An optical instrument, employed for discovering and viewing distant objects; or which magnifies their natural appearance, by representing them under a larger angle than that under which they appear to the naked eye. Telescopes are divided into two general kinds, refracting and reflecting. A refracting telescope consists of different lenses, through which the objects are seen by rays, refracted by them to the eye. A reflecting telescope, besides lenses, has a metallic speculum within its tube, by which the rays proceeding from an object are reflected to the eye. »

The principal effect of telescopes depends upon this rule, that objects appear large or small, in proportion to the angle which they subtend to the eye; and the effect is the same, whether the pencils of rays, by which the objects are rendered visible to us, come directly from the objects themselves, or from any point nearer to the eye, where they may have been united, so as to form an image of the object; because they issue again in certain directions from those points, where there is nothing to intercept them, in the same manner as they did from the corresponding points, in the objects themselves. In fact, therefore, all that is effected by a telescope, is, first, to make an image of a distant object, by means of a lens or mirror, and then to give the eye some assistance for viewing that image as near as possible; so that the angle which it shall subtend to the eye, may be very large, compared with the angle which the object itself would subtend in the same situation to the naked eye. This is effected by means of an eye-glass, which so refracts the pencils of rays, that they may afterwards be brought to their several foci, by the humours of the eye.

The forms of refracting and reflecting telescopes have been frequently varied, and they are sometimes distinguished by the names of their inventors, as the Galilean, and the Newtonian telescope; sometimes by the particular use for which they are best adapted, as the "land telescope," the "night telescope," the"astronomical telescope," etc.

The astronomical telescope consists of two convex lenses, A B, K M, each fixed at the extremity of a different tube. One of the tubes is very short, as its use is merely to adjust the focus in proportion to the distance of the object viewed, and it slides within the other. Contrary to the arrangement which takes place in the microscope, the glass which has the longest focus, is presented to the object, and therefore constitutes the object-glass. P R, respresents a very distant object, from every point of which rays come, so very little diverging to the object-lens K M of the telescope, as to be nearly parallel: p r, is the picture of the object P R, which would be formed upon a screen situated at that place. Beyond that place, the rays of every single radiant point proceed divergently to the eye-glass A B, of greater convexity, and which causes the rays of each pencil to become parallel, in which direction they enter the eye at O. The axes of the two lenses are coincident in the direction QLO; L q, is the focal distance of the object-glass, and E g, is the focal distance of the eye glass; consequently, the distance between the two glasses is equal to the sum of their focal distances.

An object viewed through this telescope, by an eye situated at O, will appear magnified and distinct, but inverted. The object seen without the telescope, will be, to its appearance, through the telescope, as q E to q L; that is, as the focal distance of the eye-glass, to the focal distance of the object-glass. For the pencils of rays, which, after their crossing at r q p, proceed divergently, fall upon the lens A B, in the same manner as if a real object were situated at r q p; and consequently, after passing through that lens, the rays of each pencil proceed parallel. To the eye at O, the apparent magnitude of the object P R, is measured by the angle BOA, or by its equal p E; but, to the naked eye at L, when the glass is removed, the apparent magnitude of the object is measured by the angle P L R, or by its equal r Lp; therefore, the apparent magnitude to the naked eye, is to the apparent magnitude through the telescope, as the angle r L p, is to the angle pEr; or, as the distance q E, is to the distance q L.

If the angles r L p, and pEr, were equal to each other, the telescope would not magnify, and they would be equal, if the lenses were of equal focal distance. Hence, as the magnifying power of the telescope is produced by making the focal distance of the eye-glass less than that of the object-glass, it will easily be perceived, that the greater the difference of the focal lengths, the greater will be the magnitying power. It is found, however, that they may be so disproportionate, that the increased magnifying power is overbalanced by the indistinctness which ensues. In order, therefore, to obtain a great magnifying power with the preservation of just proportion, these telescopes have sometimes been made one hundred feet, or upwards, in length; and, as they were mostly used for astronomical purposes, they were frequently used without a tube. The object lens was fixed on the top of a pole, in a frame capable of being moved by a cord or wire, in any required direction, and the eye-glass, fixed in a short tube, was held in the hand, or fitted to another frame, about the height of the observe!, so as to be capable of a simultaneous movement. A telescope of this description was called an atrial telescope.

Its use was evidently very incommodious; but, such were the great pains taken by philosophers, in exploring the wonders which even the imperfect telescopes, at first constructed, promised to lay open, that with such an instrument, the five satellites of Saturn, and many other remarkable objects, were discovered.

The length of common refracting telescopes must be increased in the proportion of the square of the increase of their magnifying power; so that, in order to magnify twice as much as before, with the same degree of light and distinctness, the telescope must be lengthened four times; and to magnify three times as much, nine times. On this account, their unwieldy length, when great powers are desired, is unavoidable. The breadth of an object-glass adds nothing to the magnifying power; for, whatever the latter may be, the image will be equally formed at the distance of its focal length; but the brilliancy of the image will be increased by the breadth, as a greater number of rays will then diverge from every point of the image.