Professor Forbes on the Colours of the Atmosphere. 423 



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 *. 



A very clever but little known writer, Mr. Thomas Mel- 

 vill, who died in 1753, aged twenty-seven, has left some in- 

 teresting observations exactly to our purpose, in a paper 

 published in the second volume of the Edinburgh Physical 

 and Literary Essays f. Amongst other acute remarks on op- 

 tical subjects, after approving of Newton's theory of the blue 

 colour of the sky, he objects to his explanation of the tints of 

 sunset, justly inquiring, " Why the particles of the clouds be- 

 come 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; 

 accordingly, 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 re- 

 flect the same rays in a more bright and lively manner?" This 

 he more fully illustrates, and then adds, — "Does it not greatly 

 confirm this explication, that these coloured clouds immedi- 

 ately 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 soajp-huhhle^ 

 from the particular size of their parts, they would preserve 

 nearly the same colours, though much fainter when illumi- 

 nated 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 setting, seems to incline to a 

 faint sea-green, by the mixture of his transmitted beams, which 

 are then yellowish, with aethereal blue ; at greater distances, 

 this faint green gradually changes into a reddish-brown, be- 

 cause the sun's rays, by passing through more air, begin to 

 incline to orange ; and on the opposite side of the hemisphere, 

 the colour of the horizontal sky inclines sensibly to purple, 

 because his transmitted light, which mixes with the azure, by 

 passing through a still greater length of air, becomes reddish." 

 1 have (juoted this passage, because, so far as it goes, it ex- 



• Nollct, Lcrotis dc Physique, vi. 17. 1765. 

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



