378 PROFESSOR FORBES ON THE COLOURS OF THE ATMOSPHERE. 



in a prodigious mass, such as the whole extent of the atmosphere, than that this 

 colour is to be ascribed to vapours floating in the air, which do not pertain to it. 

 In fact, the purer the air is, and the more purged from exhalation, the brighter is 

 the lustre of heaven's azure, which is a sufficient proof that we must look for the 

 reason of it in the nature of the j^roper particles of the air."* 



The Abb^ Nollet (1764) attributes the blue colour of the sky to its reflect- 

 ing those rays ; but, strangely enough, he supposes, that, in order to convey that 

 tint to the eye, they must previously have come to the earth, been reflected by it, 

 and stopped in their second transit through the atmosphere. The colour of the 

 sun in a fog he attributes to the fog stopping the blue rays, at which time, he 

 says, the atmosphere must appear blue externally to an observer in the moon.f 



A very clever but little known wi'iter, Mr Thomas Melvill, who died in 

 1753, aged twenty-seven, has left some interesting observations exactly to our 

 purpose, in a paper published in the second volume of the Edinburgh Physical 

 and Literary Essays.:]: Amongst other acute remarks on optical subjects, after 

 approving of Newton's theory of the blue colour of the sky, he objects to his ex- 

 planation of the tints of sunset, justly inquiring, " Why the'particles of the clouds 

 become just at that particular time, and never at any other, of such magnitude as 

 to separate these colours ; and why they are rarely, if ever, seen tinctured with 

 blue and green, as well as red, orange, and yellow?" "Much rather," he adds, 

 " since the atmosphere reflects a greater quantity of the blue and violet rays than 

 of the rest, the sun's light transmitted through it ought to draw towards orange- 

 yellow or red, especially when it passes through the greatest tract of air ; accord- 

 ingly, every one must have remarked that the sun's horizontal light is sometimes 

 so deeply tinctured, that objects directly illuminated by it appear of a high orange 

 or even red ; at that instant, is it any wonder that the colourless clouds reflect 

 the same rays in a more bright and lively manner." This he more fully illus- 

 trates, and then adds, — " Does it not greatly confirm this explication, that these 

 coloured clouds immediately resume that dark leaden hue which they receive from 

 the sky as soon as the sun's direct rays cease to strike upon them ? For if their 

 gaudy colours arose like those of the soap-huhhle, from the particular size of their 

 parts, they would preserve nearly the same colours, though much fainter when 

 illuminated only by the atmosphere. About the time of sunset, or a little after, 

 the lower part of the sky to some distance on each side from the place of his set- 

 ting seems to incline to a faint sea-green, by the mixture of his transmitted 

 beams, which are then yellowish, with ethereal blue ; at greater distances, this 

 faint green gradually changes into a reddish-brown, because the sun's rays, by 

 passing through more air, begin to incline to orange ; and on the opposite side of 



* Euler's Letters (translation), ii. 507. f Nollet, Leqons de Physique, vi. 17. 1765. 



X Page 81-89, &c. Edin. 1770. 



