London:-a precious memento of the ingenuity of one whose name will never die.

We are told that Newton's first reflecting telescope was 6 inches long, and had an aperture of about 1 inches. The eye-glass was inserted in the side of the tube, and was one-sixth of an inch deep. This instrument, although so small, magnified thirty-five times, and enabled Newton to see the crescent-form of Venus, and Jupiter's satellites. He then made another, which was 2 inches in diameter, and therefore gave a larger reflecting surface to the speculum at the bottom of the tube.

The following figure shows the construction of the Newtonian reflecting telescope.

A is the speculum, or reflecting surface, formed of polished metal, either spherically or parabolically curved-the latter is more correct, but the former is more easy to construct. This speculum, as we saw in the case of Herschel's telescope, is placed at the end of the tube d d d d, and has such a degree of curvature, that the rays r d, r d, from any celestial object, will,

that

rays

after reflection be conveyed to a focus at e, near the mouth of the tube. But, before they reach this point, their progress is arrested by a plane mirror, c, so placed as to make an angle of 45° both with the length and with the diameter of the tube. This plane mirror is held in its place by an arm or bracket, g, fitted to the side of the tube, but, at the same time, capable of sliding along it, in order that the distance between the two reflectors may be increased or diminished at pleasure. Let us now suppose from a celestial object are entering the tube, and that they are reflected from the surface of the speculum a: instead of meeting at e, they are reflected a second time, from the small plane mirror c, and cross each other at the point f, at which point of intersection an image of the object is formed. At this part of the process is the following adaptation:—a convex lens is fitted into a socket h, the focal power of which lens is such, that the image is in that focus, and the magnitude of the image is increased.

Many disadvantages resulted from the necessity for the observer looking in at the side of the tube; and inconvenience manifestly results from looking down the mouth of the tube. Those who have attempted to construct reflecting telescopes, have always had in view the removal or diminution of the difficulties mentioned before. Complete success, without loss of light, has not, however, attended their efforts.

From the time when Galileo made his telescope, which was of the common refracting sort, with eyepieces, one at each end of a tube, to the construction of Herschel's great reflecting telescope, a period of nearly 200 years, the knowledge of the heavens rapidly pro

gressed. The construction of the Newtonian telescope was an event mid-way; but the further extent of our knowledge of "the argent fields above," will have to depend mainly on the further improvement of our telescopes.

In concluding this part of the subject of ASTRONOMY, we cannot do better than use the thoughtful and eloquent words of Chalmers:

"The first thing which strikes a scientific observer of the fixed stars, is their immeasurable distance. If the whole planetary system were lighted up into a globe of fire, it would exceed, by many millions of times, the magnitude of this world, and yet only appear a small lucid point from the nearest of them. If a body were projected from the Sun, with the velocity of a cannon-ball, it would take hundreds of thousands of years before it described that mighty interval which separates the nearest of the fixed stars from our sun and from our system. If this Earth, which moves at more than the inconceivable velocity of a million and a half miles a day, were to be hurried from its orbit, and to take the same rapid flight over this immense tract, it would not have arrived at the termination of its journey after taking all the time which has elapsed since the creation of the world. These are great numbers, and great calculations; and the mind feels its own impotency in attempting to grasp them. We can state them in words. We can exhibit them in figures. We can demonstrate them by the powers of a most rigid

and infallible geometry. But, no human fancy can summon up a lively or an adequate conception-can roam in its ideal flight over this immeasurable largeness—can take in this mighty space in all its grandeur, and in all its immensity-can sweep the outer boundaries of such a creation or lift itself up to the majesty of that great and invisible arm on which all is suspended.

"But what can those stars be which are seated so far beyond the limits of our planetary system! They must be masses of immense magnitude, or they could not be seen at the distance of place which they occupy. The light which they give must proceed from themselves; for the feeble reflection of light from some other quarter, would not carry through such mighty tracts to the eye of an observer. A body may be visible in two ways. It may be visible from its own light, as the flame of a candle, or the brightness of a fire, or the brilliancy of yonder glorious Sun, which lightens all below, and is the lamp of the world. Or it may be visible from the light which falls upon it, as the body which receives its light from a taper, or the whole assemblage of objects on the surface of the Earth, which appear only when the light of day rests upon them-or the Moon, which, in that part of it that is towards the Sun, gives out a silvery whiteness to the eye of the observer, while the other part forms a black and invisible space in the firmament or as the planets, which shine only because the Sun shines upon them; and which, each of them, present the appearance of a dark spot on the side that is turned away from it. Now apply this question to the fixed stars. Are they luminous of themselves, or

do they derive their light from the Sun, like the bodies of our planetary system? Think of their immense distance, and the solution of this question becomes evident. The Sun, like any other body, must dwindle into a less apparent magnitude as you retire from it. At the prodigious distance even of the very nearest of the fixed stars, it must have shrunk into a small indivisible point. In short, it must have become a star itself, and could shed no more light than a single individual of those glimmering myriads, the whole assemblage of which cannot dissipate, and can scarcely alleviate, the midnight darkness of our world. These stars are visible, not because the Sun shines upon them, but because they shine of themselves, because they are so many luminous bodies scattered over the tracts of immensity; in a word, because they are so many suns, each throned in centre of his own dominions, and pouring a flood of light over his own portion of these unlimitable regions.

"At such an immense distance for observation, it is not to be supposed that we can collect many points of resemblance between the fixed stars and the solar star, which forms the centre of our planetary system. There is one point of resemblance, however, which has not escaped the penetration of our astronomers. We know that our Sun turns round upon himself in a regular period of time. We also know that there are dark spots scattered over his surface, which, though invisible to the naked eye, are perfectly noticeable by our instruments. If these spots existed in greater quantity upon one side than upon another, it would have the general effect of making that side darker; and the revolution of the Sun must, in such a case, give us a brighter and