Professor Forbes on the Colours of the Atmosphere, 31 



fleeted and transmitted light. This is more or less com- 

 pletely the opinion of Mariotte, Bouguer, Euler, Leslie, and 

 Brandes. 



(2.) That the colours of the sky are explicable by floating 

 vapours acting as thin plates do in reflecting and transmitting 

 complementary colours. This was Newton's theory, which has 

 been adopted in whole or in part by many later writers, and 

 especially by Nobili. 



(3.) On the principle of opalescence and of specific absorp- 

 tion depending on the nature and unknown constitution of 

 floating particles. To this theory in its various stages, we find 

 Fabri, Melvill, Delaval, Count Maistre, and Sir D. Brewster 

 attached. 



These different views are so easily blended, and have often 

 been so far misunderstood even by their supporters, that it is 

 impossible to draw any definite line between them. I will 

 notice a few of the leading points of difficulty which present 

 themselves to some of these opinions, and tend to restrict the 

 field of inquiry. 



1. The azure of the sky cannot, I think, with any proba- 

 bility, be referred to the existence of those vesicular vapours 

 which are supposed to act so important a part in the mecha- 

 nism of clouds. We have no evidence direct or indirect of 

 their existence, whenever the hygrometer is not affected, nor 

 indeed where it does not indicate absolute dampness. The 

 atmosphere we know to be pre-eminently transparent when 

 loaded with uncondensed vapour. That vapour may be 

 colourless, or it may not ; the presumption is, 1 think, that it 

 has no colour, since the blue of heaven is always most fully 

 developed when the dryness of the air is intense ; and that even 

 at heights which render it in the last degree improbable that 

 any condensed vapour should exist at heights still greater. 

 We are as ignorant of the constitution of the parts of pure 

 vapour, as we are of the parts of pure air : vesicles are 

 lioater, not vapour ; — to speak of films capable of reflecting 

 definite colours when no water exists in the air, or the hy- 

 grometer does not indicate absolute dampness, is to speak 

 (as Berkeley said of Fluxions) of the ghosts of departed 

 quantities. 



2. Admitting that the blueness of the reflected light of the 

 sky is an inherent quality, of which we can give no account, 

 we must next say that it is running too fast to a solution to 

 admit with Brandes that the red of evening is solely caused 

 by the colour of the air being complementary to its reflected 

 tint. His explanation of the variable redness of sunset, 

 owing to the variable opacity of white vapours allowing the 



