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-351] Astronomical Telescope.
349 glass, so that the image is always formed at the distance of distinct vision.
Opera glasses are, constructed on this principle. They are usually double, so as to produce an image in each eye, by which greater brightness is attained.
351. Astronomical telescope. In order to obtain a greater field of view, in observing the stars a telescope with two condensing lenses is used. Its invention is due to Kepler, and it is known as the astronomical telescope. It gives reversed images of objects, but this is not objectionable in observing the stars.
Fig. 269 represents an astronomical telescope with a cast iron support, and mounted with a hinge motion on a column of the same metal; so that, not only can any degree of inclination be imparted to it, but it can be directed to any part of the horizon. By means of a handle and two toothed wheels the telescope can be raised or lowered at pleasure. On the side of the telescope is
a smaller one called the finder ; for as it magnifies less than the large one, it embraces a greater extent of sky, and therefore is more suited for finding any given star, which is then observed more minutely with the large glass.
Fig. 270 represents the arrangement of the glasses, and the path of the rays in an astronomical telescope. It consists of two double convex glasses; the object glass, which is of large diameter, and but slightly convergent, gives at ab a reversed and very small image of the star towards which the telescope is directed. This image is looked at through the eyepiece, O, which acts here as a magnifying glass, and which, for that purpose, is placed so that the image, ab, is formed between this glass and the principal focus, F. Thus the observer sees a reversed and greatly enlarged image of the star at cd.
As in all telescopes, the eyetube, that is, the tube in which is the eyepiece, slides in the other, so that it can be brought nearer or further from the image, ab, which can thus be seen at the distance of
distinct vision. In powerful telescopes the eyepiece is not simple, as in the above case, but consists of a greater number of glasses, the object of which is not only to increase the magnifying power, but also to correct spherical and chromatic aberration.
The magnifying power of a telescope is greater the greater the diameter of the object glass, and the less its convexity; and the more convex, on the contrary, is the eyepiece. The greatest obstacle met with in the construction of these telescopes is the difficulty of manufacturing large object-glasses.
When the telescope is used to make an accurate observation of the stars, for example, their zenith distance, or their passage over the meridian, a cross wire is added. This consists of two very fine metallic wires or spider threads stretched across a circular aperture in a small metal plate. The wires ought to be placed in the position where the inverted image is produced by the object glass, and the point where the wires cross ought to be on the optical axis of the telescope, which thus becomes the line of sight, or collimation.
352. Terrestrial telescope.-— The terrestrial telescope differs from the astronomical telescope in producing images in their right
positions. This is effected by means of two condensing lenses, which are interposed between the object glass and the eyepiece, as seen in fig. 271. The object glass forming then, at I, a reversed image of the object, AB, the two glasses, m and n, impart such a direction to the rays traversing them, that, after having crossed between the two glasses, the rays reproduce an erect image at i. The eyepiece acts then just as in the astronomical telescope, giving a very near, erect, and magnified image, ab.
The terrestrial telescope is sometimes, mounted on a stand, and sometimes held in the hand ; its uses are too well known to need any description. · 353. Reflecting telescopes. — The telescopes previously described are refracting or dioptric telescopes. It is, however, only in recent times that it has been possible to construct achromatic
lenses of large size; before this, a concave metallic mirror was used instead of the object glass. Telescopes of this kind are called reflecting or catoptric telescopes. The principal forms are those devised by Gregory, Newton, Herschel, and Cassegrain.
Of these we shall describe the Newtonian telescope, which, after long disuse, has been restored to favour, in great measure owing to the improvements made in the construction of the concave mirror used in it.
Fig. 272 represents a section of a Newtonian telescope as modified by M. Foucault, and fig. 273 a perspective view. The principal piece of the telescope is a concave mirror, M, placed at the end of a long wooden tube. These mirrors were formerly of metal, and the difficulty of working such mirrors, so as to give them a perfect curvature, was so great, that the use of reflecting telescopes was virtually abandoned.
Foucault having discovered a method of silvering glass without
injuring its polish, and as glass is more easily worked than metal mirrors, reflectors for telescopes are now made of polished glass, silvered on the concave surface itself, the rays of light which come from the star observed are there reflected, and tend to form at the other end of the tube a real and very small image of the star ; but these rays fall upon a small rectangular prism, mn, into which they pass without being refracted, and form with the large face, mn, such an angle of incidence that they are reflected out instead of being refracted (317). The image is then formed at ab, in front of a horizontal tube, in which are a series of magnifying glasses, which act as ocular, and give of the image, ab, a very amplified virtual image, AB.
Fig. 273 shows how the instrument is worked. The right hand of the observer holds a handle which transmits the motion to an endless chain, and this to two other chains, which pass round
pulleys, and enable the tube to be more or less inclined ; with the left hand the same observer turns a small wheel, fixed to a screw, which enables him to move slowly the front part of the apparatus in a lateral direction, so that he can follow the star in its motion.
Fig. 273. A little lower than the eyepiece and above the small wheel is a milled head, which works a small rack and pinion motion : this is fixed to a inovable piece, which, at the same time, supports the prism, nm, and the eyepiece (fig. 272). By turning this milled head in
either direction, the prism and the eyepiece may be adjusted until the image, AB, is formed at the distance of distinct vision of the observer.
On the side of the tube is a smaller telescope, quite similar to the large one, but of far less magnifying power. This is the finder.
From its small magnifying power, not more than ten, it embraces a far greater extent of the sky, and is therefore more favourable for finding the desired star.
354. Herschel's telescope.—Sir W. Herschel's telescope, which, until lately, was the largest instrument of modern times, was constructed on a method differing from those described. The mirror was so inclined that the image of the star was formed on the side of the telescope near the eyepiece (fig. 274); hence it is termed the front view telescope. As the rays in this telescope only undergo a single reflection, the loss of light is less than in either of the preceding cases, and the image is therefore brighter. The magnifying power is the quotient of the principal focal distance of the mirror by the focal distance of the eyepiece.
Herschel's great telescope was constructed in 1789 ; it was 40 feet in length, the great mirror was 50 inches in diameter. The quantity of light obtained by this instrument was so great as to enable its inventor to use magnifying powers far higher than anything which had hitherto been attempted.
Herschel's telescope has been exceeded by one constructed by the late Earl of Rosse. This magnificent instrument has a focal length of 53 feet; the diameter of the mirror is 6 feet, and it weighs 8,400 pounds. It is at present used as a Newtonian telescope, but it can also be arranged as a front view telescope.