THE SUN. 



part of it is in its natural or unpolarised state, even when issuing 

 at the greatest obliquity ; and, consequently, that the luminous 

 matter is everywhere gaseous. 



22. All the phenomena which have been here described, and 

 others which our limits compel us to omit, are considered as giving 

 a high degree of physical probability to the hypothesis of Sir "W. 

 Herschel already noticed, in which the sun is considered to be a 

 solid, opaque, non-luminous globe invested by two concentric 

 strata of gaseous matter, the first, or that which rests immediately 

 on the surface, being non-luminous, and the other, which floats 

 upon the former, being luminous gas or flame. The relation and 

 arrangement of these two fluid strata may be illustrated by our 

 own atmosphere, supporting upon it a stratum of clouds. If such 

 clouds were flame, the condition of our atmosphere would represent 

 the two strata on the sun. 



The spots in this hypothesis are explained by occasional openings 

 in the luminous stratum by which parts of the opaque and non- 

 luminous surface of the solid globe are disclosed. These partial 

 openings may be compared to the openings in the clouds of our sky, 

 by which the firmament is rendered partially visible. 



23. Many circumstances supply indications of the existence of a 

 gaseous atmosphere of great extent above the luminous matter 

 which forms the visible surface of the sun. It is observed that the 

 brightness of the solar disk is sensibly diminished towards its 

 borders. This effect would be produced if it were surrounded by 

 an imperfectly transparent atmosphere, whereas if no such 

 gaseous medium surrounded it, the reverse of such an effect might 

 be expected, since then the thickness of the luminous coating 

 measured in the direction of the visual ray would be increased very 

 rapidly in proceeding from the centre towards the edges. This 

 gradual diminution of brightness in proceeding towards the borders 

 of the solar disk has been noticed by many astronomers ; but it 

 was most clearly manifested in the series of observations made by 

 Sir J. Herschel in 1837, so conclusively, indeed, as to leave no- 

 doubt whatever of its reality on the mind of that eminent observer. 

 By projecting the image of the sun's disk on white paper by 

 means of a good achromatic telescope, this diminution of light 

 towards the borders was on that occasion rendered so apparent, 

 that it appeared to him surprising that it should ever have been 

 questioned. 



But the most conclusive proofs of the existence of such an 

 external atmosphere are supplied by certain phenomena observed 

 on the occasion of total eclipses of the sun, which will be fully 

 explained in another part of this series. 



24. The heat generated by some undiscovered agency upon the 

 110 



