THE SPOTS ON THE SUN. 147 



about 460 miles. These facts give us some idea of the mighty scale 

 of solar phenomena. 



The solar spots have a general movement with the surface of 

 the revolving sun, and they have also minor movements of displace- 

 ment. They always make their first appearance on the same side 

 of it ; they travel across it in from twelve to fourteen days, and then 

 disappear on the other side. The form of a spot in its first appear- 

 ance is that of a small, dark streak, the length of which is much 

 greater than the breadth. Its motion appears slow at first ; it after- 

 ward increases, then slowly diminishes, until again assuming the form 

 of a narrow streak. Fig. 1, from Dr. Schellen's finely-illustrated work 

 on Spectrum Analysis (as are all the illustrations of the present arti- 

 cle), represents this change of appearance of a spot as it emerges, 

 passes across the field of view, and disappears. Of course, its relative 

 size, as here shown, is enormously exaggerated. 



The solar spot consists principally of a dark, almost black, central 

 portion, generally irregular in form, called the umbra. Mr. Dawes has 

 shown that, within this part of a spot, one or more still blacker spots 

 may generally be observed, to which he gives the name of nucleus. 

 The term black, however, as applied to an object on the sun, is to be 

 taken with caution, and means merely a diminished intensity of solar 

 light, which, by contrast, appears black. Zollner states that the black 

 umbra of a spot emits four thousand times as much light as that de- 

 rived from an equal area of the full moon; and Sir John Herschel says 

 that the Drummond light, which is so bright that the eye can hardly 

 endure it, when projected on the sun, appears as a black spot ! 



Surrounding the umbra is a tract less dark, usually more regular in 

 form, and of a fringe-like aspect, called the penumbra. Its appearance 

 is illustrated in Fig. 2, and is represented by the half-tints around the 

 darker umbra in all the accompanying delineations. But, before we 

 can understand the structure of the penumbra, it will be necessary to 

 refer to what is known of the surrounding bright solar surface. 



Viewed by a telescope of moderate power, the photosphere loses 

 its uniform aspect, and exhibits a coarsely mottled appearance. But, if 

 an instrument of the highest grade is used, the photosphere is seen to 

 have a far more definite structure, and exhibits appearances which are 

 variously described by different observers. Sir William Herschel 

 calls these appearances " corrugations." They are small points of un- 

 equal light, imperfectly separated from each other by rows of minute 

 dark spots called pores; the intervals between them being extremely 

 small, and occupied by a substance decidedly less luminous than the 

 general surface. Some call them "rice-grains;" Mr. Huggins names 

 them " granules," and his representation of them is given in Fig. 3. 

 The photosphere is again described as resembling a net of bright 

 meshes interwoven with dark threads. 



Certain portions of the photosphere are much brighter than others ; 



