As the fixed stars will appear to our observer of different degrees of magnitude and splendor, he will divide them into different classes. Those which seem the largest and brightest, he will call stars of the first magnitude; the smallest that we can see with the naked eye are called stars of the sixth magnitude; and the intermediate ones, according to their different apparent sizes, he will call of the second, third, fourth, or fifth magnitudes. Those stars which cannot be seen without the assistance of a telescope, are not reckoned in any of these classes, and are called telescopic stars.

By a knowledge of the fixed stars and their positions, our observer will obtain so many fixed points, by which he may observe the motions of the planets and the relation of these motions to each other; he will use them as so many land-marks, if the word may be allowed, by which the situations of other celestial bodies may be ascertained, and the varieties to which they are, subject be observed. For, from the same place, the motions of the heavenly bodies can only be estimated by the angle formed at the spectator's eye by the space which the moving body passes over.

To measure the spaces, the stars must be used, and considered as so many luminous points fixed in the concavity of a sphere, whose radius is indefinite, and of which the observer's eye is the centre. We may learn from hence the necessity of forming an exact catalogue of stars, and of determining their

positions with accuracy and care. With such a catalogue the science of astronomy begins.

Although, to those who are unacquainted with the nature of celestial observation, it might at first sight appear almost impossible to number the stars; yet their relative situations have been so carefully ob served by astronomers, that they have not only been numbered, but even their places in the heavens have been ascertained with greater accuracy than the relative situations of most places on the surface of the earth.

But

The greatest number of stars that are visible to the naked eye, are to be seen on a winter's night, when the air is clear, and no moon appears. even then a good eye can scarce distinguish more than one thousand at a time in the visible hemisphere: for, though, on such a night, they appear to be almost innumerable, this appearance is a deception, that arises from our viewing them in a transient and confused manner; whereas, if we view them distinctly, and only consider a small portion of the heavens at a time, and, after some attention to the situation of the remarkable stars contained in that portion, begin to count, we shall be surprized at the smallness of their number and the ease with which they may be enumerated,

The number of the ancient constellations was 48; in these were included 1022 stars. Many constellations have been added by modern astronomers; so that the catalogue of Flamsteed and De la Caille, when added together, are found to contain near five

thousand stars. The names of the constellations, their situation in the heavens, with other particulars, are best learned by studying the artificial representation of the heavens, a modern celestial globe.

concave.

The Galaxy or milky way must not be neglected; it is one of the most remarkable appearances in the heavens; it is a broad circle of a whitish hue, in some places it is double, but for the most part consists of a single path surrounding the whole celestial The great Galileo discovered by the telescope, that the portion of the heavens which this circle passes through was every where filled with an infinite multitude of exceeding small stars, too small to be discovered by the naked eye, but by the combination of their light, diffusing a shining whiteness through the heavens. Mr. Brydone says, that when he was at the top of Mount Etna, the milky way had the most beautiful effect, appearing like a pure flame that shot across the heavens.

The stars appear of a sensible magnitude to the naked eye, because the retina is not only affected by the rays of light which are emitted directly from them, but by many thousands more, which, falling upon our eye-lashes, and upon the visible aerial particles about us, are reflected into our eyes so strongly, as to excite vibrations, not only in those points of the retina where the real images of the stars are formed, but also in the other parts round about it. This makes us imagine the stars to be much bigger, than they would be if we saw them only by the few rays which come directly from them to our eyes,

without being intermixed with others. Any one may be made sensible of this, by looking at a star of the first magnitude through a long narrow tube; which, though it takes in as much of the sky as would hold a thousand of such stars, scarce renders that one visible.

The number of the stars almost infinitely exceeds what we have yet been speaking of. An ordinary telescope will discover, in several parts of the heavens, ten times as many stars as are visible to the naked eye, Hooke in his Micrographia says, that with a telescope of twelve feet he discovered seventyeight stars among the Pleiades, and with a more perfect telescope many more. Galileo reckoned eighty in the space between the belt and the sword of Orion, and above five hundred more in another part of the same constellation, within the compass of one or two degrees square. Antonia Maria de Rheita counted in the same constellation above ten thousand stars. Future improvements in the telescopes may enable us to discover numberless stars, that are now invisible; and many more may be which are too remote to be seen through. telescopes, even when they have received their ultimate improvement. Dr. Herschel, to whose ingenuity and assiduity the astronomical world is so much indebted, and whose enthusiastic ardor has revived the spirit of discoveries, of which we shall speak more largely in another part of this essay, has evinced what may be effected by improvements in the instruments of observation. In speaking here of his discoveries, I shall use the words of

M. De la Lande.* "In passing rapidly over the heavens with his new telescope, the universe inereased under his eye; 44000 stars, seen in the space of a few degrees, seemed to indicate, that there were seventy-five millions in the heavens." He has also shewn that many stars, which to the eye or through ordinary glasses appear single, do in fact consist of two or more stars. The Galaxy or milky way owes its light entirely to the multitude of small stars, placed so close as not to be discoverable even by an ordinary telescope. The nebulæ, or small whitish specks, discerned by means of telescopes, owe their origin to the same cause; former astrologers could only reckon 103, Dr. Herschel has discovered upwards of 1250 of these clusters, besides a species which he calls planetary nebula. But what are all those, when compared to those that fill the whole expanse, the boundless fields of ether! Indeed, the immensity of the world must contain such numbers, as would exceed the utmost stretch of the human imagination: for who can say, how far the universe extends, or where are the limits of it? where the Creator stayed "his rapid wheels;" or where he "fixed the golden compasses?"

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OF THE PLANETS, AS SEEN FROM THE SUN. Our solar observer having attained a competent knowledge of the fixed stars, will now apply him

* Memoires de l'Academie de Dijon, 1785.

In all the larger sort of telescopes, the apparent number of stars is found to be encreased, as the aperture of the tube is augmented. EDIT.