398 ANNUAL OF SCIENTIFIC DISCOVERT. 



there is no visible separation between their penumbra, as one group. 

 Hence, ho observes, the number of spots will depend i:i a great measure on 

 the excellence of the telescope ; and it often happens that clusters of many 

 hundred, nay of many thousand spots, will bo designated by one number 

 only, just as a single isolated spot will be. So great, however, is the sun's 

 tendency to present his spots in the form of clusters, that other observers 

 will, in the coarse of a year, assuredly not find any great difference between 

 their numbers and mine. But he particularly impresses on his readers that he 

 attaches importance not so much on the absolute number of the groups, as 

 on the ratio which obtains between them in different years. 



The result of his investigations has been to establish with a degree of 

 probability almost amounting to certainty, that the solar spots pass through 

 the phases of maximum and minimum frequency, and vice versa, in a period 

 not very different from ten years. 



The exact period Schvvabc does not pretend to have determined. That it 

 is liable to perturbation is evident. During twenty-seven years of the series 

 the results were extremely regular ; during the last three years they have 

 shown symptoms of disturbance. The epoch of minimum, which, consist- 

 ently with earlier indications, should have happened in 1853, did not occur 

 until 1856. 



Schwabc has not entered into speculations relative to the nature and origin 

 of the spots, though he has been careful to note all remarkable appearances 

 as they occurred ; and of these he has given an admirable summary in the 

 " Astronomisc-haNachrichtcn," number 473. There he calls attention to an 

 appearance which, he says, i< not uncommon, but which lie cannot explain 

 on the generally received theory, that the spots are po:tions of the surface 

 of a solid boily, seen through openings in a luminous atmosphere that sur- 

 rounds it at a distance, and another intervening atmosphere. This theory 

 of Sir W. Herschcl has been found adequate to explain most of the pheno- 

 mena of the s;>o">. But the case to which Schwabe alludes is this. On the 

 above hypothesis a spot surrounded by a penumbra, will, by the effect of 

 perspective, when it first makes its appearance on the disc, seem to be ex- 

 centrically situated on the penumbra, the border of the penumbra towards 

 the sun's centre appearing less broad than the other border. All this is in- 

 telligible, but why is not the penumbra equally illuminated all round ? For 

 it frequently happens that the border turned towards the sun's centre is dark 

 gray, while that towards the sun's limb is bright gray, and between the lat- 

 ter and the nucleus there is a string of liirht almost as bright as the sun's 



O O *-J 



disc. 



He also mentions having seen, though rarely, the phenomenon which fur- 

 nished Francis Wollaston and Lalande with an argument against Alexander 

 Wilson, who was the first to advance the theory of the spots being cavities. 

 The phenomenon is this : Sometimes a spot surrounded by a penumbra 

 passes over the sun's disc without the former undergoing any change of re- 

 lative position, from the beginning to the end of its course. This apparently 

 militates against the cavity theory. Arago says the objection is not insur- 

 mountable. " Suppose," says he, " in such cases, that the sides of the 



