Owing to the annual motion of the Earth, the Sun appears to move among the zodiacal constellations from West to East. Hence it is that, while the whole dome of the fixed stars seems to move round the Earth from East to West, in consequence of the Earth's diurnal, motion, the stars appear to rise about 3′ 56′′ sooner every evening, and thus seem to gain nearly one whole revolution over the Sun in a twelvemonth, which is due to the Sun having finished its circular course through the ecliptic. The Earth completes one revolution on its axis in twenty-three hours and fifty-six minutes, which is termed a sidereal day, being the time which elapses from any star coming upon the meridian till its arriving there again; this would, therefore, be the length of the day, were the Earth stationary; but, as we said before in other words, the Earth advances nearly one degree in its orbit in one day, or the Sun seems to move this space in the ecliptic.

When the Moon, in her passage round the Earth, passes over any of the fixed stars or planets, she is said to occult, or hide them; and such a phenomenon is termed an occultation, an observation of which is useful in determining longitude.

One of the latest opinions respecting the cause of shooting stars is, that they are fragments of planets, which in the course of their revolutions come, at certain times, within visible range of the Earth; and that the Earth, when at that part of her orbit where she is brought by the middle of November in each year, is in that point of space, which is nearest to the places of such revolutions.

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CHAPTER XIII.

THE REFLECTING TELESCOPE. HERSCHEL'S DISCOVERIES THEREWITH. THE TELESCOPE MADE BY NEWTON. CONCLUSION.

CONTEMPLATION OF THE UNIVERSE.

HAVING had occasion, more than once, to refer to the discoveries made by Sir William Herschel, in the celestial regions, we cannot altogether dismiss the subject, without describing the stupendous and wonderful instrument, which he himself constructed, and with which he enriched the modern state of astronomical knowledge. The magnifying powers of this instrument exceeded all that had ever been known, previously or since.

We will suppose the reader to be at least moderately acquainted with the principles and use of the common refracting telescope: he will therefore observe that the subjoined figure affords a representation of Herschel's REFLECTING Telescope:

Fig. 51.

B

F

Let us suppose A B to be a concave reflector, and r A, B to be rays proceeding from a distant object, such as a star. The reflector A B is not placed symmetrically across the tube, but has such a degree of inclina

tion given to it, as will bring the rays to a focus at F, near the margin of the mouth of the tube. The eye of the observer being now placed at that point, he will see the image formed by the focalization of the rays. The object to be obtained in thus bringing the rays to a focus at the margin of the tube, instead of at the centre of its diameter, is, that the body and head of the observer may intercept as few rays as possible; for it will be seen that it is quite impossible for him to look into the tube, without obstructing the passage of some of the light proceeding from the object. But this obstruction is thus limited as much as possible.

This telescope was begun by Sir William in the year 1785, by the pecuniary aid afforded to him by King George the Third, who was his patron. Herschel superintended the whole of the construction of it himself; and he had as many as forty workmen employed upon it at once. In February, 1787, he had the first view through this telescope, although it was not completely finished till August, 1789.

The great reflector, A B, was forty-eight inches in diameter of polished surface. A speaking-trumpet was fitted to the side of the telescope, for the observer to communicate the result of his observations to an assistant below, who was stationed in a small house, and provided with various instruments for measuring time, the positions of the stars, &c. This vast tube was moved about in any direction, by means of a large and complicated apparatus. The eye-pieces, through which the observer viewed the reflected image of an object, were held at the mouth of the tube by a sliding apparatus, by which the attention could be directed to any particular part of the speculum, a B.

The frame-work of this instrument having been found to be greatly decayed, after it had remained thirty years in its place, the whole was taken down and replaced by a smaller one, on the like construction, by Herschel's son, one of the most distinguished astronomers of modern days.

No sooner had the great telescope been erected by Sir William Herschel, than he began an important series of discoveries by its means. He discovered the sixth satellite of Saturn (five only having been previously known,) on the day that this instrument was completed. He had already discovered the planet Uranus, as before related; a circumstance, such as had never taken place since the days of the most ancient nations of the world. Moreover, by the aid of the new telescope, Herschel distinguished, one by one, six luminaries revolving about it; as also the seventh satellite of Saturn. The same able astronomer also discovered that the ring which surrounds the planet Saturn, is not only divided into concentric belts by a dark circle, as had been already noticed by Cassini, but that the ring likewise revolved about the body of the planet.

The art of constructing telescopes, has, like all other arts, advanced by degrees to its present state of comparative perfection. In the case of the reflecting telescope, which is, for astronomical purposes, a vast and efficacious improvement upon the former mode of using these instruments, there have been several sorts at different times constructed. These go by the names of the Newtonian, the Gregorian, the Cassegrainian, Brewster's, and Herschel's. The principle of them all is the same but the circumstances in which they differ from one another, are chiefly in the manner in

S

which the image of an object is conveyed to the eye of an observer. In every instance, rays of light from the celestial body pass into the tube of the telescope (which is open at the end nearest to the object,) and fall upon the surface of the speculum at the bottom of the tube, as shown in the preceding figure. These rays, after reflection from the surface of the speculum, meet again in a focus near the upper end of the tube, and there form a small image of the object; which image is conveyed to the eye of the observer by one of the five different contrivances, which thus form the points of difference between the various reflecting telescopes.

Of Herschel's telescope we have already spoken. Into the merits of the telescopes of Gregory, Cassegrain, and Brewster, it is not necessary here to enter; but we cannot refrain from giving a few brief notices of Newton's genius as developed in the construction of a reflecting telescope, for exploring those celestial tracts, which his skill and perspicacity were employed in reducing to order and harmony, for the better understanding of those who were to come after him.

It has been well remarked of Sir Isaac Newton, that it was his brilliant fortune to improve almost every subject to which he directed his gigantic mind. He was the first to put in practice the idea of employing reflected light for the formation of the image in a telescope. Vast and splendid as were the theoretical and mathematical powers of his mind, yet he did not disdain to construct with his own hands the instrument which his inductive reasoning told him would serve the purpose of a telescope. He, accordingly, made the first reflecting telescope which the world ever saw, and which is now in the museum of the Royal Society of