Their presence in the field of view is announced by a dawn like that of morning, and the astronomer who would keep his sight so delicate as to perceive small bodies must protect his eyes from this light. For this purpose the larger star is sometimes hidden by a fine needle introduced into the focus of the eye-piece of the micrometer. The needle, which looks like a black bar, is so placed that the star comes into the field of view behind it, and leaves the eye undazzled and able to observe small objects in the vicinity. The large fixed stars, though they have no discs, require as broad a bar to hide them as a planet, owing to their extreme brilliancy.

Except in that part of the sky which is very near the sun even moderately bright stars may be seen at noon-day through the telescopes. Very bright stars may even be discerned without a glass from the bottom of a well or through the shaft of a tall chimney or a mine.

§ 69. I cannot better close this account of the obstacles which practical astronomers encounter, than by a description of the manner in which they have been triumphantly overcome by Lord Rosse in the construction of his mammoth telescope. Lord Rosse commenced his labors twenty years ago. He began by attempting the improvement of the refracting telescope, but soon gave preference to the reflector. He endeavored to produce the true parabolic speculum which should be free from aberration. The exceeding delicacy needful in producing this form with mathematical accuracy may be judged from the fact, that if two specula of six feet in diameter, the one spherical and the other parabolic, were pressed into contact at the centre, the edges would not diverge from each other more than the thousandth part of an inch. He invented a grinding and polishing machine, and after repeated trials obtained the means of furnishing specula from one to six feet in diameter.

§ 70. A difficulty no less formidable impeded his operations, the casting a speculum of sufficient size and strength. After repeated trials he made one of three feet in diameter, cast in sixteen separate portions. By the experience he acquired in making this he became acquainted

with the method of casting a large speculum in a single piece. Several tormenting difficulties attended his first efforts. Small air-holes were formed in the metal, and the speculum cracked in cooling. A mould of sand, and subsequently a mould of cast-iron, failed in giving freedom from pores. The desideratum was a kind of mould which should retain the molten metal, and yet allow the air globules to escape. Such was at length discovered, and stamped Lord Rosse's name with celebrity, reducing as it does the casting of specula to a certainty. The contrivance consisted in making the bottom of the mould of layers of hoop iron, bound closely together, with the edges uppermost. The iron conducted the heat away through the bottom so as to cool the metal towards the top, while the interstices between the hoops, though close enough to prevent the metal from running out, were sufficiently open to allow the air to escape.

The first large speculum thus made in a single piece was a round plate of metal, three feet in diameter, nine inches thick, and upwards of a ton in weight. In a few minutes the metal set in a compact form, and while intensely hot, was conveyed by a railway to an annealing oven, a few feet distant from the foundry. The oven was nearly red hot when the speculum was shut up in it, and from this temperature it was allowed many weeks to become gradually cool. It was then ground to the proper parabolic curve, and polished.

§ 71. It was ascertained in the following manner when the proper parabolic curve was produced. A high tower was erected immediately over the speculum. On a pole at the top of the tower, ninety feet distant from the speculum, the dial-plate of a watch was placed, forming a small round object relieved against the sky. The reflection of the watch was seen by an eye-piece at the right focal distance. When it became perfectly distinct, the mirror had received its proper concavity. It was then placed in its box, lined with felt and pitch, so as to prevent any sudden change of temperature from greatly affecting its figure. It is singular that a nearly similar mode was devised for the bedding of specula by Lord Rosse in Ireland and by Sir John Her

schel at the Cape of Good Hope. Sir John Herschel found that a speculum supported on three metalic points in the circumference made the image of every considerable star triangular, and that a packthread stretched down the back of the mirror for support distorted the images of the stars to a preposterous extent. He employed a great many thicknesses of blanket to prevent the effect of flexure in the wooden back of the case. To keep the elasticity of the fibre the blanket must be often shaken.

§ 72. The speculum so fortunately completed by Lord Rosse was fixed or bedded on three iron plates, which gave it support, and then transferred to its appointed situation in the tube. This is three feet in diameter, and thirty feet long, and attached to an apparatus on the lawn, by which it can be brought to bear on any point of the sky from a short way above the horizon. The machinery for moving it round and raising and depressing it is simple and ingenious; and notwithstanding its size, it may be adjusted with the greatest ease. Two step-ladders form part of the apparatus, and by these we mount to a gallery, which can be raised or lowered to any required height. In order to procure an observation, the tube is first brought to bear on the star or other object, and the gallery being raised, we ascend to it by one of the ladders. On reaching the gallery, which is a small railed platform sufficient to hold several persons, we find ourselves close to the telescope, near its upper extremity; and here, on looking through a small eye-piece fixed to the tube, we at once recognise, in the obliquely-placed mirror within, the object of our observation. The tube is of wood, hooped with iron. The mouth of the tube remains permanently open. The telescope is lowered in wet weather, and the speculum is confined in a case, the cover of which is withdrawn by an exterior action when required. A vessel of quick-lime is also kept constantly in the case, for the purpose of absorbing the moisture and acid vapors, by which the speculum might be tarnished.

§ 73. The eye-pieces used with this telescope_range from 180 to 2,000 times the power of the naked eye. Unless the atmosphere be exceedingly clear, a powerful

eye-piece will magnify the globules of watery vapor, and form a haze. Different densities from contending streams of cold and warm air have a similar effect; and if the atmosphere be exceedingly cold, as in a Russian winter, floating spicula of ice, invisible to the naked eye, are magnified so as to interrupt perfect observation.

Sir John Herschel found that the excessive heat and dryness of the sandy plains at the Cape often destroyed distinct vision, and that in a very singular manner. In some cases the images of the stars are violently dilated, and converted into nebulous belts or puffs of 10" or 15" or more in diameter. In others they form soft, quiet, round pellets of 3" or 4" in diameter, very unlike the spurious discs which they present when best defined, and rather resembling planetary nebulæ. Sometimes the structure as it were of these pellets is disclosed, and they are seen to arise from an infinitely rapid vibratory motion of the central point in all possible directions, while on a few occasions the appearances are exceedingly perplexing and singular. Some of the phenomena evidently have reference to the state of the air in the tube of the telescope; the tube of a reflector being necessarily open at the mouth, ascending and descending currents of hot and cold air, usually rotating spirally, are established, and are very prejudicial to distinct vision. The remedy is to dispense with a tube altogether, substituting for it a light, strong, inflexible, iron frame work. In refracting telescopes, where the air is completely inclosed, its circulation is not nearly so injurious.

§ 74. The performances of Lord Rosse's telescope were found to be far beyond those of any previously constructed instrument. But Lord Rosse considered that something still grander could be achieved; and before the above telescope was well finished, he projected one of the extraordinary dimensions of six feet diameter in the speculum, with a tube of sixty feet long. The casting, grinding, polishing and mounting of this monster speculum were pretty nearly a repetition, on a larger scale, of what had been previously done. Its focal length is fifty-three feet; it weighs nearly four tons; and, as its diameter is six feet,

it has an area four times greater than that of the threefeet speculum. When finished, the speculum was placed in a square box, which is attached to the lower end of the tube, and by means of a door can be entered at pleasure. This box adds six feet to the length of the tube, which, like its predecessor, is of wood, hooped with iron like a barrel, and so wide that a tall man could walk through it without stooping. This huge black funnel is suspended between high and strong walls. It swings with a clear space of twelve feet on each side; and so far it can be drawn aside, giving half an hour before and after meridian. By means of a windlass, and a most skilful adjustment of chains and counterpoising weights, it can also be brought to the zenith, or turned fairly round from south to north. Enormous as are its dimensions, and although weighing altogether twelve tons, it seems to be about as easily moved as the other telescope; and it is as much in the mechanical contrivances for effecting this purpose as in anything else that the peculiar merit of the structure consists.

CHAPTER IV.

NEBULAR AND SIDEREAL SYSTEMS.

The Milky way. Comparative dimensions of the Solar and Sidereal Systems. Distances of the Fixed Stars. Classification of Stars according to their apparent Magnitudes. Distribution of the Stars. Gauging of the Heavens by Herschel. True form of the Milky Way. Clusters and Nebulæ. Forms and distribution of Nebula. Vastness of the Universe. Effect of the finite velocity of Light.

$75. In that portion of infinite space which is unveiled to the gaze of man lie clusters of countless suns, separated one from another by unimaginable intervals. Within one of these clusters, and probably nowise distinguished as to size or brightness from the other orbs, lies the sun around which our earth revolves. About this sun, at distances too small to be represented here, revolve the members of