35 



have been placed, and a hundred worlds as large as ours hurled 

 into the abyss, and yet it would not have been filled to the brim. 

 The photograph now thrown on the scene is a copy of Nasmyth's 

 drawing of this vast abyss in the luminous envelope of the sun. 

 This light-giving general surface of the sun is termed the photo- 

 sphere ; and before we can rightly understand the nature of a sun 

 spot we must know something more of this luminous envelope. 

 I do not speak, remember, of the body of the sun itself, for of 

 that we know very little, if anything. All that Ave know about 

 is the outer coating — the rind, so to speak, of the great orange, its 

 photosphere and atmosphere. You will observe, then, first of all, 

 that the suns surface is ragged, uneven, coarsely mottled with 

 patches of varying brightness. 



In the representation of a sun spot now on the screen (after 

 Nasmyth's drawing) these smaller patches of light assume some- 

 what the form of willow leaves. This was the name given them 

 by_Nasmyth. Other astronomers have designated them by the 

 words, rice-grains, straws, granules ; and others, failing to see any 

 likeness to these, have fallen back on the charmingly indefinite ' 

 name of " things. " Remember, however, that the smallest of 

 these rice grains must be at least as large as Great Britain. You 

 will get perhaps a better idea of the general surface of the sun 

 from this copy of a drawing, by Secchi, which is now before you. 

 Here also you will see how these willow leaves link themselves to- 

 .gether and shoot out a huge bridge of light, spanning the vast 

 abyss, on the edge of which they are so well defined. And now 

 I think it will be clearer to you what sun spots are. It is not, 

 however, quite enough to say that they are cavities in this liquid, 

 or moie probably densely gaseous photosphere. There are still 

 one or two other points in which they demand attention. Thej" 

 generally exhibit three shades of darkness ; the darkness increasing 

 from the general surface till the apparent centre of the spot is 

 reached. We have first the penumbra, then the umbra, then the 

 nucleus. Again, these spots are for ever changing their form. 

 The photograph now on the screen will give you an idea of the 



