336 



NA TURE 



\Ang. 22, 1872 



in his picture, we should find that they correspond with those 

 which will furnish him with daylight in the most perfectly 

 polarised condition. 



Once mare, among the many and curious plienomena which 

 are visible during a solar eclipse, there is one which has longer 

 than any other refused to lift its veil to the solicitations of science. 

 I mean that halo of light, or corona as it is called, which extends 

 beyond the darU disc of the moon, Ijeyond those red flames of 

 burning gis which the researches of Lockyer, of Janssen, and 

 of others have brought almost home to us, far away for 

 millions of miles into distant regions of space. It was pre- 

 eminently to investigate this phenomenon that the last Eclipse 

 Expedition, furnished with funds by Iler Majesty's Govern- 

 ment at the instance of this British Association, was sent 

 out. And upon this investigation all the powers of the 

 twin instruments of modern times, the spectroscope and 

 the polariscope, were turned. The spectroscope coull 

 tell us the nature of tlie substances to the combustion of 

 which the light is due, and even the conditions of temperature 

 and of pressure under which the combustion is taking place ; but 

 it coull not disentangle those parts of the phenomenon which are 

 due to direct, from those which are due to reflected or to scattered 

 light. It was for the polariscope to tell us whether the corjna 

 is a terrestrial effect, — a mere glare, in fact, from our own atmo- 

 sphere, — or a true solar phenomenon; and in the latter issue, 

 whether any of it is due to direct rays from incandescent matter, 

 or all of it to rays originating in such incandescent matter below, 

 but s;attered laterally from gases which have cooled in the upper 

 regions surrounding the sun. This question ha? not even yet 

 received a definitive answer. But the brief .account given within 

 the last few days by Mr. Lockyer, in anticipation of his more 

 complete digest of the voluminous reports from the various 

 branches of the Expedition, seems to justify us in the conclusion 

 that the corona is substantially a solar phenomenon due not to 

 direct but to reflected or scattered rays. 



Tne principle upon wliich the polariscope enables us to make 

 theie reftned distinctions in such far off phenomena is after all 

 very simple. If the corona were due wholly to the effect of our 

 atmosphere on such light .as rea-Aes us during a total eclipse of 

 the sun, the whole of that light would be similarly affected, be- 

 cause it comes very nearly from the same part of the heavens. 

 In other words, its polarisation would be uaiform, and the corona, 

 when examined by a Nicol and quartz, would appear of a uniform 

 colour. But if the phemraenon were wholly due to the sun and 

 its surroundings, the light would be affected, if at all, differently 

 in different directions drawn outwards (like spokes or radii of a 

 wheel) from the sun as a centre. In other words, its polarisation 

 would be arranged spoke-wise, or, to use the technical term, 

 radially ; and the corona, when examined as before, would vary 

 in colour on different sides of the sua. 



I have already drawn largely, perhaps toi largely, up^n your 

 patience. But it will not have been without purpose that, besides 

 witnessing the exhibition of a few experiments, you should have 

 seen, at least in ou'line, what mrnner of thing a scientific in- 

 vestigation is. Well, whatever it is (and I will not weary you 

 with a dry statement of its processes), the foundation of it must 

 always be laid in careful, accurate, and intelligent observation of 

 facts. And it is a consideration which may well stir the hearts 

 of us outsiders of science, especially on an occasion when we 

 come face to face with some of the greatest philosophers of our 

 time, than any one of us, by practising his eye and riveting his at- 

 tention, may contribute some natural fact, some fragment of know- 

 ledge, to the common stock. And surely has not this a particular 

 significance and importance to us, at a periol whei, by shorten- 

 ing the hours of labour, more leisure, as we may hope, will be 

 at the command of many? It will, I take it, be our own fault if 

 we spend that leisure in walking through dry places seeking rest ; 

 for, to those who have the eyes to see and the spirit to discern, 

 the world is neither dry nor barren ; but rather, it is like the 

 mountain as it appeared to the servant of the prophet when his 

 eyes were opened, full of beauty and wonder, of mystery and 

 power, — full of hosts from all nations, striving manfully onward 

 to promised lands of knowledge and of truth, and waging cease- 

 less warfare against ignorance and prejudice, and the long train 

 of evils which are consequent upon them. And if, as the even- 

 tide of life draws on, our eye wax dim, and our step grow weary, 

 so that we can no longer f jllow, we may still lay us down to 

 rest in some unknown spot, in the full confidence that others wUl 

 not be WiU'ing to fill our places and gain fresh ground, though 

 we may no: live to see it. 



SECTION A 



Sectional Proceedings 



All ititcrim Ri-port on the Results Ohtainctl by the British 

 Association Eclipse Expedition of 1871, by J. Nornwu Lockyer, 

 K.K.S. 



/. New Instruments 



These were as follows ; — 



1. A train of five prisms to view the corona. 



2. A large prism of small angle placed before the object glass 

 of a telescope. 



On these instruments I may remark that the Royal Astronomical 

 Society, in the first instance, invited me to take charge of an Ex. 

 pedition to India merely to conduct spectroscopic observations; 

 but although this request did me infinite honour, I declined it, 

 because the spec'roscope alone, as it had been used before, was, 

 in my opinion, not comoetent to deal with all the questions 

 now under discussion. Thus some of the most eminent .\merican 

 observers had come to the conclusion that the spectrum of hydro- 

 gen observed in the last eclipse round the sun, to a height of 8 

 minutes, was a spectrum of hydrogen "far above any possible 

 hydrogen" at the sun. Hence it was in some way reflected. 

 Now with our ordin.ary spectroscopic methods it was extremely 

 difficult, and one might say impossible, to determine whether the 

 light which the spectroscope analysed was really reflected or not ; 

 and that was the whole question. 



It became necessary, therefore, in order to give any approich 

 to hopefulness, to proceed in a somewhat different way in the 

 1S71 expedition, with regard to the spectroscope, and, to guard 

 against failure, to supplement such observations with photographs. 

 To understand the method adopted, let us suppose a train of 

 piisms. Take one prism out of the train, and consider what 

 will happen if we illuminate a slit with a monochromatic light 

 and observe it through the prism. If we render sodium vapour 

 incandescent and illuminate the slit by means of it, we get a bright 

 yellow image of the slit, due to the vapour of the metallic sodium 

 only giving us yellow light. But why is it that we get a line? 

 Because we employ a line slit. If, instead of a straight line, we 

 have a crooked line for the slit, then we see a crooked line through 

 the prism. Going one step further : Suppose that instead of a 

 line.jwhether straight or crooked, we have a slit in the shape of 

 a ring, we seen ring image through the prism. And then conies 

 this point : If, when we work in the laboratory, we examine 

 these various slits, illuminated by these various vapours, if we 

 observe the corona in the same way, we shall get a ring built up 

 by each ray of light which the corona gives to us ; since we know, 

 from the American observations, that there were bright lines 

 in the spectrum of the corona, as observed by a line slit : in 

 other words, the corona examined by means of a long train of 

 prisms should give us an image of itself painted by each ray which 

 the corona is competent to radiate towards us.* 



These were the cousidenations which led to the adoption of 

 this new attempt to investigate the nature of the corona now in 

 question. It was, to use a train of prisms, pure and simple, using 

 the corona as the slit, a large number of prisms being necessary 

 to separate the various rings we hoped to see, by reason of their 

 strong dispersion. 



This principle, good for a train of prisms such as I have referred 

 to, is good also for a single prism in front of the object-glass of 

 a telescope. Such was the method adopted by Prof. Respighi, 

 the distinguished Director of the Observatory of the Capitol of 

 Rome, who accompanied the expedition. 



This method, if it succeeded, would be superior to the ordinary 

 one in this way. If we were dealing merely with scattered light, 

 then all the rings formed by vapours of equal brilliancy at the 

 base of the chromosphere would be of the same height, while, if 

 such scattering were not at work, the rings would vary according 

 to the actual height of the vapours in the sun's atmosphere. 



3. Integrating spectroscopes driven by clochoork. 



4. A self-registering integrating spectroscope, furnished with 

 telescopes and collimators of large aperture and large prisms. 

 (This instrument wis lent by Lord Linds.ay.) 



5. A polariscope-telescope, so arranged that th? same observer 

 couM almost simultaneously observe both with the Savart and the 

 Biqua'tz. 



6. A polariscope-telescope, arranged for rapid sweeping 

 round the corona at a given distance from moon's limb. 



** Aft;r I fiad thou-jht of this arrangement, and had secured an instrument 

 to carry it out. Prof- Younj. in a comminica'ion to Natush, suggeslei the 

 simemclliodofobsef ' 



