200 THE SIN SPOTS OF THE PAST FEW YEARS. 



cleus. Careful measurement shows that when a spot 

 divides or undergoes a marked change in form a sudden 

 movement occurs, and it is always a forward movement, 

 i. e., in the direction of increasing longitude. 



Another interesting fact is, that when the nuclei are 

 round, the filaments of the penumbra are directed to- 

 ward the centre. In irregular spots, they lie at various 

 angles ; sometimes (as seen by Secchi) apparently over- 

 lying one another and implying a difference of level. 

 The luminous masses of the penumbra project more or 

 less into the dark nucleus, forming a ragged interior 

 edge ; and high power will often show the rapid dissolv- 

 ing away of those projecting extremities. As the umbra 

 of a spot approaches the circular form its career becomes 

 more stable ; and it is from round nuclei that the move- 

 ments of rotation are preferably obtained. A large spot 

 rarely disappears during a single rotation of the sun. 

 Secchi says that when a large spot disappears, another 

 often appears a short distance from the first and always 

 in advance of it. The group of July, 1883, shows the 

 train of smaller spots which usually attends the main 

 body of a group. These smaller spots are, in general, 

 a later development. They change much in form, size, 

 and arrangement, and lag more and more behind the 

 larger and earlier portion. It is a marked fact of obser- 

 vation that these trains follow, almost never precede, the 

 main body of the group. 



Another almost universal accompaniment of a spot are 

 the faculse. Faculce are lines of light brighter than the 

 photosphere, running more or less regularly about the 

 spot. They are more noticeable near the sun's limb 

 than toward its centre. It is not supposed that they ex- 

 ist in larger number near the edge, but that they are 

 seen more plainly there because of the darkening of the 

 photosphere toward the sun's borders. If they are (as 

 they are supposed to be) elevations of the photosphere 



190 



