October 1, 1886.] 



♦ KNOWLEDGE ♦ 



357 



' I had a gool look at your spot on the morning of Wed- 

 nesday, June 2, and I again had a search for it on the 

 morning of June 4 ' (when, I may observe, it had quite 

 passed oft' the limb). ' I quite agree with you,' he aiys, 

 ' that there was no appreciable difference in the breadth of 

 the penumbra on the preceding and following sides of the 

 nucleus' (umbra is here meant) 'when I observed it on the 

 morning of Wednesday, June 2. I was observing it between 

 9h. 20m. and lOh. a.m., and I thought that the penumbra 

 was a little darker on the preceding than on the following 

 side, but not very noticeably so. There was no band of 

 increased darkness such as would have been produced by a 

 column of absorbing matter rising vertically above the 

 nucleus. 1 quite agree with you that the phenomena 

 observed did not tend to support Wilson's hypothesis.' 



" With respect to this last spot. Father Perry's observations 

 do not confirm my own to the same extent, apparently, as 

 w.as the case in connection with the spot y of April la.'-t, 

 though it is on that of June 2 that I would rely, if pos- 

 sible, more firmly than on the rest. Together with some 

 small but beautifully executed copies of the sketches of spots 

 made recently at Stouyluirst, Father Perry wrote as fol- 

 lows : — ' You will notice that ne;u' the edge there appears 

 to be a slight confirmation of Wilson's theory at both limbs 

 of the sun.' And he adds, ' In some of our drawings of 

 solar spots this is much more marked ; but these probably 

 are different classes of spots, and all certainly do not behave 

 alke.' 



" But now as to actual measurements of what I would 

 term the crucial spots y of Ajjril 1 2, and X of June 2. 



" I fir.st noticed y on April 2, at 7h. .30m. a.m., when it was 

 about 2' -iS ' from the eastern limb, and perceiving that the 

 umbra was nicely central, and the whole spot neatly oval, 1 

 resolved to keep the spot under observation, with the view 

 of testing the hypothesis in question. It was then about 

 50' in length from north to south, and, as then, foreshortened 

 on the sphere, about 40 " in breadth from east to west. On 

 April 6 the size of the spot was somewhat diminished, but 

 its neatness and suitability for testing the hypothesis was 

 still more apparent, for a more perfectly central umbra or 

 symmetrical penumbra could not be wished for. The spot 

 I Oh. 21 )m. A.M. that day was not far from the centre of the 

 disc, and subtended about 45 " in length and the same in 

 breadth.* When at length, on April 12, the spot had 

 arrived very near to the western limb, I made the following 

 notes: — At Ih. 20m. y was only about 10 " from the western 

 edge of the sun, and subtended not more than 10 ' in 

 breadth. The umbra still appeared quite central, with a 

 width of not more than 2 . Each side of the penumbra 

 measured also about 2" in breadth, or possibly the 'pre- 

 ceding,' or outer, side, 2", and the ' following ' l"-8, but of 

 this I could not be quite certain. 



" I may observe also, in passing, that on May 9 a spot 

 lettered i, when about 18 ' only from the western limb, and 

 measuring 10 " in length and 8 " in breadth, presented a per- 

 fectly central umbra .3" in width ; here again militating 

 against Dr. Wilson's hypothesis. 



" But now for the spot A. of June 2, 188G. This was also 

 a remarkably symmetrical one, of a large size, and specially 

 adapted to the investigation in hand. On June 1 the spot 

 was, at 5h. 30m. p.m., just one minute from the western 

 limb, yet there was not the sh'ghtest appearance of any fore- 

 shortening of the inner side of the penumbra. In fact, if 

 there was a difference, the advantage was by about 1' or so 

 on the iiDier side. The .spot now subtended about 45' in 

 length, by about 17 in breadth. 



» It is most unfortunate that Mr. Howlett did not measure, or at 

 any rate does not mention, the breadth of the penumbra — that is, 

 in effect, the size of the umbra. 



" On June 2 (the day on which I had invited Mr. Ran- 

 yard, and also again Mr. Perry, to specially watch for 

 WOson's phenomenon) I made these notes : — At 7h. 50m. 

 A.M. the outer border of the penumbra of the spot X was 

 just 20' from the western limb. The whole spot subtended 

 45" in length of heliographical latitude, but so foreshortened 

 in directions of longitude as to subtend not more than 10" 

 in width. 



" The umbra was as nearly as possible central, and having 

 an apparent width of 3". Both the preceding and follow- 

 ing sides of the penumbra were of the same width, viz. 3", 

 though if there were a difference, the ' following,' or inner, 

 side was the wider of the two, by perhaps 'S. There was 

 at least a sensible difference (though exceedingly small, 

 truly) in the opinion both of myself and a perfectly im- 

 partkl fellow-observer, who, in fact, did not know what was 

 expected one way or the other. 



" I regret that there should have been any divergence of 

 opinion between Father Perry on the one side and Mr. 

 Ranyard and myself on the other, though that divergence 

 is evidently extremely slight. What we require, however, 

 in such investigations are actual measurements by micro- 

 metrical appliances." 



In my treatise on the " Sun " ob.!ervations of a kindred na- 

 ture by several observers are mentioned, the earliest probably 

 being Sir W. Herschel's observation of a spot in 1783, of 

 which he remarks : — " I ob.served that, contrary to what 

 usually hap|)ens, the margin of that side of the sjiot which 

 was farthest from the limb was the broadest." Nothing, 

 perhaps, can be regarded as much tetter established, or by 

 a more widely ranging series of observations, than the fact 

 that sun-spots do not systematically, even if regular in out- 

 line, present the signs of a funnel-shaped form which Dr. 

 Wilson recognised in 17G9, and the elder Henschel in 1779. 

 The latest statements, before ]\Ir. Hewlett's, on this point 

 ai-e those addressed by Professor Splirer, of Munich, to an 

 assembly of astronomers at Geneva in August 1885. He 

 there, " in a copious rtsume of his labours in connection 

 with the solar spots " (I quote from Mr. Howlett's paper), 

 " denies that the spots posse.ss the character of funnels 

 (lonnoirs), which is attributed to them in the greater number 

 of works on elementary astronomy. The appearances which 

 attend the disappearance of spots on the sun's western limb 

 cannot, he aflii'ms, be explained by such a theory. ' When 

 a spot,' he says, ' is very shortly about to disappear, by 

 virtue of the sun's rotation on his axis, the two lateral 

 borders of the penumbra vanish from sight, whilst the 

 nucleus [umbra] remains visible, together with the fragments 

 of the surroundings (puurtoiir) to the north and south ; the 

 whole phenomena,' he says, ' presenting the appeaiance of 

 a veiled brightness {eclat voile) to such a degree that one 

 might easily confound the nucleus [umbra] it.self with its 

 surroundings,' and of which typical and striking instances 

 may be seen in vol. ii., sheet 99, and still more sheet 108, 

 of my drawings. He seeks to explain this appearance by a 

 heated facular border surrounding the spot, and causing an 

 ascending current of extremely heated gas, through which 

 it is with the greater difficulty that the spot is discerned, the 

 nearer it is to the limb." 



Mr. Howlett mentions, among possible solutions of the 

 ditficulty, the possibility that the refractive action of the 

 sun's atmosphere, by which somewhat more than half his 

 sphere should be visible to us, may be in question. He 

 overlooks here the circumstance that small well-defined 

 spots have been observed to appear on the eastern limb, and 

 disappear at the western, at epochs by no means agreeing 

 with the supposition of a refractive action outside the photo- 

 sphere sufficient to show appreciably more than half the 

 solar globe. Nor could prominences be flattened or levelled 



