THE SUN'S CORONA AND HIS SPOTS. 



543 



while no approach even has been made to an ex- 

 planation of their periodic increase and diminu- 

 tion in number. This seems to me one of the 

 most interesting problems which astronomers 

 have at present to deal with ; nor do I despair of 

 seeing it satisfactorily solved within no very long 

 interval of time. Should the recognition of a 

 sympathy between the corona and the sun-spots 

 be satisfactorily established, an important step in 

 advance will have been made— possibly even the 

 key to the enigma will be found to have been dis- 

 covered. 



I propose now to consider, first, whether the 

 evidence we have on this subject is sufficient, and 

 afterward to discuss some of the ideas suggested 

 by the relations which have been recognized as 

 existing between the sun-spots, the sierra, the 

 colored prominences, and the zodiacal light. 



The evidence from the recent eclipses indi- 

 cates, beyond all possibility of doubt or questioD, 

 that during the years when sun-spots were nu- 

 merous, in 1870 and 1871, the corona, at least 

 on the days of the total solar eclipses in those 

 years, presented an appearance entirely different 

 from that of the corona seen on July 29th last, 

 when the sun was almost free from spots. This 

 will be more fully indicated further on. At pres- 

 ent it fe necessary to notice only — 1. That 

 whereas in 1870 and 1871 the inner corona ex- 

 tended at least 250,000 miles from the sun, it 

 reached only to a height of some 70,000 miles 

 last July; 2. In 1870 and 1871 it possessed a 

 very complicated structure, whereas last July the 

 definite structure could be recognized only in two 

 parts of the inner corona; 3. In 1871 the corona 

 was pink, whereas last July it was pearly white ; 

 4. Last July the corona was ten times brighter 

 than in 1871 ; lastly, in 1871 the light of the 

 corona came in part from glowing gas, whereas 

 last July the light came chiefly, if not wholly, 

 from glowing solid or liquid matter. I must here 

 point out that the evidence of change, however 

 satisfactory in itself, would be quite insufficient 

 to establish the general theory that the corona 

 sympathizes with the solar photosphere in the 

 special manner suggested by the recent eclipse 

 observations. There are few practices more un- 

 scientific, or more likely to lead to erroneous 

 theorizing, than that of basing a general theory 

 on a small number of observations. In this case 

 we have, in fact, but a single observed corre- 

 spondence, though the observations establishing 

 it form a series. It has been shown that so far 

 as the special sun-spot period from the minimum 

 of 1867 to the minimum of 1878 is concerned, 



there has been a certain correspondence between 

 the aspect of the corona and the state of the sun's 

 surface, with regard to spots. To assume from 

 that single correspondence that the corona and 

 the sun-spots are related in the same way would 

 be hazardous in the extreme. We may indeed 

 find, when we consider other matters, that the 

 probability of a general relation of this sort ex- 

 isting is so great antecedently, that but slight 

 direct evidence would be required to establish the 

 existence of the relation. But it must be remem- 

 bered that before the recent eclipse was observed, 

 with the special result I have noticed, few were 

 bold enough to assert the probable existence of 

 any such relationship ; and certainly no one as- 

 serted that the probability was very strong. I 

 believe, indeed, that no one spoke more definitely 

 in favor of the theory that the corona probably 

 sympathizes with the sun-spots than I did myself 

 before the recent eclipse ; but certainly I should 

 not then have been willing to say that I consid- 

 ered the evidence very strong. 



We must, then, look for evidence of a more 

 satisfactor)' kind. 



Now, although during the two centuries pre- 

 ceding the invention of the spectroscope, and the 

 initiation of the solar physical researches now in 

 progress, observations of eclipses were not very 

 carefully conducted, yet we have some records 

 of the appearance of the corona on different oc- 

 casions, which, combined with the known law of 

 sun-spot periodicity, may enable us to generalize 

 more safely than we could from observations 

 during the present spot-period, though these ob- 

 servations have been far more exact than the 

 older ones. I propose to examine some of these. 

 Necessarily I must make some selection. I need 

 hardly say that, even if there were no such rela- 

 tion as that which seems to be indicated by re- 

 cent observations, and if my purpose were simply 

 to prove, either that such a relation exists or that 

 it does not, I could very readily bring before the 

 reader of these pages what would seem like the 

 most satisfactory evidence that the relation is 

 real. I must ask him to believe, however, that 

 my purpose is to ascertain where the truth lies. 

 I shall neither introduce any observation of the 

 corona because it seems specially favorable to the 

 theory that the corona sympathizes with the pho- 

 tosphere, nor omit any because it seem3 definitely 

 opposed to that theory. To prevent any possi- 

 bility of being unconsciously prejudiced, I shall 

 take a series of coronal observations collected 

 together by myself on account of their intrinsic 

 interest several years ago, when I had not in my 



