TELESCOPE

M of a distant object M N, and falling on the different points of the object-gla , are refracted into a point m in (2.) the principal focus. In like manner, those proceeding from the point N are refracted into the point n; and thus an inverted image m n is formed at the focus of the object gla . The eye-gla is placed so that its focus shall coincide with the place of the image; consequently rays diverging from any point of the image, and falling on the lens C D, are refracted into a parallel direction before they enter the eye at E, and are thereby rendered fit to produce distinct vision. The length of the telescope is equal to the sum of the focal distances of the two lenses; and the magnifying power is equal to the focal distance of the object-gla divided by the focal distance of the eye-gla . This telescope was first described by Kepler in his Dioptrice (1611); but it does not appear to have been executed until about twenty or thirty years later. Terrestrial Telescope This differs from the astronomical telescope only in having two additional lenses E F, G H (fig. 3.), placed in the tube of the eye-gla for the purpose of restoring the inverted image to its erect position, and thereby accommodating the telescope to terres trial objects. The focal lengths of these additional lenses are usually the same as that of the eye-gla . The two pencils of rays proceeding from the points M and N cro each other in the anterior focus of the second lens E F, and falling parallel on E F form in its principal focus an inverted image of nm, and consequently an erect image of the object M N. This image m' n' is seen by the eye at E through the lens G H, as the rays diverging from m' and n' in the focus of G H enter the eye in parallel pencils. When the three first lenses are equal, the magnifying power is the same as that of the astronomical telescope, whose object and eye gla es are the same as A B and C D. (3.) The performance of refracting telescopes depends most e entially on the goodne of the object-gla ; for if the first image is bright and distinct, and perfectly achromatic, there is little difficulty in constructing eye-pieces to magnify it, without causing it to undergo any sensible alteration. Reflecting Telescopes. — In reflecting telescopes the speculum, or mirror, performs the same office that the object-gla does in those of the refracting kind, and is therefore called the object-mirror. The instrument is constructed in various forms; but these differ from one another chiefly in reference to the contrivances which liave been adopted for bringing the focal image into a convenient situation for being viewed by the eye-piece. The principal forms are the Newton i an, the Greg or i an, the Cas seg rain i an, and the Herschelian. Newton i an Telescope. — Let A B C D (fig. 4.) represent (4.) a section of the tube of the telescope; A B the object mirror, which would form at its focus the image a of any distant object. Now if a person attempted to view the image in its place at a by placing himself directly before the mirror, he would nece arily intercept the rays of light from the object pa ing down the tube to the mirror, and consequently there would be no image to view. Sir Isaac Newton overcame this difficulty by introducing a small diagonal plane speculum d between A B and a, which intercepting itself but a small portion of the light, reflects towards the side of the tube the rays converging from A B, and causes the image which would have been formed at a to be formed at b, where it can be conveniently viewed by the eye-piece E attached to the side of the tube. The small mirror is of an oval form, and is ftxed on a slender arm c connected with a slide, by means 1220 of which it may be made to approach or recede from the large speculum A B, according as tlie image approaches to or recedes frcm it. In this telescope the magnifying power is equal to the focal length of the object-mirror A B divided by that of the eye-gla . Greg or i an Telescope. — In this construction the object mirror A B (fig. 5.) is perforated in the middle, and the (5.) 'A rays of light from a distant object being reflected from the surface of A B cro each other in the focus, where they form an inverted image «, and are then intercepted by a small concave mirror d, which causes them again to converge to a focus at b, near the perforation of the object mirror, where they form a rein verted or direct image, which is viewed bv an eye-piece E screwed into the tube behind A B. The curvature of the small speculum should be elliptical, having the foci at a and b; but it is generally made spherical. In this case the great speculum should be slightly hyperbolic, to counteract the aberration of the small mirror. Cas seg rain i an Telescope. — The great speculum of this instrument is perforated like the Greg or i an; but the rays converging from the surface of the mirror A B (fig.6.) (G.) D iL towards the focus a are intercepted before they reach that point by a small convex mirror d, not sufficiently convex to make the rays divergent, but of such a curvature as to prevent them from coming to a focus till they are thrown back to b, near the aperture in A B, where they form an inverted image which is viewed by the eye-piece E. This construction has the advantage of requiring a shorter tube than the Greg or i an; but the inversion of the image is not corrected, and for this reason probably it has not been much used. In the two last constructions the small mirror d is adjusted by means of a rod turning on a shoulder near the eye end of the tube, and connected by a screw with the apparatus which carries the arm c to which tlie mirror is attached. Herschelian Telescope. — This construction differs from the others in having no second mirror. The large speculum AB (fig. 7.) is placed at the bottom of the tube in an inclined position, so as to bring the focal image a near the edge of the tube, where it is viewed directly by the eye-piece E without interfering with the light entering the telescope from the object observed. The magnifying power is the same as in the Newton i an. The reflecting telescope was invented by James Gregory, and is described by him in his Optica Fromoia (1663); but the first telescope of the kind was executed by Newton. Reflecting telescopes have been made on a very large scale. The celebrated intrument of Sir William Herschel, erected at Slough in 1789, was 40 feet in length. Its great speculum had a diameter of 40| inches; its thickne was about 3J inches, and its weight when cast was 2118 lbs. Its focal length was 40 feet, and it admitted of a power of 64.50 being applied to it. The e ential advantage of large telescopes of this kind consists in the immense quantity of light which they collect, whereby the observer is enabled to perceive faint nebula; and stars which are altogether invisible in ordinary instruments. Reflecting telescopes are used only for observing phenomena, and are not, like refracting telescopes, attached to circular instruments for the purpose of measuring angles with greater precision. In order to derive full [s. 1233]