the object-glass to that of the eye-piece giving its exact amount; that is to say, if the focus of the object-glass is 100 inches, and that of the eye-piece one inch, the telescope will magnify 100 times. Bearing in mind that what an astronomer wants is a good clear image of the object observed, we shall at once recognise that magnifying power depends upon the perfection of the image thrown by the object-glass and upon the illuminating power, of which we have already spoken. If the objectglass does not perform its part properly, a slight magnification blurs the image, and the telescope is useless. Hence many large telescopes are inferior to much smaller ones in the matter of magnifying power, although their illuminating power is so much greater. 472. The eye-pieces used with the astronomical telescope vary in form. The telescope made by Galileo, similar in construction to the modern opera-glass, was furnished with a bi-concave eye-piece. As the action of the eye-piece is to render the rays parallel, this eye-piece is used between the object-glass and the focus, at a point where its divergent action (Art. 462) corrects the convergent action of the object-glass. A convex eye-piece for the same reason is placed outside the focus, as shown in Fig. 48. Such eye-pieces, however, colour the light coming from the image in the same way as the object-glass would colour the light going to form the image, if its chromatic aberration were not corrected. 473. It was discovered by Huyghens, however, that this defect might be obviated in the case of the eye-piece by employing two plano-convex lenses, the flat sides next the eye, a larger one nearest the image, called the field-lens, and a smaller one near the eye, called the eye-lens. This construction is generally used, except for micrometers (Art. 519), a name given to an eye-piece with spider-webs in the focus of the eye-piece for measuring the sizes of the different objects. In this case the flat sides are turned away from the eye. 474. The telescope-tube keeps the object-glass and the eye-piece in their proper positions, and the eye-piece is furnished with a draw-tube, which allows its distance from the object-glass to be varied. LESSON XXXVIII. THE TELESCOPE (continued). POWERS OF TELESCOPES of DifferENT APERTURES. LARGE TELESCOPES. METHODS OF MOUNTING THE EQUATORIAL TELESCOPE. 475. Very many of the phenomena of the heavens may be seen with a small telescope. In our climate a telescope with an object-glass of six inches' aperture is probably the size which will be found the most constantly useful; a larger aperture being frequently not only useless, but hurtful. Still, 44 or 3 inches are useful apertures, and if furnished with object-glasses, made of course by the best makers, views of the sun, moon, planets, and double stars may be obtained sufficiently striking to set many seriously to work as amateur observers. Thus, in the matter of double stars, a telescope of two inches' aperture, with powers varying from 60 to 100, will show the following stars double : A 4-inch aperture, powers 80-120, reveals the duplicity of-B Orionis. a Lyræ. € Hydræ. έ Ursa Majoris. 8 Geminorum. ☛ Cassiopeæ. € Draconis. 476. Observations should always be commenced with the lowest power, or eye-piece, gradually increasing it until the limit of the aperture, or of the atmospheric condition at the time, is reached the former being taken as equal to the number of hundredths of inches which the diameter of the object-glass contains. Thus, 3-inch object-glass, if really good, should bear a power of 375 on double stars where light is no object; the planets, the moon, &c. will be best observed with a much lower power. 477. In the case of stars, owing to their immense distance, no increase in their size follows the application of higher magnifiers. With planets this is different, each increase of power increases the size of the image, and therefore decreases its brilliancy, as the light is spread over a larger area. Hence the magnifying power of a good telescope is always much higher for stars than for planets, although at the best it is always limited by the state of the air at the time of observation. 478. It is always more or less dangerous to look at the Sun directly with a telescope of any aperture above two inches, as the dark glasses, without which the observer would be at once blinded, are apt to melt and crack. A diagonal reflector, however, which reflects an extremely small percentage of light to the eye, and by reason of its prismatic form refracts the rest away from the telescope, affords a very handy method of solar observation. Care should be taken that the object-glass is properly adjusted. This may be done by observing the image of a large star out of focus. If the light be not equally distributed over the image, or the circles of light which are always seen in a good telescope are not perfectly circular, the telescope should be sent back to the optician for adjustment. 479. The testing of a good glass refers to two different qualities which it should possess. Its quality, as to material and the fineness of its polish, should be such that the maximum of light shall be transmitted. Its quality as to the curves should be such that the rays passing through every part of its area shall converge absolutely to the same point, with a chromatic aberration sufficient to surround objects with a faint dark blue light. 480. To give an idea of the great accuracy with which a fine object-glass refracts the light transmitted, we will take for example an object-glass of 8 inches' aperture and 10 feet focal length, which, if a fine one, will separate the components of y2 Andromedæ, whose angular distance is about half a second- that is, it will depict at its focus two minute discs of light fairly separated, the distance of whose centres, as above stated, is half a second. To come at the value of this half-second, as measured on a scale of inches and parts, we must consider the centre of the object-glass to be the centre of a circle, whose radius is the focal length of the object-glass. The focal value of a degree of such a circle is 2°0944, or nearly 2 inches; of a minute, '0349 of an inch; of a second, 0005818, or 3000 of an inch nearly; of half a second, '0002909 inch, which is little more than the fourth part of the one-thousandth of an inch. Light from a fixed star passing through four refracting surfaces, and half an inch or more in thickness of glass, and filling 50 square inches of surface, and travelling 120 inches down the tube, is so accurately concentrated at the focal point as to all pass through the smallest hole that could be made with the most delicate needle-point through a piece of fine paper. This requires a degree of accuracy in the figuring and polishing of the material of the lenses almost inconceivable. 481. We have so far confined our attention to the principles of the ordinary astronomical telescope, and we have dealt with it in its simplest form. There are other kinds; the construction of some of which depends upon reflection; that is to say, the light is reflected by a concave mirror instead of being refracted by a lens; but we need not dwell upon them. Let us next inquire what the very largest telescope really can do. The largest refractor— as the refracting telescopes are called-in the world has just been completed by Messrs. Cooke and Sons, English opticians of great eminence. The object-glass is 25 inches in diameter. Now, the pupil of our eye is th of an inch in diameter: this object-glass, therefore, will grasp 15,000 times more light than the eye can: if used when the air is pure, it should easily bear a power of 3,000 on the Moon; in other words, the Moon will appear as it would were it 3,000 times nearer to us, or at a distance of 80 miles, instead of, roughly, 240,000; measuring from the centres of the Earth and Moon, and not from their surfaces. The largest reflector in the world has been constructed by the late Earl of Rosse; its mirror, or speculum, is six feet in diameter, and its illuminating power is such that it enables us to see, 66 as clearly as the heavens shine to us on a cloudless evening, the details of a starry universe, stretching into space five hundred times further than those depths at which we are accustomed to gaze almost in oppressive silence."* 482. An astronomer wants telescopes for two kinds of work he wants to watch the heavenly bodies, and study their physical constitution; and he wants to note their actual places and relative positions; so that he mounts or arranges his telescope in several different ways. * Nichol. |