178 



KNOWLEDGE. 



[AuGtsT 1, 1900. 



did not of course include the outer streamers. His first 

 impression was that of the entire familiaa'ity of the 

 object he beheld; the photographs of 1878 and 1889 

 had so exactly presented the same leading features. The 

 great difference was that in looking at the reality and 

 not at the photographic picture a sense of perspective 

 and of relief was jierceived, whereas the photographs 

 seemed essentially plane sections. In one sense the 

 sight was disappointing; there was no structure seen 

 more detailed than the microscopic scrutiny of photo- 

 graphs had already made Mr. Wesley familiar with. 

 Probably no one else knows how much the photographs 

 really have to show. However both the photographs 

 and direct examination concur that the corona at this 

 eclipse showed much less stnicture than in 1898 and 

 other years nearer the solar maximum. 



One form in particular Mr. Wesley looked for. Round 

 the more brilliant prominences of 1893, the corona was 

 perceptibly fainter for some little distance ; a brighter 

 margin including the prominence region some way 

 further out. The prominences therefore seemed to be 

 arched over by bright coronal matter, giving somewhat 

 the effect as if they were under glass cases. Mr. Wesley 

 could not, however, recognize this " glass case " appear- 

 ance, nor do the photographs clearly show it. 



This last conclusion is more than confirmed by the 

 detailed examination made by Miss Lilian Martin-Leake 

 with a three inch telescope mounted on the roof of the 

 Hotel de la Regence, Algiers. Miss Martin-Leake used 

 a higher power than Mr. Wesley, and hence had a 

 smaller field of view, only commanding a part of the 

 corona but giving this with more detail. The portion 

 of the limb examined by Miss Martin-Leake, whose 

 drawing is reproduced in the plate, had for its centre 

 the gi-eat prominence in the south-west quadrant. It 

 will be seen from the drawing that the chief coronal 

 streamer in the region iinder examination had its polar 

 edge very sharply defined; — nothing fui'ther to the 

 south, of a coronal character, could be perceived within 

 the field of view ; a circumstance to which we shall 

 have to allude again a little later. 



Round the great prominence itself there was an 

 approximation, indeed, to the " glass case " effect, but 

 with a difference. Instead of inclosing the prominence, 

 two of the " glass cases " appeared to start from between 

 the two wings of the prominence. The taller of these 

 wings was a conical flame, red in colour, proceeding 

 straight upwards from the limb, tapering to a fine point 

 at its apex and showing strongly defined spiral markings 

 throughout the whole of its upward course. The other 

 part of the prominence consisted of a thick radial stem 

 also with strongly defined spiral markings, but about 

 three quarters of the height of the taller prominence 

 from the limb, the second one bent sharply, almost at 

 right angles, towards it. 



The same region was also examined in the telescope, 

 but only for a few seconds, by Mr. Crommelin, who 

 drew three conical projections corresponding most closely 

 to those brighter portions of the corona inclosed within 

 the outlines which Miss Martin-Leake has shown. 



11. Dark Rays in the Corona. — I may be forgiven 

 for again reverting here to the fact that my wife and I 

 had but a very small instrumental equipment both in 

 the eclipse of 1898 and in that of 1900. We felt there- 

 fore that it would be quite absurd for us to attempt to 

 do on a microscopic scale what was being done by 

 others in an infinitely more satisfactory manner, namely, 

 to get a photographic picture of the corona. If our 

 work was to have any value at all it must be somethinsr 



different from that which others were doing far more 

 efficiently than we could hope to do. In India, there- 

 fore, we set ourselves a two-fold task ; the first to give 

 a series of exijosures to the corona varying over a far 

 wider range than any that had been attempted before ; 

 the next to get, if possible, a photograph of the faint 

 outer streamers of the corona. Both attempts were 

 successful, but inasmuch as our longest Indian exposures 

 were the most successful in bringing up the rays, it left 

 it an open question whether a still further prolongation 

 of the exposure might not record those rays to a still 

 greater distance from the sun. In the eclipse just past, 

 therefore, we prolonged our exposures to the utmost 

 extent which the circumstances of the eclipse permitted, 

 with the result of finding that for this eclipse, at any 

 rate, increase of exposure did not mean increase of 

 extension. 



But these little long-exposure photographs of ours do 

 show features which ai-e not shown, or, at any rate, not 

 shown so distinctly on our photographs of shorter ex- 

 posure. Of deliberate purpose we pushed exposure and 

 development to the furthest limit that the circumstances 

 of the case allowed. Our object was not to get a photo- 

 graphic picture of the corona; we knew that was being 

 far better done elsewhere. Had we been trying for 

 such, then both our exposure and development would 

 have to be censured as extravagant. As it was, we 

 obtained a most unexpected result that could probably 

 have not been obtained in any other way. 



It will have struck anyone who has examined eclipse 

 negatives that upon these the moon very frequently 

 comes out much darker than the sky even at a great 

 distance from the sun. This is a very remarkable cir- 

 cumstance, for we must remember that in an eclipse the 

 moon is more fully illuminated by " eai-th-shine " than 

 on other occasions. The moon therefore is not black 

 or anything like it in a total eclipse, and if it appears 

 much darker than the sky, it can only be because there 

 is a very appreciable amount of diffused light round the 

 sun itself perfectly distinct from any scattering in our 

 own atmosjjhere. 



This consideration gives its significance to a very 

 curious feature of our little photographs, namely, some 

 hlack rays ; rays, that is to say, distinctly darker than 

 the general sky background, or rather what I may call 

 the general coronal glare. The largest and darkest of 

 these rays, it may be added, is that shown on the edge 

 of Miss Martin-Leake's drawing as a region free from 

 coronal light. 



These dark rays are not a mere contrast effect, for 

 though undoubtedly the northern edge of the principal 

 one corresponds to the southern edge of the great south- 

 western streamer, yet it is traced as a distinct black line 

 further than the bright coronal ray on the border of 

 which it lies, nor is there any very manifest bright ray 

 to define the other, that is to say, the southern side of 

 the dark ray. These dark rays therefore can only be 

 seen as such where the exposure and development have 

 been sufficient to bring up the general coronal glare. 



The matter is one of great importance as it regards 

 our conception of coronal structure. The explanation 

 of the great rifts which have often caught attention 

 both in the corona itself when obsei-ved directly and in 

 photographs of it, has been extremely difficult, on the 

 assumption that they are the mere intei'spaces between 

 the bright streamers, since it is inconceivable that the 

 corona is really what it appears to be, an object in two 

 dimensions only. I think our photographs, though on 

 so small a scale, afford evidence that in .some cases at 



