he would not accede to my wish; for the star, instead of disappearing the moment the moon's edge came in contact with it, apparently glided on the moon's dark face, as if it had been seen through a transparent moon, or as if the star were between me and the moon. It remained on the moon's disk nearly two seconds of time, and then instantly disappeared, at 10h. 9m. 59-72s. sidereal time. I have seen this apparent projection of a star on the moon's face several times, but from the great brilliancy of the star, this was the most beautiful I ever saw." Sir James South thus proceeds to compare the great telescopes —

"The only telescopes, in point of size, comparable with Lord Rosse's three feet and six feet, are Sir William Herschel's twenty feet and forty feet Le Mairean's. The twenty feet had a speculum of 18.8 inches diameter, and the forty feet one of four feet.

"The Le Mairean of 18.8 inches diameter, in point of light is equal to a Newtonian of 224 inches diameter.

"The Le Mairean of four feet diameter is equal to a Newtonian of 57 inches.

"The Le Mairean of three feet is equal to a Newtonian of 43 inches.

"And the Le Mairean of six feet is equal to a Newtonian of 86 inches.

“By substituting, then, the Le Mairean form for the Newtonian, the present three feet Newtonian will be made as effective as if it were 43 inches diameter, and the six feet as if it were 86 inches in diameter; or the quantity of light in each telescope, after the alteration, will be, to its present light, as seven to five nearly, or almost half as much again as it now has.

"Seeing, then, that the change from the Newtonian to the Le Mairean construction will be attended with such an accession of light, Lord Rosse, having determined geometrically the form of the curve requisite to produce with it a definition of objects equal to that which each of the telescopes at present gives, is devising mechanical means for producing it."

It is not generally understood in what way greatness of size in a telescope increases its powers; and it conveys but an imperfect idea of the excellence of a telescope to tell how much it magnifies. In the same instrument, an increase of magnifying power is always attended with a diminution of the light and of the field of view. Hence, the lower powers generally afford the most agreeable views, because they give the clearest light and

take in the largest space. The several circumstances which influence the qualities of a telescope are illuminating power, magnifying power, distinctness, and field of view. Large mirrors and large object-glasses are superior to smaller ones, because they collect a larger beam of light, and transmit it to the eye. Stars which are invisible to the naked eye are rendered visible by the telescope, because this instrument collects and conveys to the eye a large beam of the few rays which emanate from the stars; whereas a beam of these rays of only the diameter of the pupil of the eye, would afford too little light for distinct vision. In this particular, large telescopes have great advantages over small ones. The great mirror of Herschel's forty feet reflector collect and conveys to the eye a beam more than four feet in diameter. The Dorpat telescope also transmits to the eye a beam nine and a half inches in diameter. This seems small, in comparison with the reflector; but much less of the light is lost on passing through the glass than is absorbed by the mirror, and the mirror is very liable to be clouded or tarnished; so that there is not so great a difference in the two instruments, in regard to illuminating power, as might be supposed from the difference of size.

Distinctness of view is all-important to the performance of an instrument. The object may be sufficiently bright, yet, if the image is distorted, or ill-defined, the illumination is of little consequence. This property depends mainly on the skill with which all the imperfections of figure and colour in the glass or mirror are corrected, and can exist in perfection only when the image is rendered completely achromatic, and when all the rays that proceed from each point in the object are collected into corresponding points of the image, unaccompanied by any other rays. Distinctness is very much affected by the steadiness of the instrument. Every one knows how indistinct a page becomes when a book is passed rapidly backwards and forwards before the eyes, and how difficult it is to read in a carriage in rapid motion on a rough road.

Field of view is another important consideration. The finest instruments exhibit the moon, for example, not only bright and distinct, in all its parts, but they take in the whole disc at once; whereas the inferior instruments, when the higher powers especially are applied, permit us to see only a small part of the

moon at once.

Some states of the weather, even when the sky is clear, are far more favourable for astronomical observations than others, After sudden changes of temperature in the atmosphere, the medium is usually very unsteady. If the sun shines out warm after a cloudy season, the ground first becomes heated, and the air that is nearest to it is expanded, and rises, while the colder air descends, and thus ascending and descending currents of air, mingling together, create a confused and wavy medium. The same cause operates when a current of hot air rises from a chimney; and hence the state of the atmosphere in cities and large towns is very unfavourable to the astronomer on this account, as well as on account of the smoky condition in which it is usually found. After a long season of dry weather also the air becomes smoky, and unfit for observation. Indeed, foggy, misty, or smoky air, is so prevalent in some countries, that only a very few times in the whole year can be found, which are entirely suited to observation, especially with the higher powers; for we must recollect that these inequalities and imperfections are magnified by telescopes as well as the objects themselves. Thus, as I have already mentioned, not more than one hundred good hours in a year could be obtained for observation with Herschel's 's great telescope. By good hours, Herschel means that the sky must be very clear, the moon absent, no twilight, no haziness, no violent wind, and no sudden change of temperature. As a general fact, the warmer climates enjoy a much finer sky for the astronomer than the colder, having many more clear evenings, a short twilight, and less change of temperature. The watery vapour of the atmosphere also is more perfectly dissolved in hot than in cold air, and the more water air contains provided it is in a state of perfect solution, the clearer it is.

A certain preparation of the observer himself is also requisite for the nicest observations with the telescope. He must be free from all agitation, and the eye must not recently have been exposed to a strong light, which contracts the pupil of the eye. Indeed, for delicate observations, the observer should remain for some time beforehand in a dark room, to let the pupil of the eye dilate. By this means, it will be enabled to admit a larger number of the rays of light. In ascending the stairs of an observatory, visitors frequently get out of breath, and having perhaps recently emerged from a strongly lighted apartment, the eye is not in a favourable state for observation. Under these disad

vantages, they take a hasty look into the telescope, and it is no wonder that disappointment usually follows.

Want of steadiness is a great difficulty attending the use of the highest magnifiers; for the motions of the instrument are magnified as well as the object. Hence, in the structure of observatories, the greatest pains is requisite, to avoid all tremor, and to give to the instruments all possible steadiness; and the same care is to be exercised by observers. In the more refined observations, only one or two persons ought to be near the instrument.

In general, low powers afford better views of the heavenly bodies than very high magnifiers. It may be thought absurd to recommend the use of low powers, in respect to large instruments especially, since it is commonly supposed that the advantage of large instruments is, that they will bear high magnifying powers. But this is not their only, nor even their principal advantage. A good light and large field are qualities, for most purposes, more important than great magnifying power; and it must be borne in mind that as we increase the magnifying power in a given instrument, we diminish both the illumination and the field of view. Still, different objects require different magnifying powers; and a telescope is usually furnished with several varieties of powers, one of which is best fitted for viewing the moon, another for Jupiter, and a still higher power for Saturn. Comets require only the lowest magnifiers; for here, our object is to command as much light, and as large a field as possible, while it avails little to increase the dimensions of the object. On the other hand, for certain double stars (stars which appear single to the naked eye, but double to the telescope), we require very high magnifiers, in order to separate these minute objects so far from each other that the interval can be distinctly seen. Whenever we exhibit celestial objects to inexperienced observers, it is usual to precede the view with good drawings of the objects, accompanied by an explanation of what each appearance, exhibited in the telescope, indicates. The novice is told that mountains and valleys can be seen in the moon by the aid of the telescope; but, on looking, he sees a confused mass of light and shade, and nothing which looks to him like either mountains or valleys. Had his attention been previously directed to a plain drawing of the moon, and each particular appearance interpreted to him, he would then have looked through the telescope with intelligence and satisfaction.

OBSERVATORIES.

AN observatory is a structure fitted up expressly for astronomical observations, and furnished with suitable instruments for

that purpose.

The two most celebrated observatories hitherto built are those of Tycho Brahe, and of Greenwich. The observatory of Tycho Brahe was constructed at the expense of the King of Denmark, in a style of royal magnificence, and cost no less than two hundred thousand crowns. It was situated on the island of Huenna, at the entrance of the Baltic, and was called Uraniburg, or the palace of the skies.

Before giving an account of Tycho's observatory, it will be useful to relate a few particulars respecting this great astronomer himself.

Tycho Brahe was of Swedish descent, and of noble family; but having received his education at the University of Copenhagen, and spent a large part of his life in Denmark, he is usually considered a Dane, and quoted as a Danish astronomer. He was born in the year 1546. When he was about fourteen years old, there happened a great eclipse of the sun, which awakened in him a high interest, especially when he saw how accurately all the circumstances of it answered to the prediction with which he had been before made acquainted. He was immediately seized with an irresistible passion to acquire a knowledge of the science which could so successfully lift the veil of futurity. His friends had destined him for the profession of law, and, from the superior talents of which he gave early promise, added to the advantage of powerful family connexions, they had marked out for him a distinguished career in public life. They therefore endeavoured to discourage him from pursuing a path which