Nov. 25, 1886] 



NA TURE 



87 



as I can learn, no other observer of the solar atmosphere has 

 seen anything similar. I certainly have not myself. And I 

 think some of his published observations of velocities of two 

 or three thousand miles a second in the motions of the promi- 

 nences, as evidenced by the displacement of lines in the spec- 

 trum, are still more questionable. 



In two or three cases, prominences have been observed since 

 1876 considerably higher than any known previously. In Octo- 

 ber 1S7S I myself observed one which attained an elevation of 

 nearly 4CX3,oc» miles (l3i'). 



Eclipses and the Corona. — The sun's corona has been, per- 

 haps, more earnestly studied than anything else about the central 

 luminary, especially during the four eclipses which have occurred 

 since 1S76. At the eclipse of 1878, in the midst of an epoch 

 of sunspot quiescence, the corona was found less brilliant than 

 ordinary, and especially deficient in the unknown gas that pro- 

 duces the so-called 1474 line — the line which characterises the 

 spectrum of the corona, and first demonstrated conclusively 

 its solar origin in 1869. But while the corona at this 

 time was less brilliant than it had been formerly, it was far 

 more extensive. At least it seemed so ; for, at Pike's Peak and 

 Creston, Langley and Newcorab were able to follow its streamers 

 to a distance of 6° from the sun. It is possible, however, that 

 this extension was only due to the superior transparency of the 

 mountain air. 



The Egyptian eclipse of 1882 gave us some interestiiig results 

 respecting the spectrum of the prominences and the corona. It 

 appears that the light of the corona is especially rich in the 

 ultra-violet, and in the photographs of the spectrum a number 

 of bands are found which have been interpreted, with question- 

 able correctness I think, as indicating the presence of carbon. 

 The eclipse of 18S3 was observed in the Pacific Ocean by French 

 and American parties, but, I think, added very little real infor- 

 mation. Prof. Hastings made an observation which he believed 

 to establish a peculiar theory proposed by himself, viz. that the 

 corona is merely a diffraction effect produced by the moon's 

 limb, and depending on the non-continuity of phase in long 

 stretches of light-vibrations. With a peculiar apparatus pre- 

 pared expressly for the purpose, he found that at any moment 

 the 1474 line was visible to a much greater distance from the 

 sun on the side least deeply covered by the moon th.in on the 

 other : as unquestionably would happen if his theory were cor- 

 rect. But the same thing would result from the mere diffusion 

 of light by the air ; and, notwithstanding his protests, the French 

 observers who were at the same place, and nearly all others who 

 have discussed the observations, think that this was the true 

 explanation of what he saw. So far as I know, the discussion 

 of the subject which has resulted from his publication has only 

 strengthened the older view — that the corona is a true solar 

 appendage ; an intensely luminous but excessively attenuated 

 cloud of mingled gas and fog and dust surrounding the sun, 

 formed and shaped by solar forces. 



The diffraction theory has one advantage — that it relieves us 

 from stretching our conceptions as to the possible attenuation of 

 matter to the extent necessary in order to account for the fact 

 that a comet, itself mostly a mere airy nothing, experiences no 

 perceptible retardation in passing through the coronal regions. 

 There can be no question that this has happened several times : 

 the last instance having been the great comet of 1SS2. But on 

 careful consideration it will be found, I think, that our concep- 

 tions will bear the stretching without involving the least 

 absurdity ; a single molecule to the cubic foot would answer 

 every necessary condition of the luminous phenomenon observed. 

 And all the rifts and streamers, and all the radiating structure 

 and curved details of form, cry out against the diffraction 

 hypothesis. 



The observations of the eclipse of 1885 (observed only by a 

 few amateurs in New Zealand) have not proved important. 



At present the most interesting debate upon the subject 

 centres around the attempt of Mr. Huggins (first in 18S3) to 

 obtain photographs of the corona in full sunlight. He suc.-eeded 

 in getting a number of plates showing around the sun certain 

 faint and elusive ha'o forms which certainly look very coronal. 

 Plans were made and have been carried out, for using a similar 

 apparatus on the Riffelberg, in Switzerland, and at tlie Cape of 

 Good Hope. But so far nothing has been obtained much in 

 advance of Mr. Huggin.s's own first results. Since September 

 1883, until very recently, the air has been full, as every one 

 knows, of a fine haze, probably dust and vapour from Krakatab, 



which lias greatly interfered wiili all such operations. It is now 

 fast clearing away, and I for one am somewhat sanguine that a 

 much greater success will be reached next winter at the Cape, 

 and perhaps even in England during the coming summer. 



Just about the same time that Huggins was photographing in 

 England, Prof. Wright was experimenting in New Haven in 

 a different way : isolating the blue and ultra-violet rays by the 

 use of coloured media, stopping out the sun's disk, and receiving 

 the image of ihe coronal regions on a fluorescent screen. He 

 also had obtained what he believed, and still believes, to be a 

 real image of the corona, when the aerial haze intervened to put 

 an end to all such operations ; for of course it is evident that 

 whether one operates by this method or by photography, success 

 is possible only under conditions of unusual atmospheric trans- 

 parency and purity. 



I sujipose at present the predominant feeling among astro- 

 nomers is that the case is hopeless, and that Huggins and Wright 

 are mistaken. It may be so. But ray own impression is that 

 they are probably correct ; although, of course, the matter is still 

 in doubt. 



Inferior Planets. — Leaving now the sun, and passing to the 

 planetary system, we come first to the subject of intra- Mercurial 

 planets. 



Tlie general opinion among astronomers (in which I fully 

 concur) is that the question has been now fairly decided in the 

 negative, i.e. it is practically certain that within the orbit of 

 Mercui-y there is no planet of a diameter as large as five hundred 

 miles, probably not one hundred. If such a one existed, it 

 could not have failed to be discovered by the wide-angled photo- 

 graphs taken at the eclipses of 1882 and 18S3, to say nothing of 

 the visual observations. Of course, it is well kno'vn that at the 

 eclipse of 1878 Prof. Watson supposed he had discovered two 

 such bodies, and his extensive experience and his high authority 

 led, for a time, to a pretty general acceptance of his conclusion. 

 I notice that Dr. Ball, even very lately, in his " Story of the 

 Heavens," is still disposed to credit the discovery. But Dr. 

 Peters, by a masterly discussion of the circumstances of the 

 observations themselves, and a comparison with the star maps, 

 has shown that it is almost certain that Watson really saw 

 only the two stars 9 and f Cancri. In the same paper also, 

 Peters examined all the observations of small, dark spots 

 crossing the sun's disk which, up to that date (1879), had been 

 made by Leverrier and others the ground for their belief in 

 "Vulcan"; and he shows that they re.ally afford no suflicient 

 ground for the conclusion. As to Mr. Swift's supposed observa- 

 tion of two objects with large disks "both pointing to the sun," 

 they certainly were not the two seen by Ni'atson, while they 

 were in the region covered by Watson and several other ob- 

 servers. What the precise nature of the mistake or illusion may 

 have been it is perhaps not now possible to discover, but I think 

 no one, unless perhaps Mr. Swift himself, now considers the obser- 

 vation important. 



While, however, the question of a "Vulcan" is now pretty 

 definitely settled, it is not at all impossible, or even improbable, 

 that there may be intra-Mercurial asteroids, and that some of 

 them may be picked up as little stars of the sixth magnitude or 

 smaller, by the photographers at the eclipse of next August, or 

 in 1SS7. The sensitiveness of our present photographic plate is 

 now many times greater than it was even in 1882. 



As to the planet Mercury, there is very little to report. It 

 "transited" the sun in May 1878, and again in November 

 1881, and during the transits numerous measures were made of 

 its diameter, giving results substantially in accord with the older 

 values. I have already alluded, in connection with the earth's 

 rotation, to Newcomb's investigation of former transits of this 

 planet as establishing the sensible uniformity of the earth'* 

 rotation. 



The planet Venus, by her transit in 1882, has attracted much 

 attention, and much interest is felt as to the final outcome of 

 the whole enormous mass of data, photographic and visual. 

 Just how long we shall have to wait for the publication seems 

 still uncertain. I have already said, however, that probably 

 these transits will never again be considered as important as 

 hitherto. 



The most important physical obsei-vations upon the planet 

 during the der.ade seem to be those of Langley, who, during 

 the transit of 1882, observed a peculiar, and so far unexplained, 

 illumination of one point on the edge of the planet's disk, and 

 those of Trouvelot and Denning, who have observed and figured 



