TELESCOPE, an optical instrument, which is used for discovering and viewing distant objects, either directly by glasses, or by reflection. Telescopes are either refracting or reflecting ; the former con sist of different lenses, through which the objects are seen by rays refracted by them to the eye ; and the latter, of spe cula, from which the rays are reflected and passed to the eye. The lens, or glass, turned to the object, is called the object glass ; and that next the eye, the eye-glass ; and when the telescope con sists of more than two lenses, all but that next the object are called eye-glasses.
The principal effects of telescopes depend upon this maxim, " that objects appear larger in proportion to the angles which they subtend at the eye : and the effect is the same, whether the pencils of rays, by which objects are visible to us, come directly from the objects them-, selves, or from anyplace nearer to the eye, where they may have been united, so as to form an image of the object ; because they issue again from those points in cer tain directions, in the same manner as they did from the corresponding points in the objects themselves. In fact, there fore, all that is effected by a telescope is, first to make such 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 at the eye may be very large, compared with the angle which the object itself would subtend in the same situation. This is done by means of an eye-glass, of which so refracts the pencils ot rays, as that they may afterwards be brought to their several foci, by the natural hu mours of the eye. But if the eye had been so formed as to be able to see the image with sufficient distinctness, at the same distance, without an eye-glass, it would appear to him as much magnified, as it does to another person who makes use of a glass for that purpose, though he would not, in all cases, have so large a field of view.
Although no image be actually formed by the foci of the pencil without the eye, yet if, by the help of an eye-glass, the pencils of rays shall enter the pupil, just as they would have done from any place without the eye, the visual angle will be the same as if an image had been actually formed in that place.
Telescopes are of several kinds, dis tinguished by the number and form' of their lenses, or glasses, and denominated from their particular uses, &c. such are the " terrestrial, or land telescope," the " celestial, or astronomical telescope ;" to which may be added, the " Galilean, or Dutch telescope," the " reflecting te lescope ;" the achromatic telescope,' &c.
We shall proceed to describe some of these, in order to illustrate the principle.
The " astronomical telescope" consists of two convex lenses, A B, K M, Plate XVI. Miscel. fig. 1. fixed at the two ex tremities of a tube, that consists, at least, cif two parts, that slide one within the other, for adjusting the focus in propor tion to the distance of the objects that are to be seen through the telescope.
P Q represents the semi-diameter of 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 q is the picture of the object, P Q, which would fie Tann ed upon a screen situated at that place: Beyond that place, the rays of every sin gle radiant point proceed divergingly upon another lens, A B, called the eye glass, which is more convex than the for mer, and are, by this, caused to proceed parallel to one another, in which direction they enter the eye of the observer at 0.
The two lenses of this telescope have a common axis, O L Q3 L q is the focal dis tance of the object lens, and.E q is the fo cal distance of the eye lens. An object viewed through this telescope, by an eye situated at 0, will appear distinct, invert ed, and magnified; viz. the object seen without the telescope will be, to its ap pearance through the telescope, as q E to q L ; that is, as the focal distance of the. eye lens to the focal distance of the ob ject lens. For the rays, see Orrics,, which, after their crossing at the place, r gp, proceed divergingly, fall upon the lens, A B, in the same manner as if a real object were situated' at 2' q p ; and, of course, on the other side of that lens the rays of each pencil will proceed parallel: Now to the eye at 0, the apparent magni tude of the object, or of the part, P Q, is measured by the angle, E 0 A, or by its equal, q Ep.; but to the naked eye at L, when the glass is removed, the apparent magnitude of the object is measured by the angle, Q L F, or by its equal, q therefore the apparent magnitude, to the naked eye, is to the apparent magnitude through the telescope, as the angle, q Lp, is to the angle, g E p ; or as the distance, q E, is to the distance, q L. This teles cope is mostly used for astronomical ob servations : for, as it inverts the object, the representation of terrestrial objects through it would not be pleasant. It is evident, from the above explanation, that if the two lenses of this telescope have equal focal distances, the telescope will not magnify. It also appears, that, with A given object lens, the shorter the focus of the eye lens is, the greater will the meg nifying power be. But when the dispro portion of the two focal lengths is very great, then the aberration, arising from the figure of the lenses, and from the die. persive power of glass, becomes so very great as to do more damage than can be compensated by the increased magnifying power. Hence, in order to obtain a very great magnifying power, those telescopes have sometimes been made very long, as,, for instance, of 100 feet, or upwards : and as they were used for astronomical pur poses, or mostly in the night time, they were frecefently used without a tube, viz, the object lens was fixed on the top of a pole, in a frame capable of motion in any required direction, and the eye lens was fixed in a short tube that was held in the hand of the observer. The distance, as well as the direction, of the two lenses, was adjusted by a strong cord stretched between the frame of the object lens and the tube of the eye lens. In this construc tion, the instrument has been called an " aerial telescope." Its use is evidently incommodious ; but. it was with such a telescope that five satellites of Saturn, and other remarkable objects, were disco vered.