PROFESSOR FORBES ON THE COLOURS OF THE ATMOSPHERE. ^yi 



sky, and the complementary colour of that white which should belong to the 

 transmitted ray can never be red. On the contrary, he says, the colour of 

 the sun seen directly through clouds, when on the meridian, is always white, 

 and the effect even of so strong a mist as to render his disc easily viewed 

 by the naked eye, is to give it the appearance of a silver plate.* The beauty 

 of the sunset, he further observes, is in exact proportion to the purity of the 

 atmospheric blue during the day ; and the only reason, he asserts, why the sun 

 appears to set red through vapours, is because his light is by them so much 

 diluted that the colour can be more distinctly perceived. The colour of ele- 

 vated clouds, at some distance from the horizon, he imputes (as Melvill had 

 done) to the great space of air which the light must traverse before it reaches 

 them, and, after doing so, before it falls on the eye. The green colours of the 

 sky he attributes, as Leslie and most other writers have done, to the reflected 

 blue light mixing with the transmitted orange. This theory was never so ably 

 handled. 



A totally different hypothesis from any of the preceding, as regards the blue 

 of the sky, was about the same time started by Muncke. He asserts that this 

 hue is, what the German writers call purely subjective, that is, an ocular deception, 

 received by the eye on looking into vacant space, f This theory has been well 

 discussed by Brandes, but I think he has not succeeded in explaining Muncke' s 

 fundamental experiment, which is this : — If the sky be viewed by one eye directly, 

 and by the other through a long blackened tube, the colour in the latter case 

 gradually seems to vanish. Now, the explanation of this optical difficulty is to 

 be found, I conceive, in the general fact first observed by Mr Smith, X, and which I 

 have verified in a great variety of cases, that when a white object is viewed at 

 once by both eyes, one shaded, and the other powerfully illuminated, though its 

 natural colour is undoubtedly white, it appears red to the shaded eye, and green 

 to the other. The shaded eye in Muncke's experiment, therefore, superimposes 

 a red impression (by the effect of contrast with the exposed eye) on the blue 

 which it sees, and being its complementary colour, or nearly so, it must tend to 

 diminish the blueness, and finally to produce white. 



Berzelius adopts the view which considers the air itself coloured. || 

 In the older writings of Sir David Brewster, we find the theory of 

 Bouguer maintained ^ ; but since he has been led to what we must con- 

 sider, for a majority of cases, a refutation of the Newtonian doctrine of the 



* Gehler's Physikalisches Worterbuch, vol. i. p. 6, Note.. 



f Schweigger's Journal, xxx. 81 ; and article Atmosphdre in Gehler. 



% Edin. Journal of Science, v. 52. 



11 Lehrbuch der Chemie, Wohler's edit. 1825, i. 346. 



§ Edin. Encyclopaedia, art. Optics, p. 620. Compare articles Atmosphere and Cyanometer. 



