THE PHOTOSPHERE AND SUN-SPOTS, 541 



the most intricate and capricious patterns the frost ever traces on our 

 window-panes in winter; but, together with this, there was something 

 flame-like in the graceful terminal curves, and something strangely sug- 

 gestive of fern-like vegetation about the whole. This double and ap- 

 parently incompatible impression of something at once crystalline and 

 plant-like is strikingly conveyed by many of the penumbral forms, and 

 yet the description will doubtless appear incongruous to any but the 

 few who have seen for themselves. The comparison of the frost- 

 figures is the least inapt, perhaps, to be found, but it is really impos- 

 sible to obtain an accurate one, when we have no single thing on earth 

 which we can exactly liken it to. When we consider that this extraor- 

 dinary shape occupied a greater area than the North and South Amer- 

 ican Continents united, while that, over the whole, obtained a temper- 

 ature far above that of the wijite flame w^hich plays about the mouth 

 of a furnace, and that its parts turned, as the observer looked, from 

 one evanescent beautiful form to another, with a rapidity of change 

 which indicated the existence of inconceivable force, we need feel less 

 surprise that any metaphor, necessarily draw^n from our limited terres- 

 trial analogies, should so fail to convey an adequate idea of what the 

 writer is certain he has seen, but confesses he cannot properly describe. 



The xmihra^ or dark inner shade, commences as abruptly as the 

 penumbra, but the contrast between it and the penumbral edge is far 

 greater than between that and the photosphere. We possess no very 

 accurate photometric determinations of the relative light of these por- 

 tions of the spot, and nothing seems practicable beyond a rough aver- 

 aging where the umbrae are themselves of such various tints. If we 

 adopt the somewhat crude determinations of the elder Herschel, we 

 may assume that the penumbra is, as a whole, rather less than half as 

 bright as the photosphere, and the umbra about one-seventieth of the 

 brightness of the penumbra. More accurately, if we represent the 

 average brightness of the photosphere by 1,000, that of the penumbra 

 will be denoted by 469, and that of the umbra by only 7. The umbra 

 appears, at first sight, to be black, but this is only from contrast with 

 the superior brightness around it. It is certain, for instance, that sun- 

 light is at least 200,000 times brighter than moonlight (probably 

 more). The umbra, then, if it be but seven thousandths of the bright- 

 ness of the surface, is still 1.400 times (at least) as bright as the moon, 

 or far brighter than the calcium-light. The absolute depth of the 

 inner edge of the penumbra, below the surface, is not very great, ac- 

 cording to M. Faye, and probably not over from 2,000 to 4,000 miles. 

 (Every thing is relative ; and, on the sun, 2,000 miles is little for the 

 depth of a cavity which may be from ten to twenty times this wddth.) 



Somewhere about this lower level commences the umbra, which has 

 been already compared to the spout of the funnel, of which the 

 penumbra formed the upper shallow cone ; and, through these umbral 

 shades, the eye looks down to quite unknown depths. 



