WHAT THE SPOTS ARE. 



fringe is produced by the shelving of the sides of the excavation, 

 sloping down to its dark bottom. As the spot is carried toward 

 the edge of the sun, the height of the inner side is interposed 

 between the eye and the bottom of the excavation, so as to conceal 

 the latter from view. The surface of the inner shelving side also 

 taking the direction of the line of vision or very nearly, diminishes 

 in apparent breadth, and ceases to be visible, while the surface of 

 the shelving side next the edge of the sun becoming nearly per- 

 pendicular to the line of vision, appears of its full breadth. 



In short, all the variations of appearance which the spots 

 undergo, as they are carried round by the rotation of the sun, 

 changing their distances and positions with regard to the sun's 

 centre, are exactly such as would be produced by an excavation, 

 and not at all such as a dark patch on the solar surface would 

 undergo. 



16. It may be considered then as proved, that the spots on the 

 sun are excavations ; and that the apparent blackness is produced 

 by the fact that the part constituting the dark portion of the spot 

 is either a surface totally destitute of light, or by comparison so 

 much less luminous than the general surface of the sun, as to 

 appear black. This fact, combined with the appearance of the 

 penumbral edges of the spots, has led to the supposition, advanced 

 by Sir TV. Herschel, which appears scarcely to admit of doubt, 

 that the solid, opaque nucleus, or globe of the sun, is invested 

 with at least two atmospheres, that which is next the sun being, 

 like our own, non-luminous, and the superior one being that alone 

 in which light and heat are evolved ; at all events, whether these 

 strata be in the gaseous state or not, the existence of two such, one 

 placed above the other, the superior one being luminous, seems to 

 be exempt from doubt. 



TTe are not warranted in assuming that the black portion of the 

 spots are surfaces really deprived of light, for the most intense 

 artificial lights which can be produced, such, for example, as that 

 of a piece of quicklime exposed to the action of the compound 

 blow-pipe, when seen projected on the sun's disk, appear as dark 

 as the spots themselves ; an effect which must be ascribed to the 

 infinitely superior splendour of the sun's light. All that can be 

 legitimately inferred respecting the spots, then, is, not that they 

 are destitute of light, but that they are incomparably less brilliant-, 

 than the general surface of the sun. 



17. The prevalence of spots on the sun's disk is both variable and 

 irregular. Sometimes the disk will be completely divested of them, 

 and will continue so for weeks or months ; sometimes they will be 

 spread over certain parts of it in profusion. Sometimes the spots 

 will be small, but numerous ; sometimes individual spots will 



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