SUPPLEMENTARY REPORT ON METEOROLOGY. 123 
occurred in 1783 and 1831; on both occasions they extended 
from Europe to America; the former lasted a month, and 
enveloped the highest Alpine summits*. 
256. Blue Sun.—At the latter date (1831) the sun’s dise was 
seen of a blue or green colour in the South of Europe and in 
America. This extraordinary phenomenon we might be dis- 
posed to attribute, with M. Aragoy, to an ocular deception 
arising from the intense contrasted orange of the fog, did we 
not find that it has been repeatedly observed under circum- 
stances to which this explanation would perhaps hardly apply. 
M. Babinet, a skilful observer, has seen it twice himself t. He 
accounts for it by Dr. Young’s theory of mixed plates, in 
which colour is produced by the interference of two pencils of 
light which have passed through unequal thicknesses of a re- 
tarding medium. This he supposes may be the case in the 
atmosphere by the union of rays ‘‘ which have passed through 
vesicles of water or vapour with those which have passed 
through air only.” Now, though M. Babinet has ingeniously 
imitated the effect by a thin film of mixed air and water placed 
between two glasses, there is some difficulty in conceiving an 
extended medium like the atmosphere, which should present an 
analogous constitution §. 
257. Secondary Sunset Tints.—Many authors have described 
the appearance of a revival of the sunset glow upon the sum- 
mits of lofty mountains long after apparent sunset, and ten or 
fifteen minutes after the tints which accompanied it have dis- 
appeared ||. The appearance in question was once noticed with 
extraordinery effect by the writer of this report, in the case of 
the Jung Frau seen from the profound valley of Lauterbrun- 
nen, from which the sun had so long disappeared that it was 
almost night below, whilst the upper half of the snowy moun- 
tain was illuminated by a delicate but intense red tint, like that 
of a glowing coal. Prof. de la Rive has offered as an explana- 
* Annuaire, 1832, p. 244. + Annuaire, p. 249. 
t Comptes Rendus, viii. 306. Sir D. Brewster communicated at Glasgow an 
account of this phenomenon, observed by Dr. Harvey at Bermuda. See 
Athenzeum, 3rd October, 1840. The appearance referred to occurring on the 
10th August, 1831, is evidently part of the same widely-extended appearance 
quoted by M. Arago in the Annuaire for 1832. Dr. Harvey, however, does 
not appear to have observed the blueness of the swn’s disc, but only that of 
objects illuminated by it ;—a circumstance, which, had it stood alone, might pro- 
bably have been accounted for by the doctrine of accidental colours. 
§ The phenomenon of the scintillation of the stars has lately engaged the 
attention of M. Arago, who states (Comptes Rendus, 1840) that he has disco- 
vered a complete explanation of it founded on the laws of the interference of 
light. 
|| Germ. “ Gliihen der Alpen.” See Brandes in Gehler, art. 4lmosphare. 
