THE AMERICAN JOURNAL OF SCIENCE AND ARTS. [THIRD SERIES.] ART. XLVI.-On the Solar Corona; by Prof. C. A. YOUNG, of Dartmouth College. ALTHOUGH We have not yet official reports from the different parties who observed the solar eclipse of last December, it is perhaps not necessary to wait for them in order to form an idea. of the state in which the problem of the sun's corona now stands. From what has already appeared in the scientific and news journals, it is clear that while little new matter has been added to the stock of scientific knowledge, and while some questions are left in a more puzzling condition than ever, yet certain of the conclusions arrived at in 1869, but received with a good deal of reserve in certain quarters, have been fully confirmed and placed beyond further reasonable doubt. The polariscopic observations are less accordant and conclusive than might be wished; undoubtedly because at all the stations they were more or less interfered with and complicated by the presence of clouds or haze. On the whole, however, they seem to show pretty definitely that a portion of the coronal light is polarized in radial planes, and is therefore derived, not self-originated-almost certainly simple reflected sunlight-though, of course, the polariscope does not inform us whether the reflecting particles are near the sun, or the moon, or in our own atmosphere. Very perplexing also is the fact that the faint continuous spectrum, which must be in part produced by this polarized component of the corona's light, shows no discoverable traces of the dark lines of the ordinary sunlight-spectrum. Probably AM. JOUR. SCI.-THIRD SERIES, VOL. I, No. 5.—MAY, 1871.. they exist but are in some way masked so that they are not easily detected. But if we grant that the polariscope has demonstrated the derivative character of one fraction of the coronal radiance, it is far more certain that the spectroscope has proved some of the matter of the corona, if not all, to be self-luminous; for its spectrum is characterized by a bright line. This line both in 1869 and 1870 was identified with a bright line of the chromosphere-spectrum which coincides exactly with a dark Fraunhofer line at 1474 of Kirchhoff's scale (wave length 5316 according to Angström). Perhaps I may be allowed to step a little out of my way to add, that while it is customary to speak of this line simply as "a bright line near E," as if its exact position were somewhat doubtful, there is no occasion for the slightest reserve in the matter. The coincidence of the bright C line of the protuberance-spectrum with the dark C is no more accurately ascertained. In the eclipse of last December, just before the beginning of the totality, the cross wires of my spectroscope were set carefully upon this line, already plainly bright in the spectrum of the chromosphere; and as soon as the sun was covered, I traced it out into the corona more than 16' from the sun's limb. With the full dispersive power of 13 prisms, I have never been able to find the least want of correspondence between this line as seen in the chromosphere and its dark analogue, except occasionally such as was evidently due to the motion of the luminous matter, producing slight changes of refrangibility, sometimes in one direction and sometimes in the other. So far as I can learn, this line was verified last December by all the spectroscopic observers who saw anything at all, with the single exception of Lieut. Brown of Lord Lindsay's party. Two other faint lines which I saw in 1869 between this and D, and doubtfully* reported as corona-lines (see this Journal for Nov. 1869, p. 376), were not seen by any one on this occasion, unless perhaps one of them by Father Denza in Sicily. In a letter from Father Secchi which appeared in the Astronomische Nachrichten for Feb. 21, 1871, he writes, "Mon collegue le P. P. Denza directeur de l'observatoire de Moncalieri, observa avec un spectroscope, que j'avais convenablement disposé, deux raies brillantes dans le couronne, une prés de l'E de Fraunhofer, l'autre au milieu entre le vert et le jaune." Very possibly this latter ray "half way between the yellow and the green" may be one of the two. Considered as a demonstration of self-luminosity, however, one bright line is just as conclusive as many. I have experienced some annoyance during the past year at seeing these lines in several publications put upon the same footing as 1474. I was never at all confident as to their coronal character. Nor can there be any doubt as to the location of the selfluminous matter. It cannot be in our own atmosphere, for no possible reason can be assigned why the particular molecules of air, that happen to lie near the lines which join the eye of the observer with the edge of the moon, should become luminous rather than others in a different portion of the sky. Nor can it be at the moon; otherwise of course it would always be visible around her disc, since there is nothing in the mere accident of her being on the line of syzygies which could account for the phenomenon. I may add in passing, that I have often and carefully examined the neighborhood of the moon's limb, half hoping that some possible rare atmosphere of our satellite might reveal itself at some time by a faint auroral spectrum; thus far, however, without success. Accordingly it is now universally, I think I may say, acknowledged that one important element of the corona consists in a solar envelope of glowing gas, reaching to a considerable elevation. Mr. Lockyer, who is still disposed to assign to the solar element of the corona a lower relative importance than most other astromomers, concedes a thickness of from six to ten minutes. (See an article by him in Nature for Feb. 23d.) For this envelope the name of "leucosphere" has been proposed by a member of the Royal Astronomical Society at one of its recent meetings, and it seems to be a very suitable term and well worthy of adoption. It has been objected to on the ground that "chromosphere" covers the whole bright-line region around the sun; but when the latter name was first proposed, there was evidently no idea that above the envelope of hydrogen there lay another from 20 to 100 times as extensive, and it would certainly be very convenient to restrict it to the lower red hydrogen stratum of the solar atmosphere, and retain the new term to designate this more elevated mass of gaseous matter. How extensive then is this leucosphere? Perhaps the question can hardly be answered definitely as yet; but it seems likely that it will be found to be at least from 8' to 10' thick on the average, with occasional prolongations of double that extent; not impossibly it may turn out to have no upper limit whatever, but to extend outward indefinitely into space. Were it not for the interference of our atmosphere, the matter could be settled very summarily, at least as to a minor limit, by the record of the spectroscopic observations. Prof. Winlock observed the green line at a distance of more than 20' from the sun, and others nearly as far. But there can be no doubt that had the spectroscope, during the eclipse, been directed upon any reflecting surface illuminated by the corona light, the same green line would have appeared in its spectrum. The question, of course, at once arises, whether the presence of this line at such a distance from the sun was not due to atmospheric reflection, especially as the sky was full of haze and light cirrus clouds at all, or nearly all, the stations of observation. It is certainly difficult to say that such might not have been the case to some extent, for the hydrogen lines were seen at an elevation of 4' to 5', far above any possible hydrogen, and faintly even on the disc of the moon itself-evidently by just such an atmospheric reflection. There is, however, an important difference between the behavior of the hydrogen lines, and that of 1474. At the edge of the chromosphere there was a sudden and very great falling off in the brightness of the former, while no such boundary was observed for the latter; the line grew regularly and continuously more faint as the distance from the sun increased, until it simply faded out, no abrupt alteration of its brilliancy being anywhere noticed. (I now speak of my own observations; but so far as I have learned, the experience of others was the same.) In 1869 the difference between the hydrogen lines and 1474 was still more marked. The former could hardly be traced at all beyond the actual limit of the chromosphere, the air being so clear that there was very little atmospheric effect. But there is another way of approaching the subject. The spectroscopes used by our party at Jerez were of two different classes, and by a combination of their results we may obtain an indication of considerable value. In the instruments employed by Prof. Winlock and myself, and I presume by the greater number of the eclipse observers, the spectroscope is attached to the eye-end of a telescope whose object-glass throws upon the slit a distinct image of the body to be examined. Such an instrument may be called an analyz ing spectroscope, since it deals with ele mentary portions of the luminous area under investigation. Thus, if the figure represent the image of the corona, chromosphere and moon formed by the object-glass of the telescope, then if the slit be placed as at S, the spectrum seen will be produced solely by light from those elements of the luminous area whose foci fall within its jaws. The spectrum will be divided longitudinally into two portions, of which the one derived from the upper portion of S will show only the lines due to the leucosphere, while the other will exhibit the chromosphere lines plus those of the leucosphere. (Of course it must be remembered that while the hydrogen does not extend above the limit of the chromosphere, the coronal matter does reach down to the surface of the sun, and in the chromosphere is mingled with the hydrogen; indeed, except during eclipses, it is only at the very base of the chromosphere that the 1474 line can be seen reversed with our present spectroscopes.) If now we assume the chromosphere and leucosphere each to be of uniform brightness throughout (a supposition not strictly true but allowable for our present purpose), then by putting a for the angular area of the chromosphere, x for that of the leucosphere, for the intensity of the bright 1474 line as seen in an analyzing" spectroscope, and ni for the intensity of the bright C line observed with the same instrument, we shall have ix=[1474], where [1474] represents the total quantity of "1474 light" coming from the whole leucosphere. Similarly n. i. a.=[C], where [C] represents in the same manner the total "C light" from the chromosphere. [1474] Now, as nearly as I can estimate, the ratio between the brightness of the C and 1474 lines is about the same as that between a star of the first magnitude and one of the fifth or sixth. That is to say, the C line is between twenty-five and one hundred times as bright as 1474; so that in our equations we may write n=50 as a fair approximation, and we shall have In order to find x in terms of a it only remains to find the ratio of [1474] to [C]. For this purpose we must employ a spectroscope so arranged that its slit may receive at every point, light coming indiscriminately from every part of the object. This is the case when a spectroscope is used, unconnected with any telescope, by simply pointing the slit of its collimator toward the source of light. Thus used it may be called an integrating instrument, since it sums up in each bright line of its spectrum, the whole quantity of each kind of light reaching the slit from the whole luminous area included within its field. An instrument of this sort was employed by Prof. Pickering in 1869, and with results of considerable importance. Accordingly, two of the gentlemen of our party were equipped in the same manner, and one of them, Mr. Pye, by my request, noted the brightness of the lines on an arbitrary scale from ten down, with the following result: [C] 8.5, [D3] 55, [1474] 10, and [F] 3. Probably the squares of these numbers would more nearly express the true ratio of the intensities, since we seem instinctively to estimate one light as twice the intensity of another when it appears of the same brightness at a double distance, and so on. |