round with a uniform motion at the same rate as the stars. 3. We have hitherto treated only of refracting telescopes, or of such as are constructed on the principle of the convergence and divergence of the rays of light in passing through lenses of glass; but there is still a very important class of telescopes, called reflecting telescopes, in which a reflecting metallic mirror is used, instead of an object-glass. Newton assumed, rather too hastily, that, for all kinds of glass, the whole dispersion (coloured or chromatic dispersion) is proportional to the whole refraction of the rays, and, therefore, that the defect arising from want of achromatism, that is, the confusion of colours in the images formed by refracting telescopes, did not admit of correction by any combination of glasses. He, therefore, turned his attention to reflecting telescopes, and invented the telescope which bears his name, that is, the Newtonian telescope. 4. The reader will readily understand that rays of light, either parallel or proceeding from a point, and falling on a well-polished speculum, whose surface forms a small portion of a sphere, will be made to converge towards another point, and that thus an image of an object will be formed, in a similar way to that described in the case of refracting telescopes. This image can be then viewed by means of eye-pieces, similar to those already described. In the case of the Newtonian telescope, the rays, while converging, are received upon a small plane reflector, inclined at an angle 45° in the tube, and are then made to pass through the glasses of an eye-piece placed properly to receive them, in the side of the tube. An objection to this kind of telescope is that, since light is lost at each reflexion (no reflecting surface being perfectly smooth), the use of two mirrors is prejudicial to the brightness of the image. In Sir W. Herschel's construction, the large object-mirror is a little inclined to the axis of the tube, and the rays, after forming an image, are received immediately, without a second reflexion, upon an eye-piece placed at the end of the tube, near its edge. The position of the observer's head does not, therefore, interfere with the rays in their passage from the object into the tube. In Lord Rosse's large telescope, the second mirror is used for the sake of convenience. 5. The largest and most celebrated refracting telescopes at present existing, are those at Pulkowa, near St. Petersburg, in Russia, and at Cambridge, near Boston, in the United States of America. Each of these magnificent instruments has a clear aperture of the object-glass, amounting to about 15 inches, and a focal length of 22 feet: as they were both made by the same artists, Messrs. Merz and Mahler, of Munich, they are mounted in a similar manner. This mounting is, of course, that which has been described as equatorial or parallactic, but with a peculiarity common to foreign instruments. In English equatorial instruments the polar axis terminates in pivots, one of which is above and the others beneath the telescope; and the telescope, with its declination circle, revolves in the plane of the polar axis, on an axis called the declination axis, which is supported by the polar frame connecting the ends of the polar axis. On the contrary, in the foreign mounting, or Frauenhofer mounting, as it is generally called, the telescope revolves on one side or the other of the polar axis, which, therefore, does not require a length proportional to the length of the telescope, and greater strength and firmness are thus secured. There are, however, several inconveniences attending it, of which one is the necessity of reversing the instrument, or putting the telescope on the other side of the pier on which the instrument is supported, when the object under scrutiny arrives, by the diurnal motion, on the meridian. 6. Of reflecting telescopes, the most celebrated is that of Lord Rosse, at Parsonstown, in Ireland; and we doubt much whether there are many individuals of the United Kingdom who have not heard of its fame. It has been explained that the most important part of such an instrument is the object-mirror or speculum, which corresponds to the object-glass of refracting telescopes. The construction of very large telescopes, whether refracting or reflecting, is attended with very great trouble: in the former case, through the difficulty in procuring a piece of glass of the requisite size pure enough, or of sufficiently uniform substance, to let the rays of light pass properly through it at every part of the surface; and in the second case, by the very great difficulty attendant on the casting of a quantity of metal sufficient for making. specula of large diameters, and the extreme caution necessary in the processes gone through for insuring a good surface, which will receive and retain a high and uniform polish. 7. The mirrors of Lord Rosse's telescope are 6 feet in diameter, and of 53 feet focal length, and each of them weighs nearly four tons. It was only after a long and painful series of experiments, with a combination of different metals, that the proper alloy was at length discovered by his Lordship, which gave the greatest hardness and ductility combined. The works necessary for the casting and polishing of these enormous specula and for properly mounting them, so that they should not bend under their own weight in different positions, were on a vast scale, requiring the application of steam power, and the labour of a great many men for a long period. It would have been exceedingly difficult, and not attended with any adequate advantage, to attempt to give to this enormous telescope an equatorial mounting, admitting of unlimited motion on each side of the meridian, and this therefore has not been attempted. Instead of doing this, it moves chiefly in the plane of the meridian, but has a motion of several degrees on each side of it, sufficient for continuing the examination of any interesting object for the requisite length of time. 8. It will be proper to mention another reflecting telescope, because of the important discoveries which have been made with it, and to which we shall have occasion to refer. The telescope in question is that of Mr. Lassell, a merchant of Liverpool, erected at his residence at Stanfield near that town. The object-mirror of this telescope is 9 inches, and it was planned, cast, polished, and mounted by its proprietor, who has himself made many valuable discoveries with it, and has shown himself to be equally skilful as an observer and a mechanist, and to have united in himself all the qualities of a great astronomer. Another telescope (a refractor of 7 inches aperture) has been used with great effect, namely, that belonging to the Rev. W. R. Dawes, at Wateringbury, near Maidstone, and to this gentleman's discoveries also there will be frequent occasion to refer. 9. Having acquired a distinct notion of the construction of astronomical telescopes, and of the method of mounting them equatorially, so as to render the use of them easy and effective, the next consideration is their application for the extension of our knowledge of the peculiarities of the heavenly bodies. The eye-pieces are generally screwed into a tube, which can be pulled out farther from the object-glass, or pushed in towards it, so as to bring it into focus; that is, to render the image of the object under examination, which is formed at the focus of the object-glass, distinctly visible by the eye after the passage of the rays through the eye-piece. 10. If the object viewed be a planet, the edges of the disk ought to be smooth and distinct when the telescope is in focus, and, if the object be a star, it ought to be round and well defined. Owing to the effects of irradiation, the images of the large stars will not be reduced to points, but will present the appearance of disks. These disks are not real, but spurious, and arise from the want of exact convergence of all the rays which form the image into an exact point. Those telescopes are therefore best which give the smallest and neatest images of a large star. The disks of planets are also subject to the effects of irradiation, and appear larger than they really are, especially those which are very bright, such as Venus. Finally, to try whether the object-glass of a telescope is in good adjustment, push in or draw out the eye-tube, so as to put the telescope decidedly out of focus, and then look at a star of the first or second magnitude. The image will then have the appearance of a luminous ring, with a point in the centre. If this ring is complete, and of equal brightness in every part, the object-glass is rightly adjusted with respect to the tube, and the whole pencil of light passes through the eye-piece; but if the ring appear broken, the object-glass is tilted in the tube, and must be readjusted. 11. Close double stars, the distances of whose components are not above one or two seconds of space, are good tests of the defining power of large telescopes, and very faint stars are the proper tests of their space-penetrating power. If the object-glass be of an inferior description, the two stars composing the double star will appear blurred into one confused image, and very faint stars usually seen in good glasses of the same diameter will not be visible. The chief of the celestial objects subjected to scrutiny by the telescope, are- 1st. The sun, the moon, and the planets. 2nd. The comets. 3rd. The fixed stars and nebulæ. These classes we will take in their order. LESSON III. TELESCOPIC APPEARANCE OF SUN AND MOON. "God made two great lights, the greater light to rule by day and the lesser light to rule by night." 1. THE SUN.-For viewing the sun, directly, the eye must be protected by a coloured glass. Such glasses are either fastened separately (for different degrees of obscuration) into caps which screw on to the eye-piece, or are arranged in frames which slides across it. But an easier and pleasanter way of viewing the sun is to project the image upon a screen carried by the telescope, and the image will be still more striking if the dome or apartment in which the telescope is placed be previously darkened. Mr. Dawes has improved upon the method of viewing the sun directly, by inserting a perforated slide in the plane of the image formed by the objectglass the perforations are so minute as to allow only the : |