28 Professor Forbes on the Colours of the Atmosphere. 



From an attentive comparison of the beautiful series of tints, 

 identical with those of thin plates, so produced, Nobili en- 

 deavours to assign empirically, as Newton had done, the 

 orders to which the colours of Nature belong ; only, instead 

 of cautiously proposing them as guesses, like his illustrious 

 predecessor, he assigns them, with a degree of confidence but 

 ill sustained by the now almost untenable character of New- 

 ton's theory of the colour of bodies. Many of the remarks 

 are very ingenious, but whenever he contradicts Newton, he 

 seems, I think, to fall into evident inaccuracy. The general 

 question is one with which we have now nothing to do, and 

 therefore I confine myself only to the statements which con- 

 cern the present subject. Because he has banished the blue 

 of the frst order, as having no existence*, he is forced to 

 assign to the blue of a clear sky the character of the second 

 order ; whilst he attributes the tints of flocculent clouds, par- 

 tially illuminated by the sun or moon, to the first order ; in 

 other words, he supposes the vesicular vapour of which he 

 speaks, to have double the thickness in an azure sky, than in 

 the midst of a fog, whilst Newton expressly assigns the blue 

 of the first order to the air, because " it ought to be the colour 

 of the finest and most transparent skies in which vapours are 

 not arrived at that grossness requisite to reflect other colours, 

 as we find it is by experience." This is only one of the va- 

 rious contradictions into which the artist-like view of match- 

 ing colours by external resemblances, and assuming a common 

 origin, has led the ingenious author. The application of the 

 colours reflected from vapours to measure the thickness of the 

 vesicles t was, we have seen, completely anticipated by Krat- 

 zenstein, and the generality of the application disproved by 

 Melvill half a centui'y ago, when he speaks of the theory of 

 the "gaudy colours" of the clouds arising, "like those of the 

 soap bubble, from the particular size of their parts." 



1 have perused Nobili's Memoir with a most anxious wish 



* Nobili quotes Aniici's authority in confirination of this novel assertion, 

 and also for the alleged absence of green in the second order of colours. 

 I think I can speak with much confidence as to the existence of blue of the 

 first order in the depolarized tints of mica plates: but the attempt to show 

 (Bibl. Univ. xliv. p. 343 and 344, note), that there ought to be no blue, and 

 that the first colour of Newton's scale should be while, seems to me a faihn-e, 

 arising from a degree of misconaeption of first principles which it is difticult 

 to admit. 



t In the translation of the paper in Taylor's Scientific Memoirs, i. d9, 

 by an oversight, the maximum thickness of the cloudy vesicles is stated at 

 Ihc ten-inJUionth of an inch, instead of ten millionihs of an inch, or a hundred 

 times greater, as in the original. There is even a slight mistake in the latter ; 

 the tint he describes corresponding to plates of water, not of air, would 

 require a thickness of seven millionths. 



