Professor Forbes on the Colours of the Atmosphere. 27 



sun, sometimes appear richly coloured, and at other times 

 scarcely coloured at all, — a circumstance which renders it 

 questionable whether the colour is from the cloud itself, or 

 whether the cloud only reflects the light which is coloured by 

 refraction in passing through the haze of the atmosphere in 

 the evening. The former is, however, probably the case ; for 

 different clouds, in nearly the same angular position with re- 

 spect to the sun, show different colours at the same time*." 



I must quote myself as having formerly adopted the theory 

 of Bouguer, with regard at least to the celestial blue. In one 

 of a series of papers on the Bay of Naples, published about 

 ten years ago, I noticed the occurrence of a strictly purple 

 tinge (the poetic lumen piirpiireum), in a perfectly clear sky, 

 which I attributed to a part of the violet rays, mixed with the 

 blue, finding their way to the eye. There is no question 

 (notwithstanding the authority of Eustace t)j that Virgil's 

 epithet was founded on the accurate observation of Nature. 

 The fact has also been observed by Humboldt and by Leslie J. 

 We now come to the theory of M. Leopold Nobiliof 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 II 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 metallic surfaces, by 

 precipitation from solutions by means of galvanic decompo- 

 sition. This beautiful art of forming what Nobili calls his 

 *' Apparences Electro-chimiques," was first pointed out to me, 

 as well as the papers describing it, by Professor Necker of 

 Geneva, as far back as the winter 1831-2, when some mem- 

 bers of the Society may recollect that I exhibited in this room 

 specimens of Nobili's chromatic scale, prepared by myself §. 



* Researches about Atmospheric Phaenomena, 3rd edit., 1823, p. 86. The 

 continuation of the passage will be quoted further on. 



f " In the splendour of a Neapolitan firmament, we may seek in vain for 

 that purjjle light so delightful to our boyish fancy." — Tour in Italy, 



\ Encyclopaedia Britannica, art. Meteorology, 



II Bibliotheque Universelle (1830), tom. xliv. p. 337. — Translated in 

 Taylor's Scientific Memoirs, vol. i. 



§ 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 oi' 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 mentioned in one part of Nobili's paper. The explana- 

 tion of these colours, by supposing with the philosopher of Reggio (if I un- 

 derstand him aright), that they are produced by thin plates of adhering 

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



