138 



small, when compared with the sun's diameter, the difference of its 

 action on the central and lateral rays would be insensible. On the 

 other hand, the wide extension of the corona indicates that the sun"'s 

 atmosphere is of great thickness compared with his diameter ; and 

 there is, therefore, difficulty in supposing the darkness of the sun's 

 edges to arise from the general absorption of light by his atmosphere. 

 That phenomenon, however, is easily explained by supposing it to 

 arise from the absorbent action of a comparatively thin stratum of 

 cloud surrounding the sun. 



2. The faculse are generally understood to be ridges in the sun's 

 luminous atmosphere ; but the author supposes them to be apertures 

 in the envelope of cloud, through which his rays pass more freely 

 than elsewhere. The greater distinctness of the faculse when seen 

 near the sun's limb, is explained by the light shining through the 

 apertures being there contrasted with light which has suffered 

 absorption by passing obliquely through the envelope of cloud ; while 

 towards the centre the contrast is not so great, as the light passes 

 nearly perpendicularly through the envelope, and is therefore less 

 absorbed. 



3. The supposition that the larger prominences are situated on 

 the edges of apertures in the envelope of cloud is consistent with the 

 increased brightness of the corona in their neighbourhood, which was 

 observed at the late eclipse. 



4. The existence of an envelope of cloud surrounding the sun, 

 capable of absorbing light, but penetrated by apertures, and there- 

 fore transmitting light more freely at certain places than at others, 

 may serve to explain the great want of uniformity in the brightness 

 of the corona, and the brilliant beams of light which occur in it .-it 

 certain points. 



The hypothesis that an envelope of cloud surrounds the sun, thus 

 refers to one physical cause, a variety of solar phenomena, namely, 

 the darkness of the sun's limb compared with his centre, the exist- 

 ence of faculse on his disc, the discontinuous illumination of the 

 corona, the existence of the red prominences, and the occasional 

 increased brightness of the corona in their neighbourhood. 



The idea that a cloudy envelope surrounds the sun, occurred to 

 the author immediately after witnessing the eclipse of 28th July 

 1851, when he reflected on the striking want of unifonnity ho had 

 observed in the illumination of (he corona. 



