Nov. 23, 1871J 



NATURE 



61 



theGerman " VierteljahrsschriftdesAstronomischenGesellschaft" 

 within the past four or five years, and see that only a single one 

 of them all is expressly mentioned, and to note the values of the 

 parallax adopted in the astronomical ephtmerides of France, 

 Spain, Portugal, and Germany, and see that not one of them can 

 be traced in Mr. Proctor's history. If as he once said, he had 

 not room to describe the recent researches, I should have sup- 

 posed he would have condensed or omitted the older ones, which 

 these recent ones have superseded, instead of doing the contrary. 

 The importance of this matter arises from the fact that these 

 discussions and researches put a different face on a number of 

 questions connected with the determination of the solar parallax 

 from that given by Mr. Proctor, and I do not think the latter can 

 successfully argue that the astronomical world of today is nearly 

 a'l wrong in the views to which it has been led by five years of 

 discussion, experiment, and research. 



On Nos. 3, 4, and 7, of Mr. Proctor's defence, it is only need- 

 ful to remark (i) that X did not write No. 3 till I had verified 

 Foucault's result by a careful calculation not made on my thumb- 

 nail ; (2) that Mr. Proctor leaves it to be logically inferred that 

 the discussion alluded to in No. 4 was an unpublished one ; (3) 

 that, having disclaimed my interpretation of No. 7, his book 

 gives no explanation of the reason why Mr, Stone's parallax was 

 so much greater than those of Encke and Ferrer. It is only 

 necessary to refer to the paper of the latter in vol. v. of the 

 Memoirs of the Royal Astronomical Societv at pages 254 and 

 264 lo find a very full discussion of the apparent, and the so- 

 called true contacts. 



No. d involves one of the most important questions connected 

 with the determination of the solar parallax from transits of Venus, 

 .and I am sorry to see [hat Mr. Proctorsimply evadesihe issue, as 

 the misinterpretation to which he refers consists in supposing 

 him less erroneous than he really is. Let one make a drawing 

 representing the limbs of Venus and the sun in mathematical 

 contact. Un each side of ihe point of tangency there will be an 

 exceedingly thin thread of light, v.anisliing at that point. How 

 much of this thread will be visible by an ordinary telescope? 

 We must remember that the sun is viewed through a dark glas?, 

 which reduces its light to that of an ordinarily illuminated object. 

 The narrowest visible line so illuminated subtends an angle of 

 about 20". With a power of 120 this would correspond to a 

 breadth of one-sixth of a second. But it is well known that at- 

 mospheric tremors, and, with most old instruments, imperfect 

 corrections of the object-glass, prevent our seeing an object at 

 all approaching the minimum visible, and that the s; me cause 

 prevents the increase of magnifying power from giving a corre- 

 sponding increase of seeing power. It is probable that the 

 thickness of the least visible thread may have varied with the 

 telescope, the observer, the dark glass, and the atmo.s-phere, from 

 one or two tenths of a second to one or even two seconds. Let 

 us take the more favourable cases in which a thread of o'''2 is 

 visible. A simple calculation will show that there is a space of 

 3" '4 on each side of the point of tangency, in which the thread 

 will be thinner than this, and therefore invisible, and that the 

 visible cusps will be about 7" apart. How different this 7'' from 

 Mr. Proctor's invisibly thin ligament ! This explains the observa- 

 tions of Wolf and Andre, who found that the black drop when 

 seen at all continued after internal contact at ingress and pre- 

 ceded it at egress. 



In answer to Mr. Proctor's letter of October 5, I beg leave to 

 reply, if the " fringe " is something actually produced by the 

 telescope or the atmosphere, it is simply bad definition. If it is 

 not so produced, it is an optical illusion, of which the laws are 

 obscure, and the very e.vistence problemaiical under the circum- 

 stances in question. See, for instance, the celebrated paper 

 of Prof Baden Powell on Irradiation. Mr. Proctor's intimation 

 that the great mass of astronomers who have observed transits 

 of Mercury within the past forty years, among whom are included 

 Bessel, Airy, and the Sttuves, were careless and inferior ob- 

 servers, because they did not see an optical illusion according to 

 his view of it, is as good a rcductio ad absurduin of his theory 

 as I could ask for. It is comforting to know that one of his care- 

 less observers can be turned into a careful and attentive one by 

 giving him a te'escope with plenty of irradiating power. 



To prevent misapprehension, allow me to say that the theory 

 set forth in my letter of September 28 is in no way my own, but 

 was promulgated by Bessel nearly forty years ago, and has, I 

 believe, been since universally received on the continent of 

 Europe. , Simon Newcomb 



Washington, Oct. 23 



The Aurora of Nov. gth and loth 



I WITNESSED on the night of Nov. 9, at about 7.30 p.m., an 

 aurora which, for symmetry of form and other features, was very 

 remarkable ; and unless, as is very likely, some more able ob- 

 server has already sent you a description of it, you may like to 

 put my account on record. 



In the magnetic north horizon was the usual segment of 

 auroral light, very brilliant, and stretching considerably to the 

 east and west, its altitude being 20° or more. High above this, 

 and extending in a complete arch from the east to the west 

 horizon, was a remarkable and well-defined band of still brighter 

 light, about 7° in breadth, and passing about 30° from the zenith. 



Filling the space between these two arcs of light was what I 

 can call nothing else than a dark shadow, which had somewhat 

 of a mysterious appearance ; for, though decidedly darker to the 

 eye than other parts of the heavens, it did not in the le.ast ob- 

 scure even small stars, nor do I think this darkness was the 

 effect of contrast. In this dark space faint auroral streamers 

 occasionally shot up to the upper arch, but did not pa^s it. 

 This shadow was what the French observers speak of as the 

 nuie. 



The light of the upper arched band was silvery, and increased 

 much in intensity towards the horizon both east and west ; the 

 points of greatest intensiiy being about 5° above the horizon, as 

 would be expected in the direction in which the arch appeared 

 most foreshortened. 



While watching this phenomenon I was impressed by the con- 

 viction that, to an observer in space, the north magnetic pole of 

 our planet would have presented the appearance of being sur- 

 mounted by a symmetrical cap of light, streaked by one or more 

 bands, and terminated at its greatest distance from the pole by a 

 well-defined brilliant margin. 



In the hope that an observer in some other locality might have 

 made similar observations, I was preparing to measure the dis- 

 tance of the upper arch of light from the zenith, as well as the 

 positions in azimuth of the points where it toitched the horizon, 

 when the whole phenomenon was obscured by dense clouds. 



Stretton Rectory, Hereford, Nov. 15 H. C. Key 



The following brief extract from our observatory note book 

 may be interesting : — 



"Nov. 10. — For about 20° on each side of north, at 9.30 — 

 9,40 P.M., brilliant iva'-is of light followed one another rapidly, 

 from two to four in a second, moving upwards, following the 

 direction of the streamers, fading away at about 40° from the 

 horizon. Three or four waves could be seen at once, measuring 

 about 5° to S° by estimation, from crest to crest. . . ." 



I heard some of the boys remark " How close it must be ; it 

 looks like puffs of steam from behind those houses." 



Rugby J. M. W. 



As none of your correspondents who described the brilliant 

 aurora; of Nov. 9th and loth last week, speak of their being 

 seen earlier than from 7 to 10 P.M., it may be interesting to note 

 that in the Midland Counties the latter was visible at a consider- 

 ably earlier period of the evening. On the evening of the loth 

 I was walking from Reading in Berkshire to Caversham in Ox- 

 fordshire, from 5.45 to 6.5 P.M. During the whole of that time I 

 had before me the steady zi'hitc light of the coming aurora, extend- 

 ing perhaps 25° to 30° in width, and 20' in height, its centre being 

 immediately beneath Polaris. The appearance was exactly that 

 of the departing twilight in a clear winter sky, for which, in- 

 deed, but for its position and the time of the evening, it would 

 have been mistaken. As 1 noticed the light immediately on 

 leaving the railway station above the lights of the town, I have 

 little doubt that it had been visible since sunset. I had no oppor- 

 tunity of watching its progress after 6.5 p.m. ; up to that time 

 there were no coloured streamers, nothing but the white light I 

 have described. 



Alfred W. Bennett 



The Ghost of Flamstead 



I OUGHT earUer to have thanked this venerated shade for a 

 communication which will enable me to correct (at some future 

 time) an omission in my treatise on the Sun. Let me hasten to 



