6 REPORT — 1872. 



while the closer agrceineiit of the calculated results in reference to solar elements 

 is itself evidence of the intrinsic truthfulness of the method, and gives the highest 

 promise that our final deductions, whicli will he completed in the course of the 

 ensuing year, will not he unworthy of the exertions which I, in conjunction with 

 my friends B. Stewart and B. Loewy, have constantly devoted to this work during 

 a period of fully ten years. Not only will some doubtful questions be set finally at 

 rest by it, but new facts of the greatest interest will result, bearing on the laws 

 which appear to govern solar activity. 



By nothing, however, would the claims of photographic observation, as one of 

 the most important instruments of scientific research, seem to be so thoroughly 

 well established as by the history of recent solar eclipses. It will be recollected 

 that in 1860, for the first time, the solar origin of the prominences was placed 

 beyond doubt solely by photography, which preserved a faithful record of the 

 moon's motion in relation to these protuberances. The photographs of Ten- 

 nant at Guntour, and of Vogel at Aden, in 1868, and also those of the 

 American astronomers at Burlington and Ottumwa, Iowa, in 1869, under Professors 

 Morton and Mayer, have fully confirmed those results. In a similar manner the 

 great problem of the solar origin of that portion of the corona which extends more 

 than a million of miles bej'ond the body of the siui has been, by the photographic 

 observations of Col. Tennant and Lord Lindsay in 1871, set finally at rest, after 

 having been the subject of a great amount of discussion for some years. 



The spectroscopic discovery in 1869 of the now famous green line, 1474 K, 

 demonstrated undoubtedly the self-luminosity, and hence the solar origin of part 

 of the corona. Those who denied the possibility of any extensive atmosphere 

 above the chromosphere received the observation with great suspicion ; but in 

 1870 and again in 1871 it was fully verified. So far, therefore, the testimony of 

 spectroscopic observations was in favour of the solar origin of the inner corona. 



Indeed the observations of 1871 have proved hydrogen to be also an essential 

 constituent of the " coronal atmosphere," as Janssen proposes to call it, — hydrogen 

 at a lower temperature and density, of course, than in the chromosphere. Janssen 

 was further so fortunate as to catch glimpses of some of the dark lines of the solar 

 spectrum in the coronal light, an observation which goes far to show that in the upper 

 atmosphere of the sun there are also solid or liquid particles, like smoke or cloud, 

 which reflect the sunlight from below. Many problems, however, even with refer- 

 ence to the admittedly solar part of the corona, are unsettled. The first relates to 

 the nature of the substance which produces the line 1474 K. Since it coincides 

 with a line in the spectrum of iron, it is by many considered due to that metal ; 

 but then we must suppose either that iron vapour is less dense than hydrogen gas, 

 or that it is subject to some peculiar solar repulsion which maintains it at its ele- 

 vation ; or other hypotheses may be suggested for explaining the fact. Since the 

 line is one of the least conspicuous in the spectrum of iron and the shortest, and as 

 none of the others are found associated with it in the coronal spectrum, it seems 

 natural, as many have done, to^assume at once that it is due to some new kind of 

 matter. But the observations of AngstriJm, lioscoe, and Clifton, and recently those 

 of Schuster regarding the spectrum of nitrogen, render it probable that elementaiy 

 bodies have only one spectrum ; and since in all experimental spectra we necessarily 

 operate only on a small thickness of a substance, we cannot say what new lines 

 may be given out in cases where there is an immense thickness of vapour ; 

 and hence we cannot conclude with certainty that because there is an unknown 

 line in the chromosphere or corona, it implies a new substance. Another problem, 

 the most perplexing of aD, is the reconciliation of the strangely discordant obser- 

 vations upon the polarization of the coronal light ; but I will at once proceed to 

 the points on which photography alone can give us decisive information. 



The nature and conditions of the outer corona (the assemblage of dark rifts 

 and bright rays which overlies and surrounds the inner corona) was very incom- 

 pletely studied ; and the question whether it is solar was not finally settled in the 

 opinions of astronomers of high repute. Some believed it to be caused by some 

 action of our atmosphere ; and others supposed it due to cosmical dust between us 

 and the moon. The bright light of the corona and the prominences most undoubtedly 

 cause a certain amount of atmospheric glare ; and although it is difficidt to see how 



