The telescopes which we have hitherto described will only bear a small aper Lure, without exhibiting circular prismatic rings of colours, which are detrimental to their utility. Two causes contribute to this effect. 1. Spherical surfaces do not refract the rays of light accurately to a point; and 2. The rays of corn ed light being differently refrangible, come to their respective force at Went distances from the lens ; the more refrangible rays converging sooner, of course, than the less refrangible. If the image of a paper painted entirely ;ad, be cast, by means of a lens, upon a screen, it will be formed at a greater distance than the image of a blue paper east by the same lens. The image of a white object is composed of an indefinite number of coloured images, the violet being the nearest, and the red the farthest from the lens; and the es of intermediate colours at intermediate distance'. The whole name is the rein some degree confused, though its extremities are most so ; and this confusion being increased, not only by the magnifying power of the eye-glass, but also by the dispersive power which it has in common with the objeot-glam, the necessity for a certain proportion between the powers of the object and the eye-glass becomes indis pensable The late Mr. John Dolland, by making a compound lens of three different substances, of different refrangible powers, the rays of light which were too much dispersed by one convex lens, were brought nearer to a union with each. The telescope. made with an object-glass of this kind are now commonly used, and are distinguished by the name of achromatic telescopes, a term which signifies colourless. The object -Flames of Dolland's telescopes are composed of three dis tinct lenses, two of which are convex, and the other concave. The achromatic effect may be produced by the union of one convex and one concave lens, but not so perfectly as with three lenses.
The impossibility, however, of obtaining perfectly homogeneous glass, and the consequent failure of producing that complete correction of the aberration of the rays of light in the telescopes called achromatic, induced Dr. Blair to try the effects of fluid mediums ; and his success was such as to induce him to give the term aplanafic, or error, to the object lenses he thus constructed. Ile made a compound lens, consisting of a plano-convex of crown glass, with its flat side towards the object, and a ininiscus of the same material, with its convex side in the same direction, and its flatter concave next the eye; and the interval between the lenses he filled with a solution of antimony in a certain proportion of muriatic acid. The lens thus constructed did not exhibit the slightest vestige of any extraneous colour; but the invention, after a lapse of more than twenty years, has not come into general use, probably from tie diffi culty of preserving any fluid from growing turbid in the course of time.
Of reflecting twopes, 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 it. with a circular bole 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, t t and g q, are contained in the eye-tube L M N 0, and the observer applies his eye to a small hole at f, in order to view the magnified image of the distant object Y Z. The large reflector k 1 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 minor, the rays are reflected somewhat convergently, and in that state are received before they meet a plain convex lens t S. By the action of this lens, their convergency 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 nes the image under the angle d. If the lens t t 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 in; and the observer, while looking through the hole at f, 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.