74 



THE SUN. 



PHYSICAL CONSTITUTION OF THE SUN. 



What are the spots ? Two, and only two, suppositions have been proposed 

 to explain them. One supposes them to be scoriae, or dark scales of incombus- 

 tible matter floating on the general surface of the sun. The other supposes 

 them to be excavations in the luminous, matter which coats the sun, the dark 

 part of the spot being a part of the solid non-luminous nucleus of the sun. In 

 this latter supposition it is assumed that the physical constitution of the sun is 

 a solid non-luminous globe, covered with a coating of a certain thickness of lu- 

 minous matter. This latter supposition has been in a great measure demon- 

 strated by continued and accurate observations on the spots. 



That the spots are excavations, and not mere black patches on the surface, 

 is proved by the following observations : If we select a spot which is at the 

 centre of the sun's disk, having some definite form, such as that of a circle, and 

 watch the appearance of the same spot when, by the motion of the sun upon 

 its axis it is carried toward the edge, we find, first, that the circle becomes an 

 oval. This, however, is what would be expected even if the spot were a 

 circular patch, inasmuch as a circle seen obliquely is foreshortened into an oval. 

 But we find that as the spot moves toward the side of the sun's limb, the black 

 patch gradually disappears, the penumbral fringe on the inside of the spot be- 

 comes invisible, while the penumbral fringe on the outside of the spot increases 

 in apparent breadth, so that when the spot approaches the edge of the sun, the 

 only part that is visible is the external penumbral fringe. Now this is ex- 

 actly what would oocur if the spot were an excavation. The penumbral fringe 

 is produced by the shelving of the sides of the excavation, sloping down to its 

 dark basis. 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 

 takes 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 becomes nearly perpendicular to the line of vision, and, 

 consequently, appears of its full breadth. 



In short, all the variations of appearance which the spots undergo, as they 

 move across the sun's disk, changing their distances and positions with regard 

 to the sun's centre, are exactly those changes of appearance which would be 

 produced by an excavation, and not at all those which a dark patch on the 

 solar surface would undergo. 



It may be considered then as proved, that the spots on the sun are excava- 

 tions ; 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 penum- 

 bral edges of the spots have led to the supposition, which appears scarcely to 

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

 with two atmospheres, that which is next the sun being like our own, non- > 

 luminous, and the superior one being that in which alone 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 lu- 

 minous, seems to be exempt from doubt. 



By observing the magnitude of the spots, and the rate at which they increase 

 and diminish, the velocity of their edges has been ascertained, and this velocity 

 has been found to be such as can scarcely be attributed to matter except in the 

 gaseous form. 



We are not warranted in assuming that the black portion of the spots are 



