THE APPROACHING TOTAL SOLAR ECLIPSE. 



297 



employed. The telescope should be accurately 

 driven by clock-work, and a dark iris-disk — if I 

 may so describe an arrangement which would be 

 the converse of an iris-diaphragm — might be em- 

 ployed with advantage to hide the light of the 

 prominences and sierrra." An iris-diaphragm, 

 be it noted, is an arrangement by which the field 

 of view may be contracted or enlarged in the 

 same way that the pupil of the eye changes in 

 size ; that is, by what may be called circular ex- 

 pansion or contraction. The iris-disk I proposed 

 would contract and expand at will, similarly ; 

 but, instead of giving a contracting and expand- 

 ing field of view, it would give a contracting and 

 expanding circle of darkness in the middle of 

 the field of view. " If the field of view were 

 several degrees in diameter " — the moon's disk 

 being a little more than half a degree — " and the 

 dark disk at the beginning of totality concealed 

 a circular space extending a degree or so beyond 

 the eclipsed sun, the observer might first exam- 

 ine with great advantage the outer parts of the 

 corona, and gradually extend his scrutiny to the 

 very neighborhood of the prominences. Suppos- 

 ing his eyes had been kept in darkness before 

 totality began, he would be able to gain such an 

 insight into the real structure of the corona as 

 has never yet been obtained by astronomers." 



Although I would not now speak quite so 

 confidently of such an experiment as I did in 

 1870, yet I still believe that, if carefully carried 

 out, it would yield results of great interest and 

 importance. It was tried unsuccessfully during 

 the eclipse of 1870 ; but the failure of the method 

 on that occasion is not to be wondered at, as the 

 sky was hazy everywhere, and in great part cloud- 

 covered. Moreover, to obtain success by this 

 method, a station at some height above the sea- 

 level should be occupied. As the track of total 

 shadow on the 29th inst. will cross several ele- 

 vated ridges in North America, and that too in 

 a region where the air is exceptionally clear, the 

 occasion will be very well suited for the applica- 

 tion of this method. 



We cannot hope 'that photography will reveal 

 the extension of the corona to so great a distance 

 as the naked eye can trace the faint outlying 

 coronal streamers. Nevertheless, we may well 

 hope that much will be added to our knowledge 

 of the corona on this occasion by means of pho- 

 tography. During the eclipses of 1870 and 1871, 

 the only two occasions on which good photo- 

 graphs of the corona have been obtained, atten- 

 tion was chiefly directed to the question whether 

 the corona is a solar phenomenon or not. The 



perversity with which two or three persons (ig- 

 norant of mathematics, and therefore unable to 

 recognize the validity of the reasoning by which 

 the solar nature of the corona was demonstrated) 

 continued to assert their belief that the phenom- 

 enon was purely atmospheric, had a very mis- 

 chievous effect in this respect. I pointed out at 

 the time that "it would be a misfortune to as- 

 tronomy if the attention of observers should be 

 directed to the solution of a question already dis- 

 posed of — unless the most obvious considerations 

 of mathematics and optics are to be entirely neg- 

 lected " (Monthly Notices of the Royal Astronom- 

 ical Society for March, 1870). But at the same 

 time I showed how the question might be dis- 

 posed of, even to the satisfaction of the doubters, 

 by means of photography. If photographs taken 

 at distant stations showed the same coronal feat- 

 ures, then manifestly the features could not be- 

 long to our own atmosphere : and again, if pho- 

 tographs taken at the same station at the begin- 

 ning, middle, and end of totality showed the same 

 features, those manifestly could not be due to the 

 passage of the solar rays athwart irregularities on 

 the moon's edge. But although the experiment 

 was tried, and the result was such as I had indi- 

 cated beforehand as certain to follow, I consider 

 it a misfortune that time was thus wasted and 

 opportunities lost which may not be presented 

 again for many years to come. 



During the eclipse of December, 1870, indeed, 

 little was lost in this way, because the weather 

 was very unfavorable. My friend Mr. Brothers 

 took almost as good a view of the corona, at 

 Syracuse, in the time accorded to the last expos- 

 ure, as he would have obtained if nearly the 

 whole duration of totality had been devoted to a 

 single picture. 



But in the case of the Indian eclipse of 

 December, 1871, matters were different. Fine 

 weather prevailed both at Baicull, where Lord 

 Lindsay's party were stationed, and at Ootaca- 

 mund, where Colonel Tennant photographed the 

 sun. Mr. Davis, the skillful photographer at the 

 former station, obtained six views of the corona, 

 each showing a goodly amount of detail. Colo- 

 nel Tennant, at Ootacamund, was almost equally 

 successful. The twelve views thus obtained dis- 

 posed finally of the atmospheric theory of the 

 corona (though, oddly enough, while they were 

 being developed a telegraphic message was on its 

 way from Mr. Lockyer, at Baicull, to Captain 

 Tupman, who was awaiting the arrival of the to- 

 tality at Ceylon, announcing that the Baicull ob- 

 servations satisfactorily demonstrated the atmos- 



