part of which will traverse the glass, and the other will undergo a second reflection. The quantity of light reflected by the second mirror will depend on the relative position of the two surfaces of reflection. It will be at the maximum if these surfaces are parallel, but otherwise if they are perpendicular; so that, by varying the relative position of the two mirrors to each other, we may either augment or diminish gradually the intensity of the reflected rays. Such is the property of the polarized light, which is utilized for making observations of the sun. To the eye-glass of the instrument are fixed two smooth mirrors, so adjusted as to make to the direction which the light follows an angle equal to the angle of polarization. One of these mirrors can turn round to the reflected rays. Then, by putting the surface of the second almost perpendicular to that of the first, we can observe the sun as easily as we can the moon, seeing it in its natural color, and we can regulate at will the intensity of the light. It is to this new arrangement of the eyeglasses that we owe the greater part of the discoveries of which I am about to speak to you. I ought to add, however, that in the astronomical glasses we employ not only two, but three and even four, of these reflections.

But to come to the consideration of the sun. Everybody knows that it has spots; that these spots, relatively very small, are of a black color, and also, that they adhere to the body of the sun. They move in a manner leading us to the conclusion that this luminary turns on its own axis in the space of twenty-five and a quarter days, and that its equator has an inclination of seven degrees and a half on the ecliptic. These spots are far from being constant. They undergo, on the contrary, the greatest changes

both of form and size. They show themselves particularly in some zones, and appear and disappear at very irregular periods. The maximum and the minimum are reproduced at intervals of about eleven years. One of the most curious discoveries of our times is, that this periodicity of the solar spots has some correspondence with terrestrial magnetism. It is impossible to discover the point at which the two classes of phenomena unite, but the existence of the fact is incontestable. Thus, we have just seen the spots pass through the minimum. From September, 1866, to March, 1867, there were scarcely any of them; and during the same period the magnetic perturbations have been very feeble. As soon as the existence of these spots had been fully ascertained, the questions naturally arose, What is the cause of them, and what their nature? On these points there have been numerous opinions, all as diverse as possible. This is not to be wondered at; for hitherto there has been no correct observation from which could be learned the character and the particulars of the phenomena we desire to explain. So, without stopping to discuss ancient theories, I am about to bring before you the latest observations, and the conclusions at which we have arrived. The drawings of the first observers represent the spots as formed with a black centre surrounded by a gray tint of a uniform figure, which is called penumbra. It is not surprising that, with such imperfect means of observation, the theory of the spots should remain so long uncertain, and that these phenomena should have been taken for simple clouds floating in the solar atmosphere. This theory, which was put forth by Galileo, has been revived in our day. The solar spots have an aspect completely different from that which we see in the ancient

consisting of a black centre, around which is a penumbra all ragged. The first thing you will observe is, that the figure of the penumbra is far from being uniform. It is composed of filaments, very long and very thin, which converge toward the centre. These have been called wisps of straw, willow-leaves, etc. I prefer to call them currents, being aware, at the same time, that it is impossible to compare them to any known thing. They are

more scattered near the outline of the penumbra, and they become condensed near the centre, where the light is stronger and brighter. These luminous threads start from the outline of the spot, traverse the penumbra, and often run into the black space that forms the centre, where we see them floating singly, gradually becoming smaller, and disappearing after a

while.

the instruments we employ to observe the sun, the detached parts of the spots often appear to us as microscopic objects. In order to form an exact idea of their real dimensions, we must always remember that, at this distance, four fifths of a second is equal to 140 kilometres, and consequently these apparent threads, whose seeming width is at most not more than one or two seconds, are in reality immense currents, being, about the middle, of 600 or 700 kilometres in width, while their length is at least equal to the diameter of the terrestrial globe.

The drawings which you have just seen represent some of these spots in their complete form and exactly defined. But they present themselves oftener under fantastic and irregular forms. They are sometimes accompanied by a kind of tail, itself formed of black spots, and which seems to

are most instructive, and most important in a theoretical point of view. We find the centre divided in several parts by the luminous threads. This appearance was remarked by the ancient as tronomers, who explained it by supposing that on the surface of the sun solid crusts were formed, which broke into shivers like glass under a blow from a stone. Modern observations, however, do not admit of this explanation. They show us clearly that these divisions are produced by currents which, leaving opposite edges, meet in the middle of the centre, and thus divide the spot into several parts.

The formation of a spot is never instantaneous. It is ordinarily announced by the appearance of several black points, and by a kind of diminution in the thickness of the lu

a great spot which was formed almost suddenly on the 30th of July, 1865. The day preceding that of its appearance, in observing the sun as usual, we had remarked only three little cavities, of which we noted the position. On the 30th of July, at midday, we found in the place of these cavities an enormous spot, the surface of which was equal to at least ten times the size of our globe. It was so mobile, and its form changed so constantly, that we could scarcely draw it. We could discover in it four principal centres, where the movement of the matter was visible in the form of a whirlwind. In an interval of 24 hours it had undergone some conside rable changes. On the 31st of July, the four centres were completely distinct, and the matter which separated them seemed as if it were stretched out.

During the days which followed, this form became more and more marked. Soon there were four spots clearly defined, which ultimately assumed the form of four independent craters or cavities. In the interior of these craters we perceived some light shadows, whose form reminded us of that of the clouds we call cirrus. Their color was different from that of the other part of the sun which presented itself to view. As the polariscopic eyeglass does not change the color of objects, we are enabled to see that these clouds are often of a very decided red; and, as this tint is clear

it precipitate itself in the obscure space, and there dissolve in much the same way as we see the vapor which forms the mist dissolve into thin air. All that we are required to believe is, that these apparently black masses are but rents made in the luminous veil which covers the solar body, and to which we give the name of photosphere. It is this bed which transmits light and heat to us. It is suspended in the solar atmosphere, just as clouds in the terrestrial atmosphere. What appear to us as spots in the sun is simply the effect of the rents which take place in it. We are

and well marked, it is impossible to confound it with the effects due to the achromatism of the instruments. You see here a great number of spots presenting this appearance, and especially in Figure 2, where the red shadows seem intertwined with the white shadows. I have more than once seen these luminous tongues, so to speak, transform themselves into red veils.

This hasty view is, however, so complete as to convince us that the spots cannot be compared to clouds, their aspect not warranting such a comparison. If any part of them may be compared to clouds, it is more the luminous matter; for we see

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confirmed in this view by the well-ascertained fact that the spots are depressions in the solar body, and that they have the form of a funnel. This form becomes very perceptible when the spots are drawn by the rotary movement toward the solar disk. When we examine a spot situated toward the centre of the sun, wefind that the shape of the penumbra is more regular. But when the spot. moves toward the edge, we see the penumbra diminish on the sideof the centre, and increase on the opposite side, in which case it presents the appearance of a cavity in the form of a funnel looked at obliquely. This effect is very clearly indica

ted 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 faculæ.

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 so

lution 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 atmosphe