32 Professor Forbes on the Colours of the Atmosphere. 



redness to be more or less distinctly perceived, though inge- 

 nious, is palpably wrong. The simplest experiments prove 

 that the redness is not merely apparent, but depends upon the 

 admixture of the variable ingredients of the atmosphere. The 

 proof is the Prismatic Analysis of the sun's light, and we may 

 add, the observation of artificial lights in different states of 

 the atmosphere, which at some times are seen in their natural 

 condition, at others lose all their rays but the red, and finally 

 vanish in fogs with an intense red glare. 



3. If fogs and clouds modify the solar light on the principle 

 of reflecting the rays they do not transmit, why do not such 

 fogs and clouds appear vividly blue by reflected light, as 

 Nollet supposed a foggy atmosphere must do to a spectator 

 placed beyond it? 



4. If the vesicles constituting the clouds give to the colour- 

 less light falling upon them the various hues of sunset, wh}', 

 in the first place, do we not perceive bows of various hues, as 

 Kratzenstein did in operating on the small scale; and how 

 comes it that clouds, identical in structure, nay, the very same 

 clouds, do not exhibit sunset tints at any other time of day ? 

 But the most convincing proof of any, is simply to watch the 

 progress of the solar rays tinging a cloud successively with 

 diffisrent hues, just as it would a lock of wool similarly placed ; 

 or as it does the snowy Alpine summits. Forster mentions 

 an instance of detached cirrocumuli being of a tine golden- 

 yellow, but in a single minute becoming deep red. 



5. To these unanswerable difficulties the prismatic analysis 

 of the blue and sunset tints of the sky superadds one conclu- 

 sive against the theory of Newton as it at present stands. The 

 reflected blue and transmitted red-orange are not colours of 

 thin plates. They are derived from all parts of the spectrum 

 by the mysterious process of transmission, which has preserved 

 them and absorbed the rest. It is hopeless at present to in- 

 quire what is the mechanical constitution of the medium which 

 has effected this alchemy. 



One question, however, which is quite within our reach, 

 remains to be answered. The colours of the sky cannot in- 

 deed be explained, if by explanation we mean an ultimate 

 analysis of the mechanism producing them; but the theory of 

 absorption is incomplete until we can show in what part of the 

 course of the rays of light, and under what varying circum- 

 stances, the different pha^nomena of colour may be produced. 

 Hassenfratz observed, that the light of the horizontal sun was 

 deficient, when analysed by the prism, in all the violet and 

 blue rays*. Sir D. Brewster, making a similar observation 



♦ Kamtz, Lchi-buch,m. AQ. 



