CHAP. XV. scale. Its position was carefully determined by micrometrical reference to the next line, 2796 K (hydrogen y), which was very bright; h was also seen, very clear, but hardly brilliant. In all, I saw nine bright lines. The light of the corona "A faint continuous spectrum, without any traces of dark lines in it, was also visible, evidently due to the Corona. Its light, tested by a tourmaline applied next to the eye, proved to be very strongly polarized in a plane polarized. passing through the centre of the sun. I am not sure, however, but that this polarization, as suggested by Professor Pickering, may have been produced by the successive refractions through the prisms. This explanation at once removes the difficulty otherwise arising from the absence of dark lines." I have first to do with the continuous spectrum, deduced from Professor Pickering's observations. I think in such a method of observation, even if the corona were terrestrial and gave a dark line spectrum, the lines visible with such a dim light would in great part be obliterated by the corresponding bright lines given out by the long arc of chromosphere visible, to say nothing of the prominences, in which it would be strange if C, D, E, b, F, and many other lines were not reversed. This suggestion, I think, is strengthened by the statement that two bright lines were seen 46 near C" and "near E;" should we not rather read (for the "near" shows that we are only dealing with approximations) C ́and F, which is exactly what we might expect? But even this is not all that may be hazarded on the subject of the continuous spectrum, which was also seen by Professor Young under different conditions. Assuming the corona1 to be an atmospheric effect merely, as I have before asserted it to be, it seems to me that its spectrum should be continuous, or nearly so; for is it not as much due to the light of the prominences as to the light 1 See note on page 243. of the photosphere, which, it may be said roughly, are CHAP. XV. complementary to each other? With regard to the aurora theory, I gather from Professor Young's note that, if not already withdrawn, he is anxious to wait till the next eclipse for further facts. I consider that the fact that I often see the line at 1474, and often do not, is fatal to it, as it should be constantly visible on the proposed hypothesis. The observation of iron-vapour, as I hold it to be at this elevation, is of extreme value, coupled with its simple spectrum, seen during an eclipse, as it entirely confirms my observations made at a lower level in the case, not only of iron but of magnesium. [The following communication from Professor Young, printed in Nature, March 24, 1870, commenting upon some of the views advanced above, is here given : "It is not impossible that the so-called corona may be complex. Some portion of its radiance may, perhaps, originate in our own atmosphere, although I do not yet find myself able to accord with the conclusions of Dr. Gould and Mr. Lockyer in this respect, and am strongly disposed to believe that the whole phenomenon is purely solar. "This much appears certain, however, that there exists outside of the chromosphere properly so called (ie., the envelope of rea hydrogen), and as distinct from it as it is from the photosphere, an immense atmosphere of self-luminous substance, extending to a distance of from 5' to 8' from the sun's surface, and probably much further in places-phosphorescent dust or fog in a glowing gas. "In support of this idea I adduce the photograph of Mr. Whipple, taken at Shelbyville, Ky., with an exposure of 40%. On this, the photolytic corona (if I may use the expression to distinguish it from the visible corona, whose points of maximum brilliance were, according to Dr. Gould, entirely different reaches a height of 6'. Professor Harkness observed the 1474 line in the spectrum of the corona at a distance of nearly 5' from the sun's limb, and not near to any prominence. I do not know the precise elevation at which I saw it, but it was not less than 3′ or 4'. "Indirectly, also, the idea is confirmed by the spectroscopic observation of Professor Pickering, who used a single prism instrument, with the slit simply directed towards the sun, not attached to a telescope. He saw only three or four lines, the brightest in the green near E. Now, since this line, when observed by throwing a large image of the sun on the slit, is very faint as compared with C, D, and F, its intensity, as seen by him, can only be CHAP. XV. accounted for by supposing that the luminous area from which it was derived far exceeded that of the chromosphere and prominences. "I have noticed also that some of the observers of the Indian eclipse (Rayet and Pogson) speak of the intensity of the green line. Did they observe in the same manner as Professor Pickering? "I need hardly add that Professor Pickering's observation of the non-polarization of the corona concurs with what has been said. 66 As to the faint continuous spectrum, I am sure that the reported absence of dark lines was not the result of insufficient observation. "I could not have failed to see D, E, b, 1961, F and G, had they existed; for in a spectrum of similar brightness formed by a light from a cloud, not only these but many other lines are visible in my instrument. Now, the absence of some of these might, perhaps, be accounted for on the ingenious hypothesis proposed by Mr. Lockyer; but this would not apply to D, E, or G. [Why not?-J. N. L.] "But if we admit the existence of faintly luminous solid or foggy matter near the sun, either meteoric or arising from the cloudy condensation of a non-permanent gas, the whole is at once easy of comprehension." The auroral theory of the corona is thus referred to : "The objection pressed by Mr. Lockyer that the bright line 1474 is only occasionally visible, is, I think, unfounded. At any rate I have never failed to see it myself when looked for, and very seldom to make it visible to others when I have wished to exhibit to them. It is faint, and, like a difficult microscopic object, requires management to bring it out with five prisms; but by placing the slit tangential to the sun's disc, and giving the instrument a slight jar, it is seen to flash out as the limb passes off the slit. It is worth noting, too, that it is often especially plain at portions of the limb where the chromosphere is unusually shallow and faint. "But while I think it probable that this line coincides with the aurora line reported by Professor Winlock at 1550 of Mr. Huggins' scale, I am by no means sure of it. I understand its assigned position rested upon a single observation with a chemical spectroscope, and the probable error of such a determination cannot well be less than ten divisions of Kirchhoff's scale. I have naturally made many attempts to determine its position for myself, but have never seen it except thrice, and then not long enough at a time to complete a measurement. I am only sure that its position lies between 1460 and 1490 of Kirchhoff. "For this reason, although I do not at all abandon the hypothesis, which appears to have other elements of probability in the general appearance of the corona, the necessity of intense electrical disturbances in the solar atmosphere as the result of the powerful vertical currents known to exist there, as well as the curious responsiveness of our terrestrial magnets to solar storms; yet I do not feel in a position to urge it strongly, but rather await developments. "As to the substance which causes this line, I observe that Father Secchi, in a recent communication to the French Academy, is disposed to think it hydrogen; while Mr. Lockyer still believes it to be iron. I am in hopes that experiments now in progress may throw some light on the subject. "May I suggest, in closing this long communication, that it seems to me that valuable observations might be made at the Eclipse of next December, by fitting up telescopes with a ground glass sliding screen, upon which an image of the corona two or three inches in diameter should be thrown; the ground glass having the roughened side next the observer, so that he could sketch upon it with a lead pencil the outlines of the image, the glass being made long enough to allow of several such sketches. "The comparison of a series of such outlines would decide the question of changes in the coronal streamers, as the sketches, being simple tracings, could not but be accurate in their indications of position."] CHAP. XV. Professor Young's letter. THE MEDITERRANEAN ECLIPSE, 1870.1 I.—A LETTER FROM VENICE.2 CHAP. XVI. CLOUD in Sicily, cloud in Spain, cloud in Africa. Such at first sight might seem to be the only result of all the observations made on the eclipsed sun of 1870; such the reception given by Nature to those who wooed her as she Professor Parce's letter. 1 I should be wanting in gratitude if I did not record one fact here in connection with this expedition. Owing to those whose duty it was to bring the requirements of science before the Government appealing to the wrong department, and other causes, funds to aid the observations were in the first instance refused. At this juncture, Professor Peirce, the distinguished director of the expedition equipped in the most liberal manner by the American Government, arrived in this country, and hearing of the position of affairs, at once did me the honour to send me the following letter: "FENTON'S HOTEL "MY DEAR SIR,-I have been directed by the Government of the United States to have the best possible observations made of the total eclipse of next December. If I could aid the cause of astronomy by assisting the observers of England in their investigations of this phenomenon, I should be greatly pleased. I take the liberty therefore to invite your attendance, and also that of other eminent physicists of England, with either of the parties of my expedition, one of which will go to Spain and the other to Sicily. "Yours very respectfully and faithfully, "J. NORMAN LOCKYER, Esq. F.R.S." I at once accepted this generous offer. "BENJAMIN PEIRCE. I must also add, in justice to our own Government, that after some of my instruments had been shipped to Sicily another application was made, and it fell to my lot by an accidental circumstance to be called 2 Where I was snowed up for two days!! |