1868.] 



Constitution of the Sun and Stars. 



9 



prevail generally over the sun produce the gradation of light fading towards 

 the edge of the disk, and the finely granulated structure of his surface, with 

 its little bright patches, its dusky intervals, and its dark pores. Where 

 circumstances render the cloud thinner over any considerable extent while 

 the rain continues, we have a facula which is visible (if it be not lost in the 

 equal brightness of all around) when it is near the centre of the disk. 

 Where such thin parts occur in numerous small patches, they produce that 

 ordinary mottled appearance of the sun's surface which is visible in tele- 

 scopes of moderate power. If the rain stop, we have penumbra. If the 

 cloud also vanish, we have the umbra of a spot. 



18. We have thus arrived at an hypothesis which in a very satisfactory 

 manner agrees with several of the phenomena. Before, however, we trust 

 ourselves to this or any other particular hypothesis, we must retrace our 

 steps and go over the whole ground with care, retaining at each step all 

 the alternatives which up to that point are possible, and reducing the num- 

 ber by eliminating from the list every hypothesis which we find to be in- 

 consistent with any known fact. 



19. Now, in the first place, the gradation of brightness from the margin 

 to the centre of the sun's disk has usually been attributed to the action of 

 an absorbing atmosphere telling with most effect upon the edges of the 

 disk. But of course faculse cannot be referred to any action of our earth ; 

 and it is incredible, therefore, that they exist only near the edge of the disk. 

 Hence the cause of the gradation of light, whatever it is, must be such as 

 will leave the faculse of unimpaired lustre as they move from the centre to 

 the edge of the disk, while it renders other parts more dusky. We may 

 therefore discard the hypothesis that an absorbing atmosphere is the cause, 

 since it would not act in this way. It is therefore due in some way to the 

 nature of the photosphere itself. 



The telescope informs us that the photosphere consists of two parts which 

 may be distinguished : — a brighter part, seen in the centre of the disk, in the 

 faculae, in smaller bright patches, and in its purest form in the brighter 

 specks of those parts of the surface which are granulated ; and a dusky 

 part, seen towards the margin of the disk and in the interstices between 

 the bright specks of the granulation. Now, incandescent bodies radiate 

 equally in all directions, and therefore, if the light of the sun emanated 

 from a mere mathematical surface, the disk would not be brightest at the 

 centre. Hence the photosphere is a stratum, not a surface. .Again, the 

 brighter parts cannot be at the top of this stratum, since in that case the 

 margin of the sun's disk would be the brightest. Hence the bright and 

 dusky parts are either intermingled, or the dusky parts form the outer 

 layer ; and if they are intermingled, the brighter parts must be the more 

 transparent, to render this hypothesis consistent with the gradation of light 

 we find on the disk. 



Again, the observations show the whole granulated surface of the sun to 

 be in a state of incessant change, although not by any means so impetuous 



