120 REPORT—1840. 
VIII. Aurora Boreauis*. 
247. I have little information on this subject to offer in addi- 
tion to what was formerly given. It is to be regretted, that the 
system of observation, vigorously commenced by the British 
Association in 1833, and of which specimens are contained in 
the report of the Cambridge Meeting, has not been pursued. 
1X.—Opricat METEOROLOGY. 
A. Colour of the Sky and Clouds. 
248. The blue colour of the sky has, from a very early 
period, attracted attention. Leonardo da Vincif, and: many 
succeeding writers, vaguely attributed it to a mixture of light 
reflected from the matter of the atmosphere with the darkness 
of the celestial spaces beyond ; an opinion which Gothe has 
revivedt. Muncke§ has asserted that the blueness is a mere 
ocular deception arising from the structure of the eye, but such 
a doctrine can hardly now be seriously maintained ; and I have 
elsewhere|| offered an explanation of his fundamental experi- 
ment. 
249. Newton supposed that the blue of the sky is due to 
very attenuated vapours producing the first tints of the scale of 
the colours of thin plates. He further attributes the colours 
of sunset and of clouds generally, as he had done those of most 
natural bodies, to the varying thickness of such vesicles**. 
The latter opinion has been revived and illustrated by No- 
bilitt. 
250. Against this theory it may be urged, (1) that the sky 
appears intensely blue at elevations and under circumstances 
which forbid us to suppose that vapour can be present at all in 
a watery or vesicular form (which Newton’s statement di- 
stinctly supposes), otherwise the hygrometer would attest its 
existence ; (2) that with respect to clouds, were they coloured 
by the nature of their surfaces as a soap-bubble is, they would 
present iridescent bands, and they would not partake, as we see 
this sheet is passing through the press, I am enabled to add that no decided 
meteoric appearance has been observed in Noy. 1840 at Paris, or in the West 
of Europe generally. 
* See last Report, p. 254. Mahlmann, p. 230. 
+ Traité de la Peinture, quoted in Gehler’s Worterbuch, art. Atmosphare. 
+ Farbenlehre, i. 59, quoted by Humboldt. 
§ Schweigger’s Journal, xxx. 81; and Gehler, wt supra. 
|| Edinburgh Transactions, xiv. 381. { Optics, book ii., part iii., prop. 7. 
** Optics, ibid, prop. 5, end. 
tt Bibliothéque Universelle, 1830, xliv. 337; and Taylor’s Scientific Me- 
moirs, vol, i. 
