still a very appreciable diameter.

As to the intensity

of its heat and light, though reduced by a thousand times, it would still surpass to an enormous extent that of the most brilliant stars, for the light of the Sun illuminates the Earth with a force equal to that of twenty-two thousand millions of stars similar to the star Alpha Centauri, which is of the first magnitude (J. Herschel).

If it continued to travel still farther from us and penetrated beyond the limits of the planetary world which it warms and lights until it arrived at a distance comparable to that of the nearest fixed star, its apparent diameter would then be reduced to less than 0"01, and the Sun would be almost imperceptible to us. The most perfect micrometer in the best of telescopes would not enable us to measure its dimensions. At a distance corresponding to a parallel of 1", which is equivalent to 206,000 times its actual distance from the Earth, the extinction of the Sun's light would be such that it would appear to us, at most, like a star of the first or second magnitude. Its brilliancy diminished according to the square of the distance would be reduced the 42,500,000,000th part of its actual value: this would make it about half as brilliant as the star Alpha Centauri. The brilliancy of Sirius is estimated to be about four times that of the latter; so that were our Sun transported to the distance of the nearest fixed stars, it would appear eight times less brilliant than Sirius.

L

What would it be if it were removed to the distance of Sirius itself, which is about six times as considerable as that which we have just mentioned? The extinction of its light would be of course 36 times greater, and in fact Sir John Herschel considers the intrinsic brilliancy of the star Sirius to be 225 times greater than that of the Sun.

Stellar photometry, or the comparative study of the luminous intensity of the stars, is still in a very imperfect state, and the numbers we have just given do not possess a very great degree of precision. They suffice, nevertheless, to render most evident the fact that our Sun in the sidereal world would only figure as a star- a star of ordinary size. On the other hand, the above considerations oblige us to consider the stars as independent sources of light. tance at which the nearest of them shine renders it impossible that they can be lighted up by borrowed or reflected light. They are Suns like our Sun, so that we can say, if we please, that the Sun is a star or that the stars are Suns.

The enormous dis

These views, which throw so much light on the constitution of the universe, have been recently confirmed by comparing stellar light with that of our Sun. The method known to physicists as "spectral analysis" enables us to classify the various sources of light according to the nature of the spectrum which they give when this light is decomposed by means of a

prism. We shall see further on what is the signification of those dark lines which are visible in the solar spectrum; how they point to the presence of certain elementary bodies, metals or metalloids, which exist in the atmosphere of the Sun in the state of incandescent vapour.

Now when the spectra of the stars are studied in the same manner- -about sixty of the more brilliant only have as yet been submitted to this kind of examination—we find a certain similarity of constitution existing among them all, and a certain resemblance to our Sun. The dark rays which are seen in their spectra indicate that starlight passes also through an absorbing layer of incandescent metallic vapour before issuing into the depths of space; but the chemical substances which characterize each of them varies from one to the other: sodium and magnesium exist in a great number; in others we find hydrogen, iron, bismuth, mercury, &c.

There exists doubtless in these distant worlds an infinite variety. Their real dimensions, the intrinsic brilliancy of their light, their colours, their chemical nature, and the number of substances of which they are composed, necessarily vary exceedingly. But one fact remains certain, namely, the great analogy which connects them with our solar world. Does there exist around each star, as around the Sun, a planetary system, with planets, satellites, and comets? This is indeed highly

probable, but their distance from us is too enormous to allow us to indulge in the hope of ever verifying such a conjecture by means of direct observation. It is quite certain that an observer placed upon any one of these stars which shine in the depths of the firmament, and looking in the direction of our world-Sun, planets, satellites, &c.—would see it like a single star, in the midst of a multitude of other stellar points.

§ 2. THE SUN IS A STAR IN THE MILKY WAY.

What is the position of the Sun in the world of stars-The Milky Way; its form and constitution-The Sun is a Star in the Milky Way; its position in the Nebula, according to Herschel.

of

What is the position of the Sun in the sidereal universe? As it is one of the stars which comprise this universe, has it not some connection with any the others, with regard to its situation and its movements? Does it not form part of some of those groups of stars which the telescope reveals here and there at infinite distances in the depths of space?

It is certain that a considerable number of stars

are not completely isolated. Among those which appear single to the naked eye, there are thousands which show themselves as double or triple stars when seen with a telescope. And this is not often the mere effect of perspective, for in many of the double stars it

has been found that one moves or revolves round the other. Moreover, we know that a great number of those little celestial clouds, called nebula, are nothing else than an agglomeration of multitudes of stars whose enormous distance from us causes them to appear condensed into a comparatively small space. Is our Sun

not one of the stars of such a group?

Since the time of William Herschel science has replied to this question in the affirmative.

That great

astronomer, that laborious and sagacious observer, demonstrated a fact which had already been hinted at by Kent, by Lambert, and by several other philosophers : namely, that the Sun forms part of that immense stellar agglomeration known as the Milky Way.

That great zone, indeed, is seen spread over the background of the heavens nearly in the form of a great circle of the starry sphere; if we put aside certain inequalities in its form, and certain inequalities in its width at various points of its periphery: it divides the sky into two portions, which are not absolutely of the same extent, the smallest half being that which contains the constellations Pisces and Cetus, that is, the constellations near the equinoxial point of spring.

Thus, already, it is evident from the general appearance presented by the Milky Way that it everywhere surrounds the spot which our world, and consequently the Sun, occupies in space.

To the naked eye, and to the eye armed with a