PROFESSOR FORBES ON THE COLOURS OF THE ATMOSPHERE. 379 



the hemisphere, the colour of the horizontal sky inclines sensibly to purple, be- 

 cause his transmitted light, which mixes with the azure, by passing through a 

 stiU greater length of air, becomes reddish." I have quoted this passage because, 

 so far as it goes, it explains with remarkable elegance the actually observed 

 phenomena, and because it exposes the insufficiency of the theory of iridescent 

 colours to explain the hues of sunset. The theory of vesicular vapour, or floating 

 bubbles of water as constituting clouds, was prevalent even at a far earlier period 

 than this. Leibnitz had supported it in the seventeenth century,* and had cal- 

 culated the rarity of the ethereal fluid with which they were supposed to be filled. 

 Kratzenstein (1740) had, by actual experiment on the colours which they re- 

 flected, attempted to estimate their thickness by direct measurement, to find their 

 diameter, f Saussure demonstrated the existence of bodies apparently so con- 

 stituted, in clouds themselves ; but I nowhere find that he has applied it to explain 

 their coloration on the principle which Melvill justly condemns in this pas- 

 sage. Saussure' s opinion of the blue colour of the sky was, so far as I can judge, 

 that of Mariotte and Bouguer,^ although he alludes very particularly to bluish 

 vapours as foreign matters floating in the upper regions of the sky, which he says 

 were decidedly not aqueous, since they did not affect the hygrometer. § He thinks 

 this may illustrate the obscure phenomena of dry fogs. || 



The memoir of Eberhard of Berlin on this subject,*^ contains nothing to 

 detain us. The author seems to coincide in the theory of Mariotte, and spends 

 much labour in refuting that of Da Vinci. 



Del aval's elaborate Theory of the Colour of Bodies, we may also rapidly 

 dispose of. He adopts the idea of Fabri, that the foreign matters suspended in 

 the air become the means of reflecting blue light, and transmitting red, on the 

 same principle as arsenic dispersed through glass. This comparison to the ac- 

 knowledged phenomena of opalescence, is not unimportant. ** 



The greater part of the optical writers of the present century have closely 

 followed one or other of those already quoted. The writer of the article Optics 

 in the 4th edition of the Encyclopaedia Britannica, which was revised by Profes- 

 sor RoBisoN, gives, as an opinion which he considers new, that of Bouguer and 

 Melvill, with very little modification or addition. He assumes the greater mo- 



* Opera Omnia, ii. p. ii. 82. Edit. 1768. " Cur vapores eleventur non spernenda qusestio est, 

 atque inter alia non male concipiuntur in illis bullae insensibiles ex pellicula aquas et aere incluso con- 

 stantes, quales sensus in liquoribus spumescentibus ostendit." 



f Theorie de I'Elevation des Vapeurs et des Exhalaisons, &c. Bordeaux, 1740. Quoted in Saus 

 sure's Hygrometrie, § 202, and in Kamtz, Lehrbuch der Meteorologie, iii. 48. The diameter he made 

 j^Lg, and the thickness 55^55 inch. 



J Voyages dans les Alpes, iv. § 2083. § Hygrometrie, § 355. 



II Hygrometrie, § 372. IT Rozier, Introduction, i. 618. 



** Manchester Memoirs, Ist Series, ii. 214, &c. 

 VOL. XVI. PART II. 3d 



