THE APPROACHING TOTAL SOLAR ECLIPSE. 



293 



plied. For in truth the method had been de- 

 vised several months before the great eclipse took 

 place. It will be found definitely described in the 

 report of Dr. Huggins's observations which ap- 

 pears in the Monthly Notices of the Royal Astro- 

 nomical Society for February, 1SGS. 



From that time forward, then, the study of 

 the colored prominences ceased to be a chief 

 feature of the observations of total solar eclipsed. 



Accordingly, we find that the American ob- 

 servers of the eclipse of August, 1809, directed 

 their chief attention to the corona. They pho- 

 tographed it, but not very successfully. They 

 analyzed it with the spectroscope. The outcome 

 of their observations was that certainly a portion 

 of the corona's light comes from glowing gas, one 

 bright line seen by Prof. Young in the spectrum 

 belonging unmistakably to the corona. Two 

 fainter lines, seen by him and by Prof. Picker- 

 ing, were not so confidently attributed to the 

 corona, and have since been found to belong to 

 the light from the colored prominences. 



A contest arose, at this stage of the inquiry 

 into the solar corona. It had been maintained 

 that the corona does not belong to the sun at all, 

 but is simply due to the passage of the solar 

 rays through our own atmosphere. Oddly enough 

 (when the simplicity of the mathematical rela- 

 tions involved is considered), this erroneous no- 

 tion, though only definitely maintained by per- 

 sons unfamiliar with mathematics, was adopted 

 by a mathematician so skillful as Sir George Airy, 

 and even (which I find a great deal more re- 

 markable) by the greatest astronomer since W. 

 Herschel, the late Sir J. Herschel. At least both 

 Airy and J. Herschel adopted the idea that the 

 light of the corona comes from matter lying 

 nearer to us than the moon. Airy definitely 

 enunciated that idea in a lecture delivered at 

 Manchester; while J, Herschel, in his admirable 

 " Familiar Lectures," says of the solar corona that 

 " it can only be referred to vapors of excessive 

 tenuity existing at an immense height in our own 

 atmosphere." Yet it is demonstrable (and of 

 course either of the two astronomers I have 

 named would easily have seen this had it oc- 

 curred to them to study the matter in its geo- 

 metrical aspect) that a glory of light, in the 

 midst of which the moon appears dark, cannot 

 possibly be due to rays illuminating our own at- 

 mosphere, or matter nearer to us than the moon. 



I was not myself one of those who waited for 

 the results of the eclipse observations of Decem- 

 ber 1870, as likely to determine this particular 

 question. I had enunciated, long before, the rea- 



soning which showed that, whatever the solar 

 corona may be, it is a solar, not a lunar or ter- 

 restrial, phenomenon. The observations to be 

 made during the Mediterranean eclipse could no 

 more strengthen this reasoning, than they could 

 strengthen the reasoning by which we perceive 

 that the three angles of a triangle are equal to 

 two right angles. At that time I insisted, and 

 somewhat strongly, on the circumstance that this 

 general fact was already certain, and that obser- 

 vations directed to obtain evidence on the point 

 would be a waste of time (unless they at the 

 same time served to determine other facts as 

 yet not ascertained). I do not think that, as a 

 rule, it is desirable to urge very strongly and 

 persistently the evidence in favor of any conclu- 

 sion, even though such evidence may, in point of 

 fact, be demonstrative. But in this case, as in one 

 other, I followed that course, because it seemed 

 to me likely that favorable opportunities for obser- 

 vation might be wasted if attention were specially 

 directed to a point already sufficiently established. 



It is not now necessary to indicate fully the 

 nature of the reasoning by which it was already 

 clear, in 1869, that the corona is a sclar phe- 

 nomenon. But it may be well to consider the 

 matter briefly, because there is still occasion to 

 distinguish between the true solar corona and 

 features which must be regarded as partly be- 

 longing to our own atmosphere. It will indeed 

 be more specially necessary to draw such a dis- 

 tinction during the coming eclipse than on for- 

 mer occasions, because there can be no doubt 

 that one chief object of the observers will be to 

 obtain information respecting the outermost re- 

 gions of the solar corona ; and it will be very 

 desirable to avoid any doubt or confusion in de- 

 termining what belongs to the sun, and what is 

 due to the illumination of our own atmosphere. 



At the time of mid-totality in any considerable 

 eclipse — by which I mean any eclipse in which 

 totality lasts two minutes and upward — no direct 

 solar rays fall on any part of the air lying toward 

 the sun and moon. For a distance of at least 

 thirty moon-breadths on all sides of the dark 

 body of the moon there is no sunlit air. Beyond 

 some such distance the sky is partly illuminated 

 by direct solar rays, and at a considerable distance 

 from the sun there is a tolerably bright sunlit 

 sky. So far, however, as direct solar light is con- 

 cerned, no part of the region occupied by the solar 

 corona, as ordinarily seen, can be attributed to 

 the illumination of our own atmosphere. But the 

 case is different when we consider light from the 

 corona itself, and especially from its bright inner 



