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dimensions. Nevertheless, much can be done with an instru. ment of even moderate size. Messier discovered some score of comets, and upwards of a hundred nebulæ and clusters, with a five-feet telescope. The great map of the moon-the standard one by Baer and Maedler—was delineated by an instrument of scarcely greater power. M. Goldschmidt has detected upwards of a dozen of planets with instruments ranging from a common opera-glass to telescopes of about four inches in aperture; but in this case a most extraordinary keenness of sight must aid the unwearied diligence and patience of the observer. Although Maedler states that only two hundred nebulæ can be detected with a telescope of five inches aperture, yet D’Arrest assures us that he can see that number with a common hand-telescope or comet-seeker, whilst with an instrument of four inches aperture upwards of a thousand are visible. A great deal, in such cases, depends upon practice; but a few precautions being taken, the amateur will, in a short time, be encouraged to proceed, by looking at the same objects at different times. Probably, he may notice that some stars which were pointed out to him as double, appear only as hazy and ill-defined single ones on one evening, whilst a few nights afterwards, with the sky apparently no clearer, they may be transformed by some magic process into what he had been originally apprized of. The state of the atmosphere exerts a wonderful influence on the definition of objects. Herschel, we believe, only allowed a hundred hours of superfine observing weather during the year. Secchi (at Rome) confesses he is more fortunate, and owns that under the cerulean sky of Italy he is favoured with a clear fifteen nights throughout the year. And yet those two observers have been constantly on the watch for those happy moments, not one of which, the latter tells us, should be lost, but the telescope at once directed to the faintest and minutest objects. We have ourselves found, that the best definition and brightness exists when the wind is from the damp southerly and westerly quarters, and the worst when from the dry north and east, particularly the latter. It is important to avoid placing the telescope at a window, especially when the atmosphere within is of a higher temperature than without; and Herschel goes so far as to say that no roof with an opening, or even shady places in the open air, should be allowed. The same authority tells us that sudden changes from frost to thaw, or vice versa, are bad for definition. Above all, good definition need not be expected near the horizon, where undulations, mists, and scintillations give a boiling motion at the best of times, and render observation impossible. It is necessary, above all things, to keep both the object-glasses and eye-pieces
palker, the its celturbed. Jenses a little uns
scrupulously clean; for this purpose a piece of old lawn is much preferable to silk, and some say a well-crumpled piece of tissuepaper to either. When the instrument comes from a good maker, the centering of the object-glass and its proper position in its cell has doubtless been duly taken care of, and should not be disturbed. Sometimes, however, the glasses of both the object- and eye-lenses are pinched too tightly, which distorts the images fearfully ; a little unscrewing will at once set this to rights.
The ordinary achromatic refractor is composed of simply the object- and eye-lens, without any intermediate glass. This latter, however, like the former, consists generally of two separate lenses, although both are of the same sort of glass. The negative eye-piece consists of two plano-convex lenses, the plane sides being turned towards the object-glass, and when made of crown glass, separated by a distance equal to half the sum of their focal lengths. When measuring apparatus is attached to the telescope, an eye-piece is employed, formed of two plano-convex lenses of equal focus, separated by a distance equal to two-thirds of the focal length of one or other and their convexities opposed in the tube. This is called Ramsden's eye-piece, and gives a flat field of view, with the advantage already mentioned that a system of spider lines, or cross wires, can be distinctly seen at the same time as the double star, or planet, whose distance or diameter is to be measured. Both of those are inverting, but this is of small account in looking at the heavenly bodies. The erecting eye-piece, consisting of four lenses, although admirably adapted for land use, is never made use of for the latter purpose, as each lens tends to obstruct the light which is so precious when gathered to a focus. It need scarcely be added that the eye-piece must be adjusted with the greatest possible nicety for different sights. This is easily done by the rack and pinion motion after a few trials, whether upon the edge of a planet or by judging of the appearance of a double star, or by the visibility of the faintest single stars in the field of view.
With an instrument of three to four inches aperture (and a good one of the latter size will bear a magnifying power of from 300 to 350) the belts and markings on Jupiter, the snows of Mars, the ring of Saturn, may be well seen; and the spots and faculæ of the sun, and all the gorgeous phenomena of the moon, rendered most perfectly. The nebula of Orion surrounding the trapezium will likewise amply repay curiosity on clear dark nights. Some hundreds of double stars-shining in all their different lustres, colours, and distances—are a constant source of pleasure. In addition to these, many clusters and nebulæ may be seen with such an instrument. Mr. Dawes has been able to separate some exceedingly close double stars, and observe some very faint ones with such a telescope. The amateur will soon get accustomed to the difficulty of the various objects, and know what powers to apply, according to the fineness of the night or other circumstances. Admiral Smyth-one of the most successful observers of the age-advises the amateur not to be afraid of his small instrumental means, but to look at everything; and it is certain that with some costly apparatus much less is seen than with less pretentious affairs.
For small instruments, the ordinary tripod stand, with slow motion in altitude and azimuth, is as convenient as any, where merely viewing the object is all that is required. For larger ones, where measurements of the positions and distances of double stars are to be obtained, the telescope is mounted equatorially--the support of the instrument, instead of being pointed to the zenith, being placed parallel with the axis of the earth-by which means, by merely shifting the telescope to the right, the star is always in the field of view without any vertical motion. To most large instruments strong clockwork is attached, by which this motion to the west is duly regulated, and the observer has only to keep his eye to the telescope, which of itself follows the star. Although in the last century and the time of Sir W. Herschel reflectors of from six to ten feet were greatly in use, yet of late years, in the case of small telescopes, it has been almost completely superseded by the refractor, and on that account we have confined our observations to this most convenient description of optic tube. Unless extreme care is used with a reflector, the metal mirror is very liable to become tarnished, whilst dew or dust settling on a glass lens can easily be wiped off with a soft cloth. In: France, however, small reflectors are again coming into vogue, the mirror being formed of glass and coated with a thin film of silver; and as the latter has a reflective power of 91 to 67, in comparison with speculum metal, it has a great advantage in this respect; but, unfortunately, tarnishes very readily. As, however, the silver film is easily renewed, this is of the less consequence. We had the pleasure of inspecting several instruments of this description by the celebrated M. Foucault, at Secretan's establishment at Paris, some time since, which were beautifully fitted up. A reflector of about forty inches aperture has since been made by those gentlemen, and as it was with one of those glass mirrors that the companion of Sirius has been detected, it may give an idea of their merit.
The largest refracting achromatic telescope we have seen, and which we believe to be at present the largest in the world,
is that made by M. Porro, and which still remains at Paris. The object-glass is 54 centimetres in diameter (21.26 inches) and has the immense focal length of 15 metres, or nearly 50 feet. It is mounted with an azimuth and altitude motion, and at first sight looks like a gigantic fire-escape. The tube is composed of three hundred pieces of wood joined together by three thousand screws. Great controversy has arisen respecting the merits of this instrument; but M. Bulard, the astronomer at Algiers, asserts that he could see everything with it that he had with Earl Rosse's 6 feet mirror; and M. Porro states that he has detected two new stars within the trapezium of Orion. It is certain, however, that although in existence for six or eight years, it has been very little used. Comparing this immense instrument with the modest five-feet telescope of M. Goldschmidt, erected on a moveable tripod stand in his garden near Paris, and with which he has discovered so many planets and variable stars and nebulæ, and made so many curious observations, it will be seen what may be done with care and patience even with small means.
Should the observer be provided with a telescope mounted equatorially, he has simply to set the declination and hour circles to the object required, and of which he has of course the right ascension and polar distance given. In other cases, in searching for any particular object, he must make use of an ordinary star-map divided in the usual manner into right ascensions and declinations, on which having marked the position of the required nebula, comet, or double star, he guides the direction of the telescope by means of the bright neighbouring stars of the constellation in which the object is placed ; and with a little management this operation will soon become easy. In the same way, to find any particular planet, its place on the required day will be taken from the “Nautical Almanac,” or any similar work. Great assistance in the choice of stellar objects will be derived from Admiral Smyth’s “Celestial Cycle," in which the most prominent clusters, nebulæ, double and variable stars are fully described, and full particulars given of their positions, brightness, and difficulty. In pointing the telescope, the observer makes use of the small side tube, termed the finder, which, giving a large field of view, helps him in the alignement of the faintest stars marked on the map. In searching for comets, the lowest power of the telescope should be made use of; and if a nebulous object be caught sight of, its place should be transferred to the map, in order to determine whether any fixed body of that character has been before observed. In all cases it is almost useless to search for objects with high powers, which not only gives a small field but renders them very faint. It will be found that the eye will distinguish faint objects much more readily by keeping in the dark a few minutes previously. The observer will remark some anomalies in different eyesight; some which can distinguish very faint objects are unable to separate a close double star, and vice versa, and in many instances the colours of double stars will be judged differently. We need not state that a comparatively small telescope, in which the glass is pure and the figure polished to the true curves, will be found far more useful and pleasant to handle than a much larger one in which these conditions are forgotten. The excellence of the object-glass is the principal affair, and no beauty of mounting will compensate for this want.
In observing double stars, those which are well separated should be scrutinized at the commencement and the distances and magnitudes gradually diminished, until the most delicate objects which the instrument can bear are arrived at, and for which a very clear and favourable sky will of course be necessary. The observations of the double and multiple stars are the finest test for the performance of a telescope. Thus, commencing at the coarse double Delta and Kappa Herculis or Eta Lyræ, he may ascend to Theta Serpentis and Alpha Can. Venat. From Zeta Ursæ Majoris and Beta Cephei he can pass to Gamma Delphini and Eta Cassiopeiæ. Closer still are Alpha Herculis, Kappa Cephei, and Mu Cygni. Coming to those which are test objects for small instruments we have Gamma Virginis, Mu Draconis, Rho Herculis, Zeta Aquarii, &c., whilst on very favourable nights the observer may look for Delta Cygni, Pi Aquilæ, Xi Ursa Majoris, Sigma Coronæ Borealis. All these stars are visible at the present time of the year, and their places marked on a good celestial globe. The amateur observer has only to transfer those positions mentally to the real heavens, which, with a little practice, will be as simple as finding any geographical position on a terrestrial globe by knowing its longitude and latitude..