The object, which appears inverted through this telescope, will appear up right and distinct, if two more convex eye subjoined to it, at a distance from each other, which is equal to the sum of their focal distances ; and when their focal distances are equal, the object will be magnified as much as without those additional glasses ; but through them it will appear upright, and not in verted. Hence this telescope has been mostly used for viewing terrestrial ob jects, and is therefore called the "ter restrial telescope." The " Galilean telescope" consists of a convex object lens, and a concave eye lens, and derives its name from the great Galileo, who is generally reckoned the inventor of it. Fig. 2 shows, that the dis tance between the two lenses is less than the focal distance of the object lens ; viz. instead of the convex lens situated be hind the place of the image, to make the rays of each pencil proceed in a parallel direction to the eye, here a concave eye lens is placed as much before that image ; and this lens opens the rays of each pen cil that converged to q and p, and makes them emerge parallel towards the eye ; as is evident by conceiving the rays to go hack again through the eye lens, whose focal distance is E q.
The eye must be placed close to the concave lens, in order to receive as many pencils as possible ; and then supposing an emerging ray of an oblique pencil to be produced backwards along A 0, the apparent magnitude of the object is mea sured by the angle, A 0 E, or its equal, q E p, which is to the angle, q L p, or Q L P, as q L to q E, viz. as in the astro nomical telescope. It is evident, that in this telescope the objects appear erect, for the rays of do not cross each other.
The field of view, or quantity of objects that are taken in at once in this telescope, does not depend upon the breadth of the eye lens, as in the astronomical telescope, but upon the breadth of the pupil of the eye ; because the pupil is less than the eye lens, A B, and the lateral pencils do not now converge to, but diverge from,. the axis of the lenses. Upon this account the view is narrower in this than in the preceding telescope ; yet the objects through it appear remarkably clear and distinct.
"The night telescope" is a short tele scope, viz. about two feet long, which re presents the objects inverted, much en lightened, but not much magnified. Its field of view is also very extensive. This telescope, in consequence of those pro perties, is used at night mostly by navi gators, for the purpose of discovering ob-' sects that are not very distant, but which cannot otherwise. be seen, for want of sufficient light ; such as vessels, coasts, rocks, &c. On account of its extensive field, and great light, this telescope has also been adv'antageously used, by astro nomers, for discovering some celestial objects, whose situation was not exactly known, or for viewing at once the rela tive situation of several stars and other objects.
This telescope has a pretty large and simple object lens, whence it derives its great light ; for as the rays which pro ceed from every single point of the ob ject fall upon the whole lens of a tele scope, and are thence refracted to fo cus, it is evident that the larger that lens is, the greater number of rays will be thrown upon that focus, and of course the brighter will the image be. In this tele
scope, large lens may be used, because the telescope is not intended to magnify more than about four or six times in lineal extension.
Within this telescope a second lens is often used for shortening the focal length of the object lens. The eye lens is some times single, but mostly double, (viz. a combination of two plano-convex lenses placed at a little distance from each other) and pretty large : hence is derived the extensive field of view, which in some of those telescopes exceeds six or seven de grees.
We may observe, once for all, that in every telescope the distance between the object lens and the other lens or lenses must be alterable, in order that the focus may be adjusted according to the distance of the objects. Hence, every telescope consists at least of two tubes, one of which, viz, that with the eyelenses, slides within the other. To the same telescope several eye tubes, wits a Shallower or deeper lens, or with a different number of lenses, may be ,adapted successively, in order to give them different magnify. ing powers, suitable to the clearness of the air, of the objects,,&c. as also for con verting them into astronomical or terres trial telescopes.
We now proceed to the reflected tele scope, which is likewise 'called the New. tonian telescope ; for it' not the original projector, Sir Isaac Newton is, at least, the first person who executed a telescope of this sort, which consists of reflecting and refracting parts.
The general principle of this telescope is the same as that of the dioptric or re fracting telescope. In the latter, the rays which come from a distant object are, by the action of the convex object lens, col lected to a focus; and beyond that focus the rays of every single radiant point are rendered again parallel by the action, of the eye lens or eye lenses. This is other wise expressed, by saying that the object lens forms an image. of the object, which image is viewed by the eye lens. In the former, viz. in the reflecting telescope, the rays which come from a distant ob ject are, by the action of a concave re flector, sent back convergingly to a focus, where they form an image, which is view ed through the eye lens. There are se veral varieties of this telescope ; we shall content ourselves with the description of one only, viz. the Gregorian telescope, which is represented in fig. 3. The large concave speculum, B E, of this telescope, is .perforated with a hole quite through its middle. Within the tube of the tele scope, a small concave speculum, x y, is supported by the arm, H, directly facing the large speculum, B E. Two lenses, ss x and a o, are contained in the eye tube, and the observer applies his eye to a small hole at P, in order to view a mag nified distant object.