of the appearance, position, and mutual distances of the individual stars composing the system, in order that subsequent observers may have the means of detecting their con nected motions, or any changes which they may exhibit. Professor Struve has also taken notice of 52 triple stars, among which No. 11 of the Unicorn, Zeta of Cancer, and Zi of the Balance, appear to be ternary systems in motion. Quadruple and quintuple stars have likewise been observed, which also appear to revolve about a common center of gravity; in short, every region of the heavens furnishes examples of these curious phe. nomena. COLOR OF THE STARS. 248. Many of the double stars exhibit the curious and beautiful phenomenon of contrasted colors, or complimentary tints. In such instances, the larger star is usually of a ruddy or orange hue, while the smaller one appears blue or green, probably in virtue of that general law of optics, which provides that when the retina is under the influence of excitement by any bright colored light, feebler lights, which, seen alone, would produce no sensation but that of whiteness, shall for the time appear colored with the tint complimentary to that of the brighter. Thus, a yellow color predominating in the light of the brighter star, that of the less bright one, in the same field of view, will appear blue; while, if the tint of the brighter star verge to crimson, that of the other will exhibit a tendency to green-or even appear a vivid green. The former contrast is beautifully exhibited by Iota, in Cancer; the latter by Almaack, in Andromeda-both fine double stars. If, however, the colored star be much the less bright of the two, it will not materially affect the other. Thus, for instance, Eta Cassiopeiæ exhibits the beautiful combination of a large white star, and a small one of a rich ruddy purple. 249. It is not easy to conceive what variety of illumination two suns—a red and a green, or a yellow and a blue one-must afford to a planet revolving about either; and what charming contrasts and grateful vicissitudes-a red and a green day, for instance, alternating with a white one and with darkness—might arise from the presence or absence of one or the other, or both, above the horizon. Insulated stars of a red color, almost as deep as that of blood, occur in many parts of the heavens, but no green or blue star (of any decided hue) has, we believe, ever beer. noticed, unassociated with a companion brighter than itself. CLUSTERS OF STARS. 250. When we cast our eyes over the concave surface of the heavens in a clear night, we do not fail to observe that there are, here and there, groups of stars which seem to be compressed together more densely than those in the neighboring parts; forming bright patches or clusters. ogues! Their object? What triple stars? Ternary systems? Quadruple stars, & ? 248. What said of the colors of the stars? What law of optics referred to? What illus 'rations? 249. What remarks respecting red and green suns, &c.? Of insulated star? of a red colorf 250. What said of clusters? What specimen referred to ? Pleiades? The Pleiades are an instance of this kind, in which six or seven stars may be seen in near proximity, by the naked eye; and even more if the eye be turned carelessly upon it; for it is a remarkable fact that the center of the eye is far less sensible to feeble impressions of light, than the exterior portion of the retina. Rheita affirms that by the aid of a telescope he counted over 200 stars in this small cluster. See Map VIII., Fig. 28. In the constellation called Coma Berenices there is another group more diffused, and consisting of much larger stars. In Cancer there is a nebulous cluster of very minute stars, called Præsepe, or the Beehive, which is sufficiently luminous to be seen by the naked eye, in the absence of the moon, and which any ordinary spyglass will resolve into separate stars. In the sword-handle of Perseus, also, is another such spot, crowded with stars. It requires, however, rather a better telescope to resolve it into individual stars. See p. 65, and Map VIII., Fig. 39. Whatever be the nature of these clusters, it is certain that other laws of aggregation prevail in them, than those which have determined the scattering of stars over the general surface of the sky. Many of them, indeed, are of an exactly round figure, and convey the idea of a globular space filled full of stars, and constituting, in itself, a family or society apart, and subject only to its own internal laws. "It would be a vain task," says the younger Herschel, "to attempt to count the stars in one of these globular clusters. They are not to be reckoned by hundreds; for it would appear that many clusters of this description must contain, at least, ten or twenty thou and stars, compacted and wedged together in a round space, not more than a tenth part as lave as that which is covered by the moon. NEBULE. 251. The Nebula, so called from their dim, cloudy appearance, form another class of objects which furnish matter for curious speculation and conjecture respecting the formation and struc ture of the sidereal heavens. When examined with a telescope of moderate powers, the greater part of the nebulæ are tinctly perceived to be composed of little stars, imperceptible to the naked eye, because, on account of their apparent proximity, the rays of light proceeding from each are blended together, in such a manner as to produce only a confused luminous appear ance. In other nebulæ, however, no individual stars can be perceived, even through the best telescopes; and the nebulæ exhibit only the appearance of a self-luminous phosphorescent patch of gaseous vapor, though it is possible that even in this case, the appearance may be owing to a congeries of stars so minute, or so distant, as not to afford, singly, sufficient light to make an impression on the eye. Remarks upon their nature and the laws that govern them? Remarks of Herschel? 251. What are nebula, and why so called? How appear through telescopes? Are they all resolvable into stars? 252. One of the most remarkable nebula is in the swordhandle of Orion. It is formed of little flocky masses, like wisps of cloud, which seem to adhere to many small stars at its outskirts. It is not very unlike the mottling of the sun's disc, but of a coarser grain, and with darker intervals. These wisps of light, however, present no appearance of being composed of small stars; but in the intervals between them, we fancy that we see stars, or that, could we strain our sight a little more, we should see them. These intervals may be compared to openings in the firmament, through which, as through a window, we seem to get a glimpse of other heavens, and brighter regions, beyond. See page 45, and Map VIII., Fig. 32. 253. Another very remarkable nebula is that in the girdle of Andromeda, which, on account of its being visible to the naked eye, has been known since the earliest ages of astronomy. It is often mistaken for a comet, by those unacquainted with the heavens. See page 20, and Map VIII., Fig. 22. Marius, who noticed it in 1612, describes its appearance as that of a candle shining through horn; and the resemblance is certainly very striking. Its form is a long oval, increasing, by insensible gradations of brightness, from the circumference to a central point, which, though very much brighter than the rest, is not a star, but only a nebula in a high state of condensation. It occupies an area comparatively large-equal to that of the moon in quadrature. This nebula may be considered as a type, on a large scale, of a very numerous class of nebulæ, of a round or oval figure, increasing more or less in density toward the center. 254. Annular nebula are those in the form of a ring, but are among the rarest objects in the heavens. The most conspicuous of this class is to be found exactly half-way between the stars Beta and Gamma Lyræ, and may be seen with a telescope of moderate power. It is small, and particularly well defined, appearing like a flat oval ring. The central opening is not entirely dark, but is filled with a faint, hazy light, uniformly spread over it, like a fine gauze stretched over a hoop. 255. Planetary nebula are very extraordinary objects. They have, as their name imports, the appearance of planets, with round or slightly oval discs, somewhat mottled, but approaching, in some instances, to the vividness of actual planets. Some of them, upon the supposition that they are equally distant from us with the stars, must be of enormous magnitude. That one, for instance, which is situated in the left hand of Aquarius, must 252. What remarkable nebula mentioned? Describe it? Point out on the map. 253. What other? How long known, and why? Show on the map. How described by Marius? Its form and extent? How considered? 254. What are Annular Nebula? ace they common? What specimen referred to? 255. Planetary nebule? Their 3haracter and magnitule? Specimen? Stellar nebulæ ? General remarks respecting have a volume vast enough, upon the lowest computation, to fill the whole orbit of Herschel ! In some instances a nebula presents the appearance of a faint, luminous atmosphere, of a circular form, and of large extent, surrounding a central star of considerable brilliancy. These are denominated Stellar Nebula. The nebulæ furnish an inexhaustible field of speculation and conjecture. That by far the larger number of them consists of stars, there can be little doubt; and in the interminable range of system upon system, and firmament upon firmament, which we thus catch a glimpse of, the imagination is bewildered and lost. Sir William Herschel conjectured that the nebulae might form the material out of which nature elaborated new suns and systems, or replenished the wasted light of older ones. But the little we know of the physical constitution of these sidereal masses, is altogether insufficient to warrant such a conclusion. (For a Spiral Nebula recently discovered by Lord Rosse, see Map IX. Fig. 68.) CHAPTER XIV. VIA LACTEA (THE MILKY-WAY). "Throughout the Galaxy's extended line, Unnumber'd orbs in gay confusion shine: Where every star that gilds the gloom of night With the strong influence of a radiant sun."-Mrs. Carter. 256. THE VIA LACTEA, or Milky-Way, is that luminous zone or pathway of singular whiteness, varying from 4° to 20° in width, which passes quite around the heavens. The Greeks called it GALAXY, on account of its color and appearance: the Latins, for the same reason, called it VIA LACTEA, which, in our tongue, is Milky Way. Of all the objects which the heavens exhibit to our view, this fills the mind with the most indescribable grandeur and amazement. When we consider what unnumbered millions of mighty suns compose this stupendous girdle, whose distance is so vast that the strongest telescope can hardly separate their mingled twilight into distinct specks, and that the most contiguous of any two of them may be as far asunder as our sun is from them, we fall as far short of adequate language to express our ideas of such immen. uity, as we do of instruments to measure its boundaries. 257. It is one of the achievements of astronomy that has resolved the Milky-Way into an infinite number of small stars, whose confused and feeble luster occasions that peculiar whiteness which we see in a clear evening, when the moon is absent. It is also a recent and well-accredited doctrine of astronomy, 256. What is the Via Lactea? the Nebula? Sir Wm. Herschel's conjecture? Its Greek name? What said of its magnificence and grandeur? 257. What said of the achievements of astronomy? Its doctrine respecting the structure of the arivers? Of the sun, and its relation to the fixed stars? that all the stars in the universe are arranged into clusters, oi groups, which are called NEBULE or STARRY SYSTEMS, each of which consists of myriads of stars. The fixed star which we call OUR SUN, belongs, it is said, to that extensive nebula, the Milky Way; and although apparently at such an inmeasurable distance from its felicws is, doubtless, as near to any one of them, as they are to one another. 258. Of the number and economy of the stars which compose this group, we have very little exact knowledge. Dr. Herschel informs us that, with his best glasses, he saw and counted 588 stars in a single spot, without moving his telescope; and as the gradual motion of the earth carried these out of view and introduced others successively in their places, while he kept his telescope steadily fixed to one point, "there passed over his field of vision, in the space of one quarter of an hour, no lest than one hundred and sixteen thousand stars, and at another time, in forty-one minutes, no less than two hundred and fifty-eight thousand." In all parts of the Milky-Way he found the stars unequally dispersed, and appearing to arrange themselves into separate clusters. In the small space for example, between Beta and Sad'r, in Cygni, the stars seem to be clustering in two divisions; each division conta ning upwards of one hundred and sixty-five thousand stars. At other observations, when examining a section of the Milky-Way, not apparently more than a yard in breadth, and six in length, he discovered fifty thousand stars, large enough to be distinctly counted; and he suspected twice as many more, which, for want of sufficient light in his telescope, he saw only now and then. 259. It appears from numerous observations, that various changes are taking place among the nebula-that several nebulæ are formed by the disolution of larger ones, and that many nebulæ of this kind are at present detaching themselves from the Milky-Way. In that part of it which is in the body of Scorpio, there is a large opening, about 4° broad, almost desti tute of stars. These changes seem to indicate that mighty movements and vast operations are continually going on in the distant regions of the universe, upon a scale of magnitude and grandeur which baffles the human understanding. More than two thousand five hundred nebulæ have already been observed; and, if cach of them contains as many stars as the Milky-Way, several hundreds of millions of stars must exist, even within that portion of the heavens which lies open to our obser. vation. "O what a confluence of ethereal fires. From urns unnumber'd down the steep of heaven 260. Although the Milky-Way is more or less visible at all seasons of the year, yet it is seen to the best advantage during 258. Number and economy of the stars? Dr. Herschel's statements? What number passed the field of his instrument in a quarter of an hour? In forty-one minutes? In space apparently only a yard in breadth? 259. What changes observed in the nebuWhat do they indicate? Number of nebulæ? Estimated number of stars? 260. When is the Via Lactea scen to the best advantage? Direction when Lyra je mu bę |