[Footnote 154: This lecture was delivered by M. Secchi to the scholars of the school of Saint Genevieve, on the 28th of July last, at a scientific soirée, presided over by Mgr. Chigi. It occupied two hours in the delivery, during the whole of which time the lecturer held captive the attention of his distinguished audience, who testified their appreciation of its scientific and literary merits by warm applause. The lecture will speak for itself. But in publishing it, there is one thing which cannot be reproduced; that is, the deep interest which necessarily attaches to the hearing a learned man himself explain his experiments and his discoveries. A number of figures were necessary for the illustration of certain parts of the lecture; and these, prepared from M. Secchi's designs by M. Duboscq, optician, were projected on a screen, by the aid of the electric light, thus enabling the spectators to follow the learned astronomer with greater ease. Of these designs, etc., only the most essential have been given in the published lecture.]
Gentlemen: From the beginning of my stay in Paris, I was invited by persons to whom I owe great deference to lecture to you on some of the subjects which are studied at the Observatory of the Roman College. This invitation I felt to be in the nature of a command, which I would readily have obeyed long before, had I not been prevented by numerous and incessant cares. I cannot, however, leave France without discharging the debt; and it is for this purpose that we have met together, on the present occasion. I propose to speak to you of the sun, and to show you what science teaches us of its physical constitution. For eighteen years I have studied the sun, and observed all that passes over its surface. I hope, also, to interest you in acquainting you not only with the fruit of my own labors, but also with the discoveries of my learned contemporaries.
What is the sun? Such is the question which has been frequently asked me. I confess it has always perplexed me to reply to it. I should not be pardoned, perhaps, if I should say I know nothing of the matter; nevertheless, it is impossible for me to give a complete and satisfactory answer. You yourselves have addressed this question to me with an eagerness which I appreciate as a particular honor; and, in responding to your desire, I am going to place before you the very interesting results which we have obtained in the study of this luminary, to which, after God, its creator, we owe all the physical blessings we enjoy here below.
To deal with this vast subject in something like an orderly form, let us speak first of the new means of observation with which modern science has furnished us; after which we shall see what advantage we have derived from them, and in what way they have served to make us better acquainted with the sun.
Astronomers, gentlemen, are not privileged beings. Like simple mortals, they are dazzled by the sun. Far from sharing the penetrating sight which poets accord to the eagle, they cannot fix their gaze on the bright orb of day without exposing their eyes to the greatest danger; and this danger becomes more serious if they employ their instruments for this purpose without taking proper precautions. Until recently, two means have been employed to protect the eyes of the observer: first, the reduction of the objective aperture of the glasses; and second, providing strongly-colored glasses. These two expedients present the most serious inconveniences. The first deprives the observer of the advantages which he would gain from the large apertures, and the confusion of the image is greatly augmented by the diffraction which the small diaphragms cause the light to undergo; while the second will not permit of our distinguishing the different colors which may meet in the sun; and on this account the observer is liable to fall into very grievous errors. The means now in use effectually obviate this double inconvenience, inasmuch as they allow of the use of the entire aperture of the glasses, and leave to the different parts of the sun their natural color. The first means consists of the employment of the reflective glass. A rectangular prism of crystal is disposed in such a manner as that its hypothenuse has an inclination of 45 degrees on the axis of the glass. The light, on reaching the surface, divides itself into two very unequal parts. The reflected rays are rather feeble, but of sufficient brightness to make them pass through a glass faintly colored, falling perpendicularly on one of the faces of the prism, without reaching the eye of the observer. The colored glass, not having to sustain so high a temperature, is not so liable to break, as often happened in the old method.
If the colored glass is completely done away with, we shall succeed by adopting a method which rests on the properties of polarized light. When the light is reflected by a glass mirror under an angle of 35 degrees 25 minutes, it undergoes a modification which is called polarization. If the rays thus polarized are received on a second glass mirror under the same inclination of 35 degrees 25 minutes, they will divide into two parts, one 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 eye-glasses 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 cuts. I am going to show the drawing of several of them as observed at the Roman College. I designed them myself, by a very rapid process, such a process being very important for objects essentially variable, and which change their form with great rapidity, and in a short space of time. Here is, first, one of the most common forms. (Figure 1.)
Figure 1.
It is a round spot, consisting of a black centre, around which is a penumbra all ragged. The first thing you wall 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.