Professor Forbes on the Colours of the Atmosphere. 25 



But it appears to me that the powers and riches of these 

 extraordinary districts remain yet to be fully developed. 

 They exhibit an immense number of mighty steam-engines, 

 furnished by nature at no cost, and applicable to the produc- 

 tion of an infinite variety of objects. In the progress of time 

 this vast macliinery of heat and force will probably become 

 the moving central point of extensive manufacturing establish- 

 ments. The steam, which has been so ingeniously applied to 

 the concentration and evaporation of the boracic acid, will 

 probably hereafter, instead of wasting itself in the air, be em- 

 ployed to move huge engines, which will be directed to the 

 infinite variety of production which engages the attention of 

 labouring and intelligent artisans; and thus, in the course of 

 time, there can be little doubt, that these lagoons, which were 

 fled from as objects of danger and terror by uninstructed 

 man, will gather round them a large intelligent population, 

 and become sources of prosperity to innumerable individuals 

 through countless generations. 



VI. The Colours of the Atmosphere considered 'with reference 

 to a previous Pajjcr " On the Colour of Steam under certain 

 circumstances^ By James D. Forbes, Esq.^ F. B. SS. L. 

 ^ Ed., Professor of Natural Philosophy i7i the University of 

 Edinburgh. 



[Continued from vol. xiv. p. 426.] 



A TOTALLY different hypothesis from any of the preceding, as 

 regards the blue of the sky, was about the same time started 

 by Muncke. He asserts that this hue is, what the German 

 writers call purely subjective, that is, an ocular deception, re- 

 ceived by the eye on looking into vacant space*. This theory 

 has been well discussed by Brandes, but I think he has not 

 succeeded in explaining Muncke's fundamental experiment, 

 which is this :— If the sky be viewed by one eye directly, and 

 by the other through a long blackened tube, the colour in the 

 lattei- case gradually seems to vanish. Now, the explanation 

 of this optical difficulty is to be found, I conceive, in the ge- 

 neral fact first observed by Mr. Smith j, and which I have 

 verified in a great variety of cases, that when a white object 

 is viewed at once by both eyes, one shaded, and the other 

 powerfully illuminated, though its natural colour is undoubt- 

 edly white, it appears red to the shaded eye, and green to the 

 other. The shaded eye in Muncke's experiment, therefore, 

 superimposes a red impression (by the effect of contrast with 



• Schweigger's Journal, XXX. 81 J and article y/^?H05/'^ioV«' in Gehler. 

 t Edin. Journal of Science, v. 52. 



