on the Optical Phenomena of the Atmosphere. 99 



of the increase at heights above 6000 feet coincides, in the Alps, 

 with the disappearance of the larger valleys. From this forwards 

 the alterations, as we ascend, are very uniform*. A correspond- 

 ing point of greater acceleration is not therefore to be expected 

 at the same height on mountains whose mean altitudes are un- 

 like. This point would move upwards as the heights of the 

 mountains increase. The latter assertion is corroborated by a 

 comparison of the observations made by Alexander von Humboldt 

 on the Andes and on the Alpsf. 



We have heretofore confined ourselves to a comparison of the 

 maxima depths of colour at different elevations; although a 

 regular increase is here exhibited, the higher and lower situations 

 approximate very near with regard to the degree of shade to 

 which the blue of the sky can sink. We observed in some in- 

 stances, at a height of more than 7000 feet, 35 per cent, and 

 less of cobalt when no trace of cloud or fog was to be perceived 

 either with the naked eye or with the telescope. A singular 

 clearness of the heavens is observed in the zenith itself, even at 

 the greatest elevations, at the beginning of the morning twilight. 

 The minimum occurs on ordinary days between 2 and 3 o'clock 

 in the morning, the maximum a little before noon ; the firma- 

 ment is afterwards in general lighter on account of the ascent of 

 vapours. 



The difference between the maximum and the minimum bright- 

 ness within twenty-four hours increases with the height, because 

 the minima in high and in low situations are very similar; for 

 the same reason the differences between max. and min. are greater 

 in the tropic regions than in higher latitudes %. j 



* How considerably the presence of valleys can alter the increase of blue 

 as we ascend is very evident from the observation of Saussure, that Cha- 

 mouni has often a less deeply coloured sky than Geneva. 



t With regard to the different attempts made to explain the colour of 

 the atmosphere, see the elaborate memoir of Forbes, "The Colours of the 

 Atmosphere with reference to a previous paper, ' On the colour of Steam 

 under certain circumstances.' " — Philosophical Magazine, vol. xiv. pp. 121, 

 419. And Pogg. Ann. 1842; Supplementary volume, vol. i. pp. 49-/8. 

 Compare also the interesting memoirs of Clausius, "Ueber die Natur 

 derjenigen Bestandtheile der Erdatmosphare durch welche die Lichtre- 

 flexion in derselben bewirkt wird." — Pogg. Ann., vol. lxxvi. pp. Hil-188; 

 and " Ueber die blaue Farbe des Himmels und (he Morgen und Aben- 

 drbthc." — Pogg. Ann., vol. lxxvi. pp. 188-195. It is shown in the last 

 memoir, that in the reflexion of light from thin plates the blue has the 

 advantage ; in the transmission of the light, the orange has the advan- 

 tage. That the blue of the heavens is due to reflected light is also corro- 

 borated by experiments on polarization. In this respect there is scarcely 

 any difference observed between higb and low situations. Forbes, who 

 examined the polarization on the summit of the .Tungfrau, found it "nor- 

 mal, but somewhat less strong than in the depths below." — Uesor. Excur- 

 sions, p. 405. 



X Alex. von. Humboldt's observations, Voyages, vol. ii. p. 123, and vol. xi, 

 p. [3. 



If 2 



