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RED PROMINENCES SEEN DURING TOTAL ECLIPSES OF THE SUN. 467 
4. On the Faculee and Luculi seen on the Sun’s Disc. 
The supposition that the sun is surrounded by an envelope of cloud, occasion- 
ally penetrated by apertures, may serve to explain the facule and Juculi seen on 
the solar disc. These are portions of the sun’s surface brighter than the rest; 
and they have been supposed to be ridges in his luminous atmosphere, indicating 
violent agitation in their neighbourhood.* May they not, however, be simply 
apertures in the envelope of cloud? If such’ apertures exist, the sun’s surface 
seen through them will appear more luminous than elsewhere; for his light pass- 
ing through the apertures will escape, more or less, the absorption suffered by 
the rays which traverse the envelope itself. This explanation of the faculee is quite 
consistent with the well-known fact, that they are best seen when near the sun’s 
edge. Thus, if the dotted line /7 (fig. 11) represent the surface of the envelope, 
and ef ¢ f’ two apertures in it, seen from a distant point in the prolongation of 
CA; the ray g will be contrasted with A h, which has passed perpendicularly 
through the envelope, while the ray g’ is contrasted with B7, which has passed 
obliquely through the envelope, and therefore suffered more absorption than A a. 
It is evident, then, that the nearer a facula approaches the sun’s limb, the more _ 
strongly will it contrast with the brightness of the surface in its neighbourhood, 
and the more distinctly will it be seen. 
If, however, the faculze be regarded as ridges in the sun’s luminous atmosphere, 
their brightness, compared with the rest of the sun’s disc, and their increased dis- 
tinctness when near his limb, may also be explained by supposing that a thin en- 
velope of cloud surrounds the sun; for if A Bd/a’ be the envelope, and Aa the 
height of aridge, the point @ will appear brighter than the rest of the surface, be- 
cause its light has only traversed the thickness aa’ of the absorbing medium, while 
that from the rest of the sun’s surface has passed through the whole thickness 
Aa’, and has consequently been more absorbed. Again, if a ridge has the posi- 
tion bd, so as to be seen projected on the sun’s limb,—supposing all the atmo- 
_ sphere above a) removed,—we should have the light from @ and 4, which would 
not then suffer absorption, contrasted in the one case with light which had passed 
through the thickness A a, and in the other through the greater thickness B 6 of ab- 
sorbent matter. The contrast at 6 would evidently then be greater than at a. 
If now the rays from A a traverse together the remaining portions of the sun’s at- 
mosphere, they will be almost equally absorbed, and the relative brightness of the 
points a A will not be sensibly altered; the same will be true with regard to Bd. 
Hence the contrast of the brightness of the ridge, when compared with that of the 
sun’s general surface, will still be greatest at the sun’s limb.+ 
* Sir Witiram Herscuer. Philosophical Transactions, 1801. 
¢ Sir Wiziiam Herscuer, who regards the facule as ridges, does not seem to hold that 
opinion very confidently. In his paper, already noticed, he only once states that they “have 
the appearance of elevations,”’—(Phil. Trans. 1801, p. 84.) Mr Dawes, however, has recently 
