382 PROFESSOR FOBBES ON THE COLOURS OF THE ATMOSPHERE. 



colours of bodies, he was naturally induced to view with doubt the composition 

 of the celestial blue, and especially of the colours of clouds. That the re- 

 flected and transmitted tints should be complementary, as Newton's theory 

 assigns, is well known to be rather the exception than rule in coloured bodies 

 generally ; and a very simple prismatic analysis, which it seems difi&cult to mis- 

 construe, proves that the composition of colours — the green of leaves, for in- 

 stance, — is widely different from that which the doctrine of thin plates would 

 infer.* " I have analyzed too," he says, " the blue light of the sky, to which 

 the Newtonian theory has been thought peculiarly applicable, but, instead of 

 finding it a blue of the first order, in which the extreme red and extreme 

 violet rays are deficient, while the rest of the spectrum was untouched, I found 

 that it was defective in rays adjacent to some of the fixed lines of Fraun- 

 HOFER, and that the absorptive action of our atmosphere widened, as it were, 

 these lines. Hence, it is obvious, that there are elements in our atmosphere 



which exercise a specific action upon rays of definite refrangibility 



I have obtained," he adds, " analogous results in analyzing the yellow, orange, 

 red, and purple Ught which is reflected from the clouds at sunset." f Such a 

 prismatic analysis as is here referred to, is even more satisfactory than in the 

 case of the juices of plants, because here the very reflected light itself is exa- 

 mined in the state it reaches the eye. I need hardly add, that this experiment 

 is not less conclusive against the subjective theory of Muncke, than against the 

 theory of thin plates of water of Newton and his followers. 



FoRSTER, in his treatise on Atmospheric Phenomena, maintains the doctrines 

 of Melvill respecting the colour of clouds. " We observe," he says, " that 

 clouds of the same variety, having the same local or angular position with re- 

 spect to the sun, sometimes appear richly coloured, and at other times scarcely 

 coloured at all, — a circumstance which renders it questionable whether the co- 

 lour is from the cloud itself, or whether the cloud only reflects the light which 

 is coloured by refi-action 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 respect to the sun, shew different colours 

 at the same time." :j: 



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 purpureum), in a perfectly clear sky, which 

 I attributed to a part of the violet rays, mixed with the blue, finding their way to 



* Life of Newton, p. 78. 1831. Ed. Trans, xii. 538. 



t Ed. Trans, xii. 544. Compare Encyc. Brit, new Edition, art. Optics, p. 510. 

 X Researches about Atmospheric Phenomena, 3d edit., 1823, p. 86. The continuation of the 

 passage will be quoted further on. 



