Figure 3.
This effect is very clearly indicated in the drawing (Figure 3) which you have now before you, and for which we are indebted to M. Tacchini, the astronomer, of Palermo. We have observed this same spot at Rome, and we have made a drawing of it similar to that you now see; but I would rather exhibit that of M. Tacchini, because it cannot be objected that it was made under the influence of a preconceived idea. You see that in this spot the edge of the aperture is raised much in the same way as in the craters of the moon, and around these apertures are elevations, clearer and more luminous, which we call faculas.
The conclusions which I have just presented to you are also those to which M. Faye arrived, in studying the apparent perturbations in the movements of the spots. In short, what settles the question definitively is the study of the spots of exceptional grandeur when they reach the edge of the solar disk. It is then very easy to prove that the centre is lower than that part of the outline from which radiates the facule. Both M. Tacchini and I proved this at Rome, in studying the grand spot of July, 1865, at the moment in which it disappeared behind the disk of the sun.
The spots, then, are apertures, rents made in the photosphere. But how is it that these spaces do not fill up immediately? This is a serious difficulty, and it leads us to study the structure of the photosphere. If the photosphere was solid, all the movements which take place in it would be impossible. It is, then, fluid. But, on the other hand, a fluid would naturally spread itself until all points of the surface were on the same level, and it would require very little time to fill a gap having the dimensions of even the largest of the spots. The celebrated William Herschel saw this difficulty, and he met it by a solution which we still adopt, because it has been confirmed by observations and discoveries; so that what to Herschel was but a conjecture has become to us a demonstrated truth. The photospheric matter is like our clouds, gauze-like and transparent as ours. We often see among the clouds differences of level—disruptions which enable us to perceive the blue of the sky in the space which separates them. The same thing happens in the sun; and this hypothesis, which is so useful in explaining the phenomena I have just set before you, accords perfectly with all the particulars observed.
We have seen, in effect, the luminous matter remain suspended and floating in the midst of the centre, and the photospheric currents melt in obscure parts, just as our clouds dissolve, apparently dispersing themselves in a space completely deprived of vapor, when the temperature is sufficiently elevated. The little white veil in Figure 1 is a cloud about to be dissolved. Without this dissolving force, the matter which radiates from the circumference to the centre would not be long in filling up this gap. As I told you just now, we have been able to seize the fact of this dissolution of the solar atmospheric matter, and to see these cloud-like forms change into red veils occupying a large surface in the centre.
One thing alone remains to be proved—the existence of a transparent atmosphere. We have for a long time presumed its presence and its action to explain a well-established fact, namely, that the edges of the sun impart to us less of heat and light than the centre. This fact, inexplicable by any known laws of radiation, is easily explained by the action of an absorbing atmosphere; for the rays part at the edge before passing through a thicker atmospheric stratum, proving necessarily an absorption more considerable than that which flows to the centre. The existence of a solar atmosphere, which was formerly regarded as probable, has been reduced to certainty by the observation of eclipses, and it has been shown that veritable clouds float in this gauze-like bed.
Everybody has heard of the magnificent aureola which surrounds the moon during the total eclipse of the sun. It is a truly solemn moment when, the last rays having just disappeared, we see the shadow of the moon projected on a sky of leaden hue, with a perfectly black disk surrounded by a magnificent luminous glory, like that which we see represented around the heads of the saints. This aureola, at least the part nearest the disk, is owing to the atmosphere of the sun. This spectacle is magnificent, but it becomes much more instructive when we examine it through a good telescope. We then perceive around the disk of the moon gigantic flames, of a lively red, the height of which is incomparably greater than the diameter of the earth. Some are suspended without any support, and others take a horizontal direction, like the smoke that comes out of our chimneys. These flames were designated protuberances; but we knew not how to explain them. It was even doubted whether they were real; and we were quite disposed to attribute them to an optical illusion. These doubts have disappeared since the observations we made in Spain during the eclipse of 1860. On that occasion we were stationed at Desertio de las Palmas, on the coast of the Mediterranean, while M. De la Rue took up his post at Riva Bellosa, at a short distance from the ocean. We succeeded at both these stations in photographing the sun at the period of the total eclipse, and a comparison of the two photographs has proved that the protuberances have a real existence, that they have a form so fixed as to give identical images at two points distant from each other by several hundreds of kilometres. The perfect resemblance of the two photographs is the more remarkable, from their not having been executed at the same moment. Between the two operations an interval of ten minutes elapsed. These protuberances, considering their distance and their bent forms, can be nothing but clouds suspended in the solar atmosphere, and it is these which form the red veils that we have seen in the centre. The observation of eclipses proves to us conclusively that the sun is really surrounded by a stratum of this red matter, which we ordinarily see only on the most elevated summits.
In the photograph taken at Desertio de las Palmas during the total eclipse, the exterior form of the atmosphere is perfectly visible. We see that it is more extended at the equator than at the polar regions, which is a natural effect arising from the movement of rotation which the sun possesses. We see, in short, that this atmosphere is livelier in its action in the two zones on each side of the equator, in which the spots ordinarily show themselves. The existence of a solar atmosphere being perfectly in accordance with all known principles and with all ascertained facts, there is no longer any room for calling it in question. We describe the sun, then, as surrounded by a dense atmosphere in which floats the photospheric matter. The surface of the photosphere is far from being uniform and regular. It is, on the contrary, wrinkled all over, and again covered with granulations. These granulations, first perceived by Herschel, have been carefully studied in later times.
When our atmosphere is calm and the observation very precise, the whole bottom of the solar disk appears covered with small luminous grains, separated by a very fine and very dark net-work, resembling in appearance partially desiccated milk, examined through a microscope. These points, or white grains, are of different sizes. Where there are openings, we see around each of them some lines of grains in the form of leaves, more or less oval. Their mean dimension is about the third of a second. These grains are only the upper part of the flame which inclines toward the openings, thus proving that there is a very sensible power of attraction in the apertures. We may even say that these granulations resemble the appearance which the clouds known as cumuli present when, from the summit of a mountain, their upper part is examined. The largest spots would be, then, but an exaggeration of this net-work, ordinarily so fine, produced by the force which caused the flame, or rather, the stratum of the cumulus.