PROFESSOR FORBES ON THE COLOURS OF THE ATMOSPHERE. 383 



the eye. There is no qiiestion (notAvithstanding the authority of Eustace*), that 

 Virgil's epithet was founded on the accurate observation of Nature. The fact 

 has also been observed by Humboldt and by Leslie.! 



We now come to the theory of M. Leopold Nobili of Reggio, and which, after 

 what has been stated, may be very briefly expounded. In quoting M. Nobili's 

 speculations on this subject as new to me, I must observe, that they are contained 

 in a memoir t on a certain uniform scale of colours, for the use of artists, produced 

 by the elegant method of depositing thin layers of transparent substances on me- 

 tallic surfaces, by precipitation from solutions by means of galvanic decomposition. 

 This beautiful art of formingwhat Nobili calls his " Apparences Electro-chimiques," 

 was first pointed out to me. as weU as the papers describing it, by Professor 

 Necker of Geneva, as far back as the winter 1831-2, when some members of the 

 Society may recollect that I exhibited in this room specimens of Nobili's chromatic 

 scale, prepared by myself || From an attentive comparison of the beautiful series 

 of tints, identical with those of thin plates, so produced, Nobili endeavom-s to as- 

 sign 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 Newton'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 concern the present subject. Because he has banished the blue of the first 

 order, as having no existence, ^ he is forced to assign to the blue of a clear sky 



* " In the splendour of a Neapolitan firmament, we may seek in vain for that purple light so de- 

 lightful to our boyish fancy." — Tour in Italy. 



t EncyclopEedia Britannica, art. Meteorology. 



\ Bibliotheque Universelle(1830), torn. xliv. p. 337. — Translated in Taylor's Scientific Memoirs, 

 vol. i. 



II It is a curious circumstance, which I have never heard remarked, that Dr Priestley in a great 

 measure anticipated the experiment of Nobili ; for, by successive electric discharges on the surface of 

 many kinds of metal, he produced rings identical with those of Newton — Priestley, Phil. Trans. 

 1778. These colours were no doubt produced by the heat developed in the same way as those men- 

 tioned in one part of Nobili's paper. The explanation of these colours, by supposing with the philo- 

 sopher of Reggio (if I understand him aright), that they are produced by thin plates of adhering oxy- 

 gen gas, is too evidently founded in error to require any notice. 



§ Nobili quotes Amici's authority in confirmation 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 shew 

 (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 white, seems to me a failure, arising from a degree of misconception of first 

 principles which it is difl[icult to admit. 



VOL. XIV. PART II. 3 E 



