Of reflecting telescopes, the Gregorian is the one most generally used. The construction of this instrument is represented below.

At the bottom of the tube A B C D, is placed the large concave reflector k 1. with a circular hole through the middle of it, in the direction of its axis. Within the tube of the telescope, and directly facing the perforation, is placed the small concave speculum g h, supported by the arm i. Two lenses, tt and q q, are contained in the eye-tube LMNO, and the observer applies his eye to a small hole atf, in order to view the magnified image of the distant object Y Z. The large reflector k l receives the rays from the distant object, and reflects them to its focus, where they form an inverted image E F. Diverging from the points of union, the pencils of rays proceed onwards, and cross each other a little before they reach the small mirror g h: the focus of which is at n, or a little further from the large speculum, than its principal focus. From the small mirror, the rays are reflected somewhat convergently, and in that state are received before they meet a plain convex lens tt. By the action of this lens, their convergeucy is increased, and they form a second image, a b, which is erect like the object. This second image is magnified by the lens q q, through which the rays of each pencil pass nearly in a parallel direction to the eye.

To exclude all extraneous light, the eye is applied to a small hole, and sees the image under the angle cfd. If the lens tt were removed, the image would be formed somewhat larger at r, but the area, or field of view, would be smaller and less pleasant, for which reason it is not usual to omit the second lens. In this, and other reflecting telescopes, containing two curved reflectors, it is necessary to have the power of altering the distance between the two mirrors. This is usually done by a wire, e s, passing along the outside of the tube and with a screw at the end of it, which works in an external projection w, of the arm i, within the tube. The other end of the wire passes through a small stud affixed to the tube of the telescope at m; and the observer, while looking through the hole atf, turns the milled head p, of the wire, which is near him, and thus regulates the distance of the small speculum, as he finds requisite. A section of the Newtonian Reflecting Telescope is shown below.

ABCD, is the tube, which is open at the end A B, opposite the large speculum n o. The large concave speculum n o, is not perforated as in the Gregorian telescope, but the small speculum q, is set aslant, so as to direct the rays received from the large speculum, through an aperture g, at the side of the tube, where they are received and refracted to the eye by a lens or lenses in a tube h. The speculum q, is suspended within the tube, by an arm p, with its centre opposite the centre of the speculum no; it is not curved, but plane, and has therefore no other effect than that of changing the direction of the rays. Without the small reflector, the rays from the large speculum would be converged at R, and the observer might have an eye-glass placed to view the image formed there, with his face towards the speculum no; but in this case his head would intercept the greater part of the rays, unless the instrument were very large. The Newtonian telescope, as first described, is very convenient for viewing objects in the zenith; as they may be seen while the observer retains his ordinary position of looking forward horizontally.

The best and most powerful reflecting telescopes, however, which have ever been constructed, are those of Dr. Herschel, who is so well known by his labours, as one of the most eminent astronomers of the present day. The largest reflecting telescope made by Dr Herschel, is forty feet in length, and the polished surface of the large speculum is four feet in diameter. It has no 6econd reflector; a circumstance that adds much to the brightness of the objects viewed in c. The observer, who looks through an eye-glass, as in other telescopes, has, of course, his back to the object; but it is so contrived that little or no light is intercepted by this means. We may use the foregoing diagram of the Newtonian telescope, on page 773, to illustrate the position of the observer, by this telescope, more particularly. Supposing the speculum q, and its support to be removed, the rays n o, as before observed, would be converged at R; but if the observer were placed there, he would intercept a large portion of the light, even when facing this gigantic telescope.

Supposing the upper part n, of the speculum, to be inclined downwards, that is, to be set at an angle to the axis of the tube, the rays may be directed to S, or any other point nearer the tube, where the spectator may be placed, and will occasion no sensible dimness of the image thrown by the large mirror. In Dr. Herschel's large telescope, the converging rays reflected by the mirror pass the extremity of the tube, at the distance of four inches from it, and come into the air; by this means the observer scarcely at all interferes with the incident light, as the diameter of the tube exceeds that of the mirror, by about ten inches. The mirror has a magnifying power of six thousand times the diameter of an object.