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    Half hours with the Telescope
    Half hours with the Telescope
    Half hours with the Telescope
    Ebook124 pages1 hour

    Half hours with the Telescope

    By Richard A. Proctor

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    About this ebook

    The object which the Author and Publisher of this little work have proposed to themselves, has been the production, at a moderate price, of a useful and reliable guide to the amateur telescopist.
    Among the celestial phenomena described or figured in this treatise, by far the larger number may be profitably examined with small telescopes, and there are none which are beyond the range of a good 3-inch achromatic.
    The work also treats of the construction of telescopes, the nature and use of starmaps, and other subjects connected with the requirements of amateur observers.
    LanguageEnglish
    PublisherBooks on Demand
    Release dateMar 11, 2019
    ISBN9783749432066
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      Book preview

      Half hours with the Telescope - Richard A. Proctor

      Half hours with the Telescope

      Half hours with the Telescope

      PREFACE.

      CHAPTER I.

      CHAPTER II.

      CHAPTER III.

      CHAPTER IV.

      CHAPTER IV B

      CHAPTER V.

      CHAPTER VI.

      CHAPTER VII.

      FOOTNOTES:

      Copyright

      Half hours with the Telescope

      Richard A. Proctor

      PREFACE.

      The object which the Author and Publisher of this little work have proposed to themselves, has been the production, at a moderate price, of a useful and reliable guide to the amateur telescopist.

      Among the celestial phenomena described or figured in this treatise, by far the larger number may be profitably examined with small telescopes, and there are none which are beyond the range of a good 3-inch achromatic.

      The work also treats of the construction of telescopes, the nature and use of starmaps, and other subjects connected with the requirements of amateur observers.

      R.A.P.

      CHAPTER I.

      A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE.

      There are few instruments which yield more pleasure and instruction than the Telescope. Even a small telescope—only an inch and a half or two inches, perhaps, in aperture—will serve to supply profitable amusement to those who know how to apply its powers. I have often seen with pleasure the surprise with which the performance even of an opera-glass, well steadied, and directed towards certain parts of the heavens, has been witnessed by those who have supposed that nothing but an expensive and colossal telescope could afford any views of interest. But a well-constructed achromatic of two or three inches in aperture will not merely supply amusement and instruction,—it may be made to do useful work.

      The student of astronomy is often deterred from telescopic observation by the thought that in a field wherein so many have laboured, with abilities and means perhaps far surpassing those he may possess, he is little likely to reap results of any utility. He argues that, since the planets, stars, and nebulæ have been scanned by Herschel and Rosse, with their gigantic mirrors, and at Pulkova and Greenwich with refractors whose construction has taxed to the utmost the ingenuity of the optician and mechanic, it must be utterly useless for an unpractised observer to direct a telescope of moderate power to the examination of these objects.

      Now, passing over the consideration that a small telescope may afford its possessor much pleasure of an intellectual and elevated character, even if he is never able by its means to effect original discoveries, two arguments may be urged in favour of independent telescopic observation. In the first place, the student who wishes to appreciate the facts and theories of astronomy should familiarize himself with the nature of that instrument to which astronomers have been most largely indebted. In the second place, some of the most important discoveries in astronomy have been effected by means of telescopes of moderate power used skilfully and systematically. One instance may suffice to show what can be done in this way. The well-known telescopist Goldschmidt (who commenced astronomical observation at the age of forty-eight, in 1850) added fourteen asteroids to the solar system, not to speak of important discoveries of nebulæ and variable stars, by means of a telescope only five feet in focal length, mounted on a movable tripod stand.

      The feeling experienced by those who look through a telescope for the first time,—especially if it is directed upon a planet or nebula—is commonly one of disappointment. They have been told that such and such powers will exhibit Jupiter's belts, Saturn's rings, and the continent-outlines on Mars; yet, though perhaps a higher power is applied, they fail to detect these appearances, and can hardly believe that they are perfectly distinct to the practised eye.

      The expectations of the beginner are especially liable to disappointment in one particular. He forms an estimate of the view he is to obtain of a planet by multiplying the apparent diameter of the planet by the magnifying power of his telescope, and comparing the result with the apparent diameter of the sun or moon. Let us suppose, for instance, that on the day of observation Jupiter's apparent diameter is 45, and that the telescopic power applied is 40, then in the telescope Jupiter should appear to have a diameter of 1800, or half a degree, which is about the same as the moon's apparent diameter. But when the observer looks through the telescope he obtains a view—interesting, indeed, and instructive—but very different from what the above calculation would lead him to expect. He sees a disc apparently much smaller than the moon's, and not nearly so well-defined in outline; in a line with the disc's centre there appear three or four minute dots of light, the satellites of the planet; and, perhaps, if the weather is favourable and the observer watchful, he will be able to detect faint traces of belts across the planet's disc.

      Yet in such a case the telescope is not in fault. The planet really appears of the estimated size. In fact, it is often possible to prove this in a very simple manner. If the observer wait until the planet and the moon are pretty near together, he will find that it is possible to view the planet with one eye through the telescope and the moon with the unaided eye, in such a manner that the two discs may coincide, and thus their relative apparent dimensions be at once recognised. Nor should the indistinctness and incompleteness of the view be attributed to imperfection of the telescope; they are partly due to the nature of the observation and the low power employed, and partly to the inexperience of the beginner.

      It is to such a beginner that the following pages are specially addressed, with the hope of affording him aid and encouragement in the use of one of the most enchanting of scientific instruments,—an instrument that has created for astronomers a new sense, so to speak, by which, in the words of the ancient poet:

      Subjecere oculis distantia sidera nostris,

      Ætheraque ingenio supposuere suo.

      In the first place, it is necessary that the beginner should rightly know what is the nature of the instrument he is to use. And this is the more necessary because, while it is perfectly easy to obtain such knowledge without any profound acquaintance with the science of optics, yet in many popular works on this subject the really important points are omitted, and even in scientific works such points are too often left to be gathered from a formula. When the observer has learnt what it is that his instrument is actually to do for him, he will know how to estimate its performance, and how to vary the application of its powers—whether illuminating or magnifying—according to the nature of the object to be observed.

      Let us consider what it is that limits the range of natural vision applied to distant objects. What causes an object to become invisible as its distance increases? Two things are necessary that an object should be visible. It must be large enough to be appreciated by the eye, and it must send light enough. Thus increase of distance may render an object invisible, either through diminution of its apparent size, or through diminution in the quantity of light it sends to the eye, or through both these causes combined. A telescope, therefore, or (as its name implies) an instrument to render distant objects visible, must be both a magnifying and an illuminating instrument.

      Fig. 1.

      Let EF, fig. 1 , be an object, not near to AB as in the figure, but so far off that the bounding lines from A and B would meet at the point corresponding to the point P. Then if a large convex glass AB (called an object-glass ) be interposed between the object and the eye, all those rays which, proceeding from P, fall on AB, will be caused to converge nearly to a point p . The same is true for every point of the object EMF, and thus a small image, emf , will be formed. This image will not lie exactly on a flat surface, but will be curved about the point midway between A and B as a centre. Now if the lens AB is removed, and an eye is placed at m to view the distant object EMF, those rays only from each point of the object which fall on the pupil of the eye (whose diameter is about equal to mp suppose) will serve to render the object visible. On the other hand, every point of the image emf has received the whole of the light gathered up by the large glass AB. If then we can only make this light available , it is clear that we shall have acquired a large increase of light from the distant object. Now it will be noticed that the light which has converged to p , diverges from p so that an eye, placed that this diverging pencil of rays may fall upon it, would be too small to receive the whole of the pencil. Or, if it did receive the whole of this pencil, it clearly could not receive the whole of the pencils proceeding from other parts of the image emf . Something would be gained, though, even in this case, since it is clear that an eye thus placed at a distance of ten inches from emf (which is

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