prisms, refract to the eye some of the transmitted light from the interior, 

 and it then appears green, (Comptes Rendus, 23d July 1838.) Most authors have admitted the intrin- 

 sically blue or green colour of pure water, as Newton (Optics, b. i., part ii-, prop, x.), Mariotte (al- 

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



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



J Annuaire 1832, p. 248. Whilst this Paper is passing through the press, I have seen a notice by 

 M. Babinet (Comptes Rendus, 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. 



II Germ. " Gliihen der Alpen." 



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



