Volume 8 — The Eclipse Of The Sun Of August 18, 1868. A Report Addressed By M. Janssen To The Marshal Of France, President Of The Bureau Of Longitudes.

Calcutta, November 3, 1868. M. Le Maréchal Et Ministre:

I have the honor of addressing to you, as President of the Bureau of Longitudes, my report on the eclipse of the 18th of last August, and upon some subsequent observations, which led me to the discovery of a method of observing the solar protuberances when the sun is not eclipsed. I will beg you to have the kindness to communicate this to the Bureau.

I have the honor to be, etc. etc.,

Janssen.

Mr. President:

I had the privilege of writing to you on the 19th of September last, to give you a brief account of my expedition. I am now able to furnish you with a more complete report of my observations during the great eclipse of the 18th of August.

The steamer of the Messageries Imperiales, in which I left France, landed me, on the 16th of July, at Madras, where I was received by the English authorities with great courtesy. Lord Napier, the governor of the province, gave me passage to Masulipatam upon a government boat. Mr. Grahame, an assistant collector, was sent with me to remove any difficulties which I might meet with in the interior.

On arriving, I had to select my station.

A chart of the eclipse shows that the central line, after crossing the Bay of Bengal, enters the peninsula of India at Masulipatam, and crossing the great plains formed by the delta of the Kistna, passes into a hilly country, containing several chains of mountains, on the frontier of the independent state of Nizzam. After receiving and considering much information on the subject, I determined to choose the city of Guntoor, situated on this central line, half-way between the mountains and the sea. I thus avoided the sea-fogs, very frequent at Masulipatam, as well as the clouds which often hang about lofty peaks.

Guntoor is quite an important place, being the centre of a large cotton trade. This cotton comes mostly from Nizzam, and is shipped to Europe from the ports of Cocanada and Masulipatam. Several French merchants, with their families, live at Guntoor; they are descended, generally speaking, from those ancient and numerous families which in former times were the glory of our beautiful Indian colonies.

My observatory was at the residence of M. Jules Lefaucheur, who was so kind as to place at my disposal all the first story of his house, which is in the highest and best part of the city. The rooms of this first story communicated with a large terrace, upon which I erected a temporary structure suitable for the observations intended.

The instruments were several achromatic lenses of six inches aperture, and a Foucault telescope of twenty-one centimetres. The former were all mounted upon one stand. The general movement was given by a mechanism constructed by Messrs. Brunner Bros., which enabled one to follow the sun by a simple rotation.

The apparatus was furnished with finders of two and two and three quarter inches aperture, which were themselves good astronomical glasses. In spectral analysis, these finders have a peculiar importance; for by means of them the precise point of the object under examination is known, to which the slit of the spectroscope in the principal telescope is directed. It is therefore necessary that the cross-wires, or in general the sights placed in the field of the finder, should correspond with great exactness with the slit of the spectral apparatus, and I had, of course, taken great care to secure this essential point. Special micrometers were also provided, to measure rapidly the height and angle of position of the protuberances. As for the spectroscopes, I had chosen them of different magnifying powers, so as to answer to the different requirements of the various phenomena. Finally, the apparatus carried, at the eye-piece end, screens of black cloth, forming a dark chamber, in order to preserve the sensibility of the eye.

Besides these instruments, intended for the principal observations, I had brought a full set of very delicate thermometers, made with great skill by M. Baudin; also some portable spy-glasses, hygrometers, barometers, etc. Thus I was able to turn to account the kindness of MM. Jules, Arthur, and William Lefaucheur, who offered their services for the subsidiary work. M. Jules, who is a good draughtsman, undertook to sketch the eclipse. An excellent telescope, of three inches aperture, furnished with cross-wires, was assigned to his use; he practised with it the representation of the expected phenomena by means of artificial imitations of eclipses. The thermometric observations were given to M. Arthur, who was also directed to ascertain the brilliancy of the protuberances and of the corona at the moment of totality, by a very simple photometric process.

I was assisted in my own operations by M. Redier, a young subaltern, whom the commander of the steamer L'Imperatrice had supplied to me. The services of M. Redier, who has excellent observing qualities, were very useful to me.

The time which remained before the eclipse was employed in preliminary study and practice, which served to familiarize us with the handling of our instruments, and suggested to me various improvements in them.

The day approached, but the weather did not promise to be favorable. It had rained for some time all along the coast. These rains were considered as extraordinary and exceptional. Fortunately, they moderated gradually before the 18th; and on that day the sun rose unclouded, and dimmed only by a mist out of which it soon passed; and at the time when our telescopes showed us that the eclipse began, it was shining with its full splendor.

Every one was at his post, and the observations immediately commenced. During the first phases some thin vapors passed before the sun, which interfered somewhat with the thermometric measurements; but, as the moment of totality approached, the sky became sufficiently clear.

Meanwhile the light diminished sensibly, surrounding objects appearing as if seen by moonlight. The decisive moment was near, and we waited for it with some anxiety; this anxiety took nothing from our powers of observation, it rather stimulated and increased them; and it was, besides, fully justified by the grandeur of the spectacle which nature was preparing for us, and by the consciousness that the fruits of our thorough preparations and of a long voyage would depend on the use now made of a few minutes.

The solar disc was soon reduced to a narrow bright arc, and we redoubled our attention. The slits of the spectroscopes were kept precisely upon the part of the moon's limb where the last light of the sun would be seen, so that they would be directed to the lower regions of the solar atmosphere at the moment of contact of the discs.

The total obscuration occurred instantaneously, and the spectral phenomena also changed immediately in a very remarkable manner. Two spectra, formed of five or six very bright lines—red, yellow, green, blue, and violet—occupied the field in place of the prismatic image of the sun which had just disappeared. These spectra, about one minute (of arc) long, corresponded line for line, and were separated by a dark space in which I could see no lines.

The finder showed that these two spectra were caused by two magnificent protuberances which were now visible on each side of the point of contact. One of them, that on the left, was more than three minutes (or one tenth of the sun's diameter) in height; it looked like the flame of a furnace, rushing violently from the openings of the burning mass within, and driven by a strong wind. The one to the right presented the appearance of a mass of snowy mountains, with its base resting on the moon's limb, and enlightened by a setting sun. These appearances have been carefully drawn by M. Jules Lefaucheur. I will therefore only remark before quitting the subject, which I shall have to treat subsequently under a special aspect, that the preceding observation shows at once:

1st. The gaseous nature of the protuberances, (the lines being bright.)

2d. The general similarity of their chemical composition, (the spectra corresponding line for line.)

3d. Their chemical species, (the red and blue lines of their spectrum being no other than the lines C and F of the solar one, and belonging, as is well known, to hydrogen gas.)

Let us now return to the dark space which separated the spectra of these protuberances. It will be remembered that, at the moment of the total obscuration, the slits were tangent to the solar and lunar discs, and were therefore directed toward the circumsolar regions immediately above the photosphere, in which regions M. Kirchhoff's theory places the atmosphere of vapors, which produces by absorption the dark lines of the solar spectrum. This atmosphere, when shining by its own light, should, according to the same theory, give a reversed solar spectrum, that is to say, one composed entirely of bright lines. This is what we were expecting and trying to verify, and it was to make the proof decisive that I had used so many precautions. But we have just seen that only the protuberances gave positive or bright-line spectra. Now, it is very certain that, if an atmosphere formed of the vapors of all the substances which have been found in the sun really existed above the photosphere, it would have given a spectrum at least as brilliant as that of the protuberances, which were formed of a gas much less dense and less luminous. It must, then, be admitted that, if this atmosphere exists, its height is so small that it has escaped notice.

I must also add that this result did not much surprise me; for my investigations on the solar spectrum had led me to doubt the reality of any considerable atmosphere around the sun, and I am more and more inclined to think that the phenomena of elective absorption, ascribed by the great physicist of Heidelberg to an atmosphere exterior to the sun, are clue to the vapors of the photosphere itself, in which the solid and liquid particles forming the luminous clouds are floating. This view is not merely in harmony with the beautiful theory on the constitution of the photosphere which we owe to M. Faye, but even seems to be a necessary deduction from it.

In fine, the eclipse of the 18th of August appears to me to show that the formation of the solar spectrum cannot be explained by the theory heretofore admitted, and I propose a correction to this theory as above indicated.

To return to the protuberances. During the total obscuration, I was much impressed by the extreme brilliancy of their spectral lines. The idea immediately occurred to me that they might be seen even when the sun was unobscured; unfortunately the weather, which became cloudy after the eclipse, did not allow me to try the experiment on that day. During the night, the method and the means presented themselves clearly to my mind. Rising the next morning at three, I prepared for these new observations. The sun rose quite clear; as soon as it had risen from the haze of the horizon, I began to examine it, placing the slit of the spectroscope, by means of the finder, upon the same place where, the day before, I had seen the protuberances.

The slit, being placed partly on the solar disc and partly outside, gave, of course, two spectra, that of the sun and that of the protuberances. The brilliancy of the solar spectrum was a great difficulty; I partially avoided it by hiding the yellow, the green, and the blue portions, which were the most brilliant. All my attention was directed to the line C, dark for the sun, bright for the protuberance, and which, coming at a rather faint part of the spectrum, was seen with comparative ease.

I had not examined the right hand or western part of the protuberant region long when I suddenly noticed a small bright red line, forming an exact prolongation of the dark line G of the sun. Moving the slit so as to sweep methodically the region which I was exploring, this line remained, but changed its length and its brilliancy in the different parts, showing a great inequality in the height and brightness of the various parts of the protuberance. This examination was resumed at three different times, and the bright line always appeared in the same circumstances. M. Redier, who assisted me with much interest in these experiments, saw it as well as I, and soon we could even predict its appearance by merely knowing what region we were examining. Soon after, I ascertained that the line F showed itself simultaneously with G.

In the afternoon, I returned to the region examined in the morning; the bright lines again showed themselves, but they indicated great changes in the distribution of the protuberant matter; the lines broke up sometimes into isolated fragments which would not unite with the principal one, notwithstanding the shifting of the slit. This suggested the existence of scattered clouds formed during the forenoon. In the region of the great (or left hand) protuberance, I found some bright lines, but their length and arrangement showed that great changes had also occurred here.

These first observations already showed that the coincidence of the lines G and F was real, and that hydrogen was certainly the most important element in these circumsolar masses. They also established the rapidity of the changes which these bodies undergo, which cannot be perceived during the short duration of an eclipse.

The following days, I availed myself of all the opportunities allowed by the weather to apply and perfect the new method, at least as far as was permitted by the character of the instruments, which had not been constructed to suit this new idea.

Observing very attentively the lines of the protuberances, I have sometimes noticed that they penetrated into the dark lines of the solar spectrum, showing thus that the protuberance extends over part of the sun's disc. This result was naturally to be expected; but the interposition of the moon has always made its proof impossible during eclipses.

I will also detail here an observation made on the 4th of September at a favorable time, which shows how rapidly the protuberances change their form and position.

At 9h. 50m., the examination of the sun showed a mass of protuberant matter in the lower part of the disc. To determine its shape, I used a method which may be called chronometric, since time is employed in it as the standard of measure.

In this method, the telescope is placed in a fixed position, so chosen that by the diurnal movement of the sun all parts of the region to be explored shall come in turn into the field of the spectroscope; and at determinate times the length and situation of the spectral lines successively produced are noted.

The time occupied by the sun's disc in passing before the slit gives the value of a second of time in minutes of arc. This, combined with the length of the lines estimated in the same unit, gives the means for a graphic representation of the protuberance.

The application of this method to the study of the solar region just mentioned as seen on this occasion, showed a protuberance extending over about thirty degrees (or one twelfth) of the sun's circumference, ten of which were east of the vertical diameter, and twenty west. Near the extremity of the western part, a cloud was lying, distant one and a half minutes, or one twentieth of the sun's diameter, from its limb. This cloud, about two minutes long and one high, was parallel to the limb. One hour afterward, a new drawing showed that the cloud had risen rapidly, and taken a globular form. But its movements soon became still quicker; for ten minutes later, at eleven o'clock, the globe was enormously extended in a direction perpendicular to the limb and to its previous position. A little mass of matter was also detached from the lower part, and hung between the sun and the main body of the cloud. Thick weather coming on prevented further observations.

To resume our remarks. Considered in regard to its principle, the new method is based upon the difference of the spectral properties of the protuberances and of the photosphere. The light of the latter emanates from solid or liquid particles, which are incandescent, and is incomparably brighter than that of the former; so that these have hardly been visible hitherto, except during eclipses. But the case is quite altered when we use the spectra of these bodies. For the solar light is spread over the whole extent of its spectrum, and thus much weakened; while that of the protuberances, on the contrary, is condensed into a few lines whose intensity bears some proportion to that of the corresponding solar ones. Hence their lines are quite easily seen in the field, together with those of the sun, though their ordinary images are entirely effaced by the dazzling light of the photosphere.

Another very fortunate circumstance for the new method comes to the support of the one just mentioned, namely, that the bright lines of the protuberances answer to the dark ones of the solar spectrum. Hence they are not only more easily seen in their own proper field, outside and on the edge of the solar spectrum, but they can even be followed into the interior of the latter, and by this means the protuberances can be traced upon the globe of the sun itself.

As regards the determination of chemical composition, the methods followed during total eclipses always carried with them some uncertainty; since, in the absence of the sun's light, graduated scales had to be employed to fix the position of the lines. The new method enables us to compare the two spectra directly.

As to the results obtained during the brief period in which this method has been used, they are as follows:

1st. That the luminous protuberances observed during total eclipses belong unquestionably to the circumsolar regions.

2d. That these bodies are mainly or entirely composed of incandescent hydrogen gas.

3d. That they are subject to movements of which no terrestrial phenomenon can give us any idea; since, though they are masses of matter having several hundred times the volume of the earth, they change completely their form and position in the course of a few minutes.

Such are the principal facts arrived at. I hope, notwithstanding the state of my eyes—fatigued by protracted experiments upon the subject of light—that I shall be able to continue my labors, and have the honor of submitting the results to the Bureau.

In conclusion, I will add, that I have also had an opportunity to continue my researches on the spectrum of the vapor of water. The climate of India, which is very moist at present, is quite favorable to these investigations. I am inclined to attribute to this spectrum a continually increasing importance. The whole series of my observations here and at Paris has made me confident of an elective action upon all the solar rays as far as the extreme violet, though in the latter such an action is much more difficult to establish with certainty. These experiments will form the subject of a separate communication.