this principle, though with an additional reflection, that the mightiest telescope in existence has been constructed. This renowned instrument, known wherever science is known, was built, forty years ago, by the late Earl of Rosse at Parsonstown. The colossal dimensions of this instrument have never been surpassed; they have, indeed, never been rivalled. The reflector in this case is a thick metallic disc, consisting of an alloy of two parts of copper to one of tin, forming a hard and brittle metal intractable for mechanical operations, but admitting of a brilliant polish, and of receiving and retaining an accurate figure. The great reflectorsix feet in diameter-reposes at the end of a tube sixty feet long. This tube is mounted between two castellated walls of masonry, which form an imposing feature on the lawn at Birr Castle, as represented in Fig. 6. This instrument does not admit of being directed towards any part of the sky like the equatorials we have recently been considering. The great reflector is only capable of an up and down movement along the meridian, and of a small lateral movement east and west of the meridian. A little consideration will, however, show that, though the telescope cannot at any moment be directed to any particular star, . yet that each star visible in the latitude of Parsonstown can be observed when looked for at the right time.

As the object is approaching the meridian, be it planet or comet, star or nebula, the telescope is raised to the right height. This is accomplished by a chain passing from the mouth of the instrument to a windlass at the northern end of the walls. By this windlass the telescope can be raised or lowered, and an ingenious system of counterpoises renders the movement equally easy at all altitudes. The observer then takes his station in the lofty gallery which gives access to the eye-piece; and when the right moment has arrived, the object enters the field of view. A vast clockwork mechanism at the lower end of the tube gives movement to the great instrument, so that the object can be followed by the observer until he has made his measurements, or finished his drawing.

It will thus be seen that, notwithstanding the stupendous size

of this telescope (the tube is large enough for a tall man to walk through without stooping), it is comparatively easy to observe with. It must not, however, be assumed that for all the purposes of

astronomy an instrument so colossal is the most suitable. The mighty reflector is chiefly of use where very faint objects are to be sought for; but where accurate measurements are required of objects not unusually faint, telescopes of smaller dimensions and of different construction are more suitable. Among the other great reflectors, we may mention that constructed by

Mr. Common, of Ealing, three feet in aperture, which possesses great optical perfection and has done excellent astronomical work.

The fundamental truths of the movements of the heavenly bodies have been chiefly learned from the work of instruments of comparatively moderate telescopic power, specially arranged to enable precise measures of position to be secured. Indeed, in the early stages of astronomy, important observations of position were obtained by contrivances which showed the direction of the object without any telescopic aid.

In our modern observatories the most important measurements are those obtained by that most accurate of all instruments of precision, known as the meridian circle. It would be out of place to attempt to give here any minute description of this instrument, even in any of its multitudinous forms. It is, however, equally impossible, in any adequate account of the Story of the Heavens, to avoid some reference to this fundamental instrument; and therefore we shall give a very brief account of one of the simpler forms, choosing for this purpose a great instrument in the Paris Observatory, which is represented in the illustration (Fig. 7).

The telescope is attached at its centre to an axis at right angles to its length. The pivots at the extremities of this axis rotate in fixed bearings, so that the movements of the telescope are completely restricted to the plane of the meridian. Inside the eye-piece of the telescope extremely fine vertical lines are stretched. The observer watches the moon, or star, or planet, or whatever may be the object, enter the field of view; and he notes the second, or fraction of a second, by the clock, as the star passes over each of the lines. The circle attached to the telescope is divided into degrees and subdivisions of a degree, and this circle, which moves with the telescope, will indicate the elevation at which the telescope is pointed. For the accurate reading of the circle, microscopes are used. These microscopes are shown in the sketch, each one being fixed into an aperture in the wall which supports one of the pivots. At the opposite side is a lamp,

the light from which passes through the perforated pivot and is thence deflected to illuminate the lines at the focus.

The lines, which the observer sees stretched over the field of view of the telescope, demand a few words of explanation. We require for this purpose a line which shall be very fine and durable, elastic, and of little or no weight. These conditions cannot be completely fulfilled by any metallic wire, but they are most exquisitely fulfilled in the beautiful thread which is spun by the spider. These gossamer threads are stretched with nice skill across the field of view of the telescope, and secured in their proper places. With instruments so furnished, it is easy to understand the precision of modern observations. The telescope is directed towards a star, and the image of the star is a minute point of light. When that point is made to coincide with the intersection of the two central spider lines, the telescope is properly sighted.

We use the word sighted designedly, because we wish to suggest a comparison between the sighting of a rifle at the target and the sighting of a telescope at a star. Instead of the large bull's-eye of a rifle-target, suppose that the target only contained an ordinary watch-dial; the rifleman would not be able to sight the dial. But with the telescope of the meridian circle we could easily see the watch-dial at the distance of a mile. The meridian circle has, indeed, such delicacy as a sighting instrument, that it could be pointed separately to each of two stars, which subtend at the eye an angle no greater than that subtended by an adjoining pair of the sixty minute dots around the circumference of a watch dial a mile away.

This delicacy of sighting would be of little use were it not combined with arrangements by which, when the telescope has been pointed correctly, its position can be ascertained and recorded. One element is secured by the astronomical clock, which gives the moment when the object crosses the central vertical wire; the other element is given by the graduated circle which reads the zenith distance.

Superb meridian instruments adorn our great observatories, and