468 MR WILLIAM SWAN ON THE 
It may be further observed, that if the red prominences are portions of the 
envelope of cloud, thrown upwards by currents in the sun’s atmosphere, which 
tend to produce faculze by ultimately rupturing the envelope, the prominences 
will have a physical connexion with the faculee, but no necessary connexion with 
the solar spots. 
It is well known, however, that conspicuous faculz often occur in the neigh- 
bourhood of groups of spots; and the same upward currents in the sun’s atmo- 
sphere, which are supposed to occasion the spots, by removing a portion of his lu- 
minous envelope, may also occasion the faculee, by removing a portion of the super- 
incumbent cloudy strata. The occurrence, immediately before an eclipse, of faculee 
near the sun’s limb, with or without accompanying spots, ought therefore to indi- 
cate a quarter where red prominences might be expected to appear; and at least 
one coincidence in the position of spots and red prominences was actually observed 
at the late eclipse. A group of spots was seen by me immediately before the eclipse, 
about 15 from the sun’s limb, and to the west of his vertex. The angle of posi- 
tion of this group, reduced to the sun’s north point, was ascertained to be 288° 47’; 
and the conspicuous hook-shaped prominence, which appeared during the totality, 
was found to be situated at 282° 8’ from the sun’s north point. The hook-shaped 
prominence may therefore have been connected with faculz in the neighbourhood 
of the group of spots. The region where spots occur is limited to a zone, extend- 
ing about 25° or 30° on either side of the sun’s equator; and Mr Lasse observes, 
that, as at the late eclipse “‘ some prominences appeared on parts of the sun’s 
limb not usually traversed by spots, the connexion between the two is not made 
out.”* This remark, however, does not apply to the supposed occasional relation 
between the prominences and the faculz; for although facule are seen most 
abundantly in the neighbourhood of spots, they also occur by themselves, and 
have been observed all over the sun’s disc; so that the prominences which are 
supposed to be connected with them might appear at any point in the sun’s limb. 
5. On the Irregular Illumination of the Corona seen during Total Eclipses of the Sun. 
Let us next inquire how the hypothesis of an envelope of cloud surrounding the 
sun’s luminous atmosphere, consists with the phenomena of the corona. 
There are remarkable appearances in the corona, which seem quite inexpli- 
observed a facula projecting beyond the sun’s limb (R. Ast. Soe. Notice for April 1852), With 
reference to that observation, it may be permitted to remark,—that considering the rapid degrada- 
tion in brightness of the sun’s dise towards the edges,—it is evident that the dark glasses used 
in observing the sun may, to some extent, diminish its apparent diameter, the light from the extreme 
edge of the dise being possibly too feeble to be seen through the glass. A portion of the sun’s limb 
brighter than the rest would be less encroached on from this cause, and would thus appear to project 
beyond the general outline. Irradiation would also conspire to increase this effect. The action of 
the dark glasses, in diminishing the sun’s apparent diameter, may also be one of the causes of the 
anomalous variations in observations of the sun’s diameter. 
* R. Ast. Soc. Notice, p. 53. 
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