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THE TELESCOPE—PRACTICAL

Having thus briefly sketched the history of the telescope, we turn now to consider the optical means which are most likely to be in the hands or within the reach of the beginner in astronomical observation. Let us, first of all, make the statement that any telescope, good, bad, or indifferent, is better than no telescope. There are some purists who would demur to such a statement, who make the beginner's heart heavy with the verdict that it is better to have no telescope at all than one that is not of the utmost perfection, and, of course, of corresponding costliness, and who seem to believe that the performance of an inferior glass may breed disgust at astronomy altogether. This is surely mere nonsense. For most amateurs at the beginning of their astronomical work the question is not between a good telescope and an inferior one, it is between a telescope and no telescope. Of course, no one would be so foolish as willingly to observe with an inferior instrument if a better could be had; but even a comparatively poor glass will reveal much that is of great interest and beauty, and its defects must even be put up with sometimes for the sake of its advantages until something more satisfactory can be obtained. An instrument which will show fifty stars where the naked eye sees five is not to be despised, even though it may show wings to Sirius that have no business there, or a brilliant fringe of colours round Venus to which even that beautiful planet can lay no real claim. Galileo's telescope would be considered a shockingly bad instrument nowadays; still, it had its own little influence upon the history of astronomy, and the wonders which it first revealed are easily within the reach of anyone who has the command of a shilling or two, and, what is perhaps still more important, of a little patience. The writer has still in his possession an object-glass made out of a simple single eyeglass, such as is worn by Mr. Joseph Chamberlain. This, mounted in a cardboard tube with another single lens in a sliding tube as an eye-piece, proved competent to reveal the more prominent lunar craters, a number of sunspots, the phases of Venus, and the existence, though not the true form, of Saturn's ring. Its total cost, if memory serve, was one shilling and a penny. Of course it showed, in addition, a number of things which should not have been seen, such as a lovely border of colour round every bright object; but, at the same time, it gave a great deal more than thirteen pence worth of pleasure and instruction.

Furthermore, there is this to be said in favour of beginning with a cheap and inferior instrument, that experience may thus be gained in the least costly fashion. The budding astronomer is by nature insatiably curious. He wants to know the why and how of all the things that his telescope does or does not do. Now this curiosity, while eminently laudable in itself, is apt in the end to be rather hard upon his instrument. A fine telescope, whatever its size may be, is an instrument that requires and should receive careful handling; it is easily damaged, and costly to replace. And therefore it may be better that the beginner should make his earlier experiments, and find out the more conspicuous and immediately fatal of the many ways of damaging a telescope, upon an instrument whose injury, or even whose total destruction, need not cause him many pangs or much financial loss.

It is not suggested that a beginning should necessarily be made on such a humble footing as that just indicated. Telescopes of the sizes mainly referred to in these pages—i.e., refractors of 2 or 3 inches aperture, and reflectors of 4½ to 6 inches—may frequently be picked up second-hand at a very moderate figure indeed. Of course, in these circumstances the purchaser has to take his chance of defects in the instrument, unless he can arrange for a trial of it, either by himself, or, preferably, by a friend who has some experience; yet even should the glass turn out far from perfect, the chances are that it will at least be worth the small sum paid for it. Nor is it in the least probable, as some writers seem to believe, that the use of an inferior instrument will disgust the student and hinder him from prosecuting his studies. The chances are that it will merely create a desire for more satisfactory optical means. Even a skilled observer like the late Rev. T. W. Webb had to confess of one of his telescopes that 'much of its light went the wrong way'; and yet he was able to get both use and pleasure out of it. The words of a well-known English amateur observer may be quoted. After detailing his essays with glasses of various degrees of imperfection Mr. Mee remarks: 'For the intending amateur I could wish no other experience than my own. To commence with a large and perfect instrument is a mistake; its owner cannot properly appreciate it, and in gaining experience is pretty sure to do the glass irreparable injury.'

Should the beginner not be willing or able to face the purchase of even a comparatively humble instrument, his case is by no means desperate, for he will find facilities at hand, such as were not thought of a few years ago, for the construction of his own telescope. Two-inch achromatic object-glasses, with suitable lenses for the making up of the requisite eye-pieces, are to be had for a few shillings, together with cardboard tubes of sizes suitable for fitting up the instrument; and such a volume as Fowler's 'Telescopic Astronomy' gives complete directions for the construction of a glass which is capable of a wonderful amount of work in proportion to its cost. The substitution of metal tubes for the cardboard ones is desirable, as metal will be found to be much more satisfactory if the instrument is to be much used. The observer, however, will not long be satisfied with such tools as these, useful though they may be. The natural history of amateur astronomers may be summed up briefly in the words 'they go from strength to strength.' The possessor of a small telescope naturally and inevitably covets a bigger one; and when the bigger one has been secured it represents only a stage in the search for one bigger still, while along with the desire for increased size goes that for increased optical perfection. No properly constituted amateur will be satisfied until he has got the largest and best instrument that he has money to buy, space to house, and time to use.

Let us suppose, then, that the telescope has been acquired, and that it is such an instrument as may very commonly be found in the hands of a beginner—a refractor, say, of 2, 2½, or 3 inches aperture (diameter of object-glass). The question of reflectors will fall to be considered later. Human nature suggests that the first thing to do with it is to unscrew all the screws and take the new acquisition to pieces, so far as possible, in order to examine into its construction. Hence many glasses whose career of usefulness is cut short before it has well begun. 'In most cases,' says Webb, 'a screw-driver is a dangerous tool in inexperienced hands'; and Smyth, in the Prolegomena to his 'Celestial Cycle,' utters words of solemn warning to the 'over-handy gentlemen who, in their feverish anxiety for meddling with and making instruments, are continually tormenting them with screw-drivers, files, and what-not.' Unfortunately, it is not only the screw-driver that is dangerous; the most deadly danger to the most delicate part of the telescope lies in the unarmed but inexperienced hands themselves. You may do more irreparable damage to the object-glass of your telescope in five minutes with your fingers than you are likely to do to the rest of the instrument in a month with a screw-driver. Remember that an object-glass is a work of art, sometimes as costly as, and always much more remarkable than, the finest piece of jewellery. It may be unscrewed, carefully, from the end of its tube and examined. Should the examination lead to the detection of bubbles or even scratches in the glass (quite likely the latter if the instrument be second-hand), these need not unduly vex its owner's soul. They do not necessarily mean bad performance, and the amount of light which they obstruct is very small, unless the case be an extreme one. But on no account should the two lenses of the object-glass itself be separated, for this will only result in making a good objective bad and a bad one worse. The lenses were presumably placed in their proper adjustment to one another by an optician before being sent out; and should their performance be so unsatisfactory as to suggest that this adjustment has been disturbed, it is to an optician that they should be returned for inspection. The glass may, of course, be carefully and gently cleaned, using either soft chamois leather, or preferably an old silk handkerchief, studiously kept from dust; but the cleaning should never amount to more than a gentle sweeping away of any dust which may have gathered on the surface. Rubbing is not to be thought of, and the man whose telescope has been so neglected that its object-glass needs rubbing should turn to some other and less reprehensible form of mischief. For cleaning the small lenses of the eye-pieces, the same silk may be employed; Webb recommends a piece of blotting-paper, rolled to a point and aided by breathing, for the edges which are awkward to get at. Care must, of course, be taken to replace these lenses in their original positions, and the easiest way to ensure this is to take out only one at a time. In replacing them, see that the finger does not touch the surface of the glass, or the cleaning will be all to do over again.

FIG. 8.

a, O.G. in perfect adjustment; b, O.G. defectively centred.

Next comes the question of testing the quality of the objective. (The stand is meanwhile assumed, but will be spoken of later.) Point the telescope to a star of about the third magnitude, and employ the eye-piece of highest power, if more than one goes with the instrument—this will be the shortest eye-piece of the set. If the glass be of high quality, the image of the star will be a neat round disc of small size, surrounded by one or two thin bright rings (Fig. 8, a). Should the image be elliptical and the rings be thrown to the one side (Fig. 8, b), the glass may still be quite a good one, but is out of square, and should be readjusted by an optician. Should the image be irregular and the rings broken, the glass is of inferior quality, though it may still be serviceable enough for many purposes. Next throw the image of the star out of focus by racking the eye-piece in towards the objective, and then repeat the process by racking it again out of focus away from the objective. The image will, in either case, expand into a number of rings of light, and these rings should be truly circular, and should present precisely the same appearance at equal distances within and without the focus. A further conception of the objective's quality may be gained by observing whether the image of a star or the detail of the moon or of the planets comes sharply to a focus when the milled head for focussing is turned. Should it be possible to rack the eye-tube in or out for any distance without disturbing the distinctness of the picture to any extent, then the glass is defective. A good objective will admit of no such range, but will come sharply up to focus, and as sharply away from it, with any motion of the focussing screw. A good glass will also show the details of a planet like Saturn, such as are within its reach, that is, with clearness of definition, while an inferior one will soften all the outlines, and impart a general haziness to them. The observer may now proceed to test the colour correction of his objective. No achromatic, its name notwithstanding, ever gives an absolutely colourless image; all that can be expected is that the colour aberration should have been so far eliminated as not to be unpleasant. In a good instrument a fringe of violet or blue will be seen around any bright object, such as Venus, on a dark sky; a poor glass will show red or yellow. It is well to make sure, however, should bad colour be seen, that the eye-piece is not causing it; and, therefore, more than one eye-piece should be tried before an opinion is formed. Probably more colour will be seen at first than was expected, more particularly with an object so brilliant as Venus. But the observer need not worry overmuch about this. He will find that the eye gets so accustomed to it as almost to forget that it is there, so that something of a shock may be experienced when a casual star-gazing friend, on looking at some bright object, remarks, as friends always do, 'What beautiful colours!' Denning records a somewhat extreme case in which a friend, who had been accustomed to observe with a refractor, absolutely resented the absence of the familiar colour fringe in the picture given by a reflector, which is the true achromatic in nature, though not in name. The beginner is recommended to read the article 'The Adjustment of a Small Equatorial,' by Mr. E. W. Maunder, in the Journal of the British Astronomical Association, vol. ii., p. 219, where he will find the process of testing described at length and with great clearness.

In making these tests, allowance has, of course, to be made for the state of the atmosphere. A good telescope can only do its best on a good night, and it is not fair to any instrument to condemn it until it has been tested under favourable conditions. The ideal test would be to have its performance tried along with that of another instrument of known good quality and of as nearly the same size as possible. If this cannot be arranged for, the tests must be made on a succession of nights, and good performance on one of these is sufficient to vindicate the reputation of the glass, and to show that any deficiency on other occasions was due to the state of the air, and not to the instrument. Should his telescope pass the above tests satisfactorily, the observer ought to count himself a happy man, and will until he begins to hanker after a bigger instrument.

The mention of the pointing of the telescope to a star brings up the question of how this is to be done. It seems a simple thing; as a matter of fact, with anything like a high magnifying power it is next to impossible; and there are few things more exasperating than to see a star or a planet shining brightly before your eyes, and yet to find yourself quite unable to get it into the field of view. The simple remedy is the addition of a finder to the telescope. This is a small telescope of low magnifying power which is fastened to the larger instrument by means of collars bearing adjusting screws, which enable it to be laid accurately parallel with the large tube (Fig. 10). Its eye-piece is furnished with cross-threads, and a star brought to the intersection of these threads will be in the field of the large telescope. In place of the two threads crossing at right angles there may be substituted three threads interlacing to form a little triangle in the centre of the finder's field. By this device the star can always be seen when the glass is being pointed instead of being hidden, as in the other case, behind the intersection of the two threads. A fine needle-point fixed in the eye-piece will also be found an efficient substitute for the cross-threads. In the absence of a finder the telescope may be pointed by using the lowest power eye-piece and substituting a higher one when the object is in the field; but beyond question the finder is well worth the small addition which it makes to the cost of an instrument. A little care in adjusting the finder now and again will often save trouble and annoyance on a working evening.

The question of a stand on which to mount the telescope now falls to be considered, and is one of great importance, though apt to be rather neglected at first. It will soon be found that little satisfaction or comfort can be had in observing unless the stand adopted is steady. A shaky mounting will spoil the performance of the best telescope that ever was made, and will only tantalize the observer with occasional glimpses of what might be seen under better conditions. Better have a little less aperture to the object-glass, and a good steady mounting, than an extra inch of objective and a mounting which robs you of all comfort in the using of your telescope. Beginners are indeed rather apt to be misled into the idea that the only matters of importance are the objective and its tube, and that money spent on the stand is money wasted. Hence many fearful and wonderful contrivances for doing badly what a little saved in the size of the telescope and expended on the stand would have enabled them to do well. It is very interesting, no doubt, to get a view of Jupiter or Saturn for one field's-breadth, and then to find, on attempting to readjust the instrument for another look, that the mounting has obligingly taken your star-gazing into its own hands, and is now directing your telescope to a different object altogether; but repetition of this form of amusement is apt to pall. A radically weak stand can never be made into a good one; the best plan is to get a properly proportioned mounting at once, and be done with it.

FIG. 9.—SMALL TELESCOPE ON PILLAR AND CLAW STAND.

For small instruments, such as we are dealing with, the mounting generally adopted is that known as the Altazimuth, from its giving two motions, one in altitude and one in azimuth, or, to use more familiar terms, one vertical and the other horizontal. There are various types of the Altazimuth. If the instrument be of not more than 3 feet focal length, the ordinary stand known as the 'pillar and claw' (Fig. 9) will meet all the requirements of this form of motion. Should the focal length be greater than 3 feet, it is advisable to have the instrument mounted on a tripod stand, such as is shown in Fig. 10. In the simpler forms of both these mountings the two motions requisite to follow an object must be given by hand, and it is practically impossible to do this without conveying a certain amount of tremor to the telescope, which disturbs clearness of vision until it subsides, by which time the object to be viewed is generally getting ready to go out of the field again. To obviate this inconvenience as far as possible, the star or planet when found should be placed just outside the field of view, and allowed to enter it by the diurnal motion of the earth. The tremors will thus have time to subside before the object reaches the centre of the field, and this process must be repeated as long as the observation continues. In making this adjustment attention must be paid to the direction of the object's motion through the field, which, of course, varies according to its position in the sky. If it be remembered that a star's motion through the telescopic field is the exact reverse of its true direction across the sky, little difficulty will be found, and use will soon render the matter so familiar that the adjustment will be made almost automatically.

FIG. 10.—TELESCOPE ON TRIPOD, WITH FINDER AND SLOW MOTIONS.

A much more convenient way of imparting the requisite motions is by the employment of tangent screws connected with Hooke's joint-handles, which are brought conveniently near to the hands of the observer as he sits at the eye-end. These screws clamp into circles or portions of circles, which have teeth cut on them to fit the pitch of the screw, and by means of them a slow and steady motion may be imparted to the telescope. When it is required to move the instrument more rapidly, or over a large expanse of sky, the clamps which connect the screws with the circles are slackened, and the motion is given by hand. Fig. 10 shows an instrument provided with these adjuncts, which, though not absolutely necessary, and adding somewhat to the cost of the mounting, are certainly a great addition to the ease and comfort of observation.

FIG. 11.—EQUATORIAL MOUNTING FOR SMALL TELESCOPE.

The Altazimuth mounting, from its simplicity and comparative cheapness, has all along been, and will probably continue to be, the form most used by amateurs. It is, however, decidedly inferior in every respect to the equatorial form of mount. In this form (Fig. 11) the telescope is carried by means of two axes, one of which—the Polar axis—is so adjusted as to be parallel to the pole of the earth's rotation, its degree of inclination being therefore dependent upon the latitude of the place for which it is designed. At the equator it will be horizontal, will lie at an angle of 45 degrees half-way between the equator and either pole, and will be vertical at the poles. At its upper end it carries a cross-head with bearings through which there passes another axis at right angles to the first (the declination axis). Both these axes are free to rotate in their respective bearings, and thus the telescope is capable of two motions, one of which—that of the declination axis—enables the instrument to be set to the elevation of the object to be observed, while the other—that of the polar axis—enables the observer to follow the object, when found, from its rising to its setting by means of a single movement, the telescope sweeping out circles on the sky corresponding to those which the stars themselves describe in their journey across the heavens. This single movement may be given by means of a tangent screw such as has already been described, and the use of a telescope thus equipped is certainly much easier and more convenient than that of an Altazimuth, where two motions have constantly to be imparted. To gain the full advantage of the equatorial form of mounting, the polar axis must be placed exactly in the North and South line, and unless the mounting can be adjusted properly and left in adjustment, it is robbed of much of its superiority. For large fixed instruments it is, of course, almost universally used; and in observatories the motion in Right Ascension, as it is called, which follows the star across the sky, is communicated to the driving-wheel of the polar axis by means of a clock which turns the rod carrying the tangent screw (Plate II.). These are matters which in most circumstances are outside the sphere of the amateur; it may be interesting for him, however, to see examples of the way in which large instruments are mounted. The frontispiece, accordingly, shows the largest and most perfect instrument at present in existence, while Plate II., with Figs. 4 and 12, give further examples of fine modern work. The student can scarcely fail to be struck by the extreme solidity of the modern mountings, and by the way in which all the mechanical parts of the instrument are so contrived as to give the greatest convenience and ease in working. Comparing, for instance, Plate II., a 6-inch refractor by Messrs. Cooke, of York, available either for visual or photographic work, with the Dorpat refractor (Fig. 3), it is seen that the modern maker uses for a 6-inch telescope a stand much more solid and steady than was deemed sufficient eighty years ago for an instrument of 9⁶⁄10 inches. Attention is particularly directed to the way in which nowadays all the motions are brought to the eye-end so as to be most convenient for the observer, and frequently, as in this case, accomplished by electric power, while the declination circle is read by means of a small telescope so that the large instrument can be directed upon any object with the minimum of trouble. The driving clock, well shown on the right of the supporting pillar, is automatically controlled by electric current from the sidereal clock of the observatory.

PLATE II.

6-inch Photo-Visual Refractor, equatorially mounted. Messrs. T. Cooke & Sons.

We have now to consider the reflecting form of telescope, which, especially in this country, has deservedly gained much favour, and has come to be regarded as in some sense the amateur's particular tool.

FIG. 12.—8-INCH REFRACTOR ON EQUATORIAL MOUNTING.

As a matter of policy, one can scarcely advise the beginner to make his first essay with a reflector. Its adjustments, though simple enough, are apt to be troublesome at the time when everything has to be learned by experience; and its silver films, though much more durable than is commonly supposed, are easily destroyed by careless or unskilful handling, and require more careful nursing than the objective of a refractor. But, having once paid his first fees to experience, the observer, if he feel so inclined, may venture upon a reflector, which has probably more than sufficient advantages to make up for its weaker points. First and foremost of these advantages stands the not inconsiderable one of cheapness. A 10½-inch reflector may be purchased new for rather less than the sum which will buy a 4-inch refractor. True, the reflector has not the same command of light inch for inch as the refractor, but a reflector of 10½ inches should at least be the match of an 8-inch refractor in this respect, and will be immeasurably more powerful than the 4-inch refractor, which comes nearest to it in price. Second stands the ease and comfort so conspicuous in observing with a Newtonian. Instead of having almost to break his neck craning under the eye-piece of a telescope pointed to near the zenith, the observer with a Newtonian looks always straight in front of him, as the eye-piece of a reflector mounted as an altazimuth is always horizontal, and when the instrument is mounted equatorially, the tube, or its eye-end, is made to rotate so that the line of vision may be kept horizontal. Third is the absence of colour. Colour is not conspicuous in a small refractor, unless the objective be of very bad quality; but as the aperture increases it is apt to become somewhat painfully apparent. The reflector, on the other hand, is truly achromatic, and may be relied upon to show the natural tints of all objects with which it deals. This point is of considerable importance in connection with planetary observation. The colouring of Jupiter, for instance, will be seen in a reflector as a refractor can never show it.

Against these advantages there have to be set certain disadvantages. First, the question of adjustments. A small refractor requires practically none; but a reflector, whatever its size, must be occasionally attended to, or else its mirrors will get out of square and bad performance will be the result. It is easy, however, to make too much of this difficulty. The adjustments of the writer's 8½-inch With reflector have remained for months at a time as perfect as when they had been newly attended to. Second, the renewal of the silver films. This may cause some trouble in the neighbourhood of towns where the atmosphere is such as to tarnish silver quickly; and even in the country a film must be renewed at intervals. But these may be long enough. The film on the mirror above referred to has stood without serious deterioration for five years at a time. Third, the reflector, with its open-mouthed tube, is undoubtedly more subject to disturbance from air currents and changes of temperature, and its mirrors take longer to settle down into good definition after the instrument has been moved from one point of the sky to another. This difficulty cannot be got over, and must be put up with; but it is not very conspicuous with the smaller sizes of telescopes, such as are likely to be in the hands of an amateur at the beginning of his work. There are probably but few nights when an 8½-inch reflector will not give quite a good account of itself in this respect by comparison with a refractor of anything like equal power. On the whole, the state of the question is this: If the observer wishes to have as much power as possible in proportion to his expenditure, and is not afraid to take the risk of a small amount of trouble with the adjustments and films, the reflector is probably the instrument best suited to him. If, on the other hand, he is so situated that his telescope has to be much moved, or, which is almost as bad, has to stand unused for any considerable intervals of time, he will be well advised to prefer a refractor. One further advantage of the reflecting form is that, aperture for aperture, it is very much shorter. The average refractor will probably run to a length of from twelve to fifteen times the diameter of its objective. Reflectors are rarely of a greater length than nine times the diameter of the large mirror, and are frequently shorter still. Consequently, size for size, they can be worked in less space, which is often a consideration of importance.

FIG. 13.—FOUR-FOOT REFLECTOR EQUATORIALLY MOUNTED.

The mountings of the reflector are in principle precisely similar to those of the refractor already described. The greater weight, however, and the convenience of having the body of the instrument kept as low as possible, owing to the fact of the eye-piece being at the upper end of the tube, have necessitated various modifications in the forms to which these principles are applied. Plates III. and IV., and Fig. 13, illustrate the altazimuth and equatorial forms of mounting as applied to reflectors of various sizes, Fig. 13 being a representation of Lassell's great 4-foot reflector.

PLATE III.

20-inch Reflector, Stanmore Observatory.

And now, having his telescope, whatever its size, principle, or form of mounting, the observer has to proceed to use it. Generally speaking, there is no great difficulty in arriving at the manner of using either a refractor or a reflector, and for either instrument the details of handling must be learned by experience, as nearly all makers have little variations of their own in the form of clamps and slow motions, though the principles in all instruments are the same. With regard to these, the only recommendation that need be made is one of caution in the use of the glass until its ways of working have been gradually found out. With a knowledge of the principles of its construction and a little application of common-sense, there is no part of a telescope mounting which may not be readily understood. Accordingly, what follows must simply take the form of general hints as to matters which every telescopist ought to know, and which are easier learned once and for all at the beginning than by slow experience. These hints are of course the very commonplaces of observation; but it is the commonplace that is the foundation of good work in everything.

If possible, let the telescope be fixed in the open air. Where money is no object, a few pounds will furnish a convenient little telescope-house, with either a rotating or sliding roof, which enables the instrument to be pointed to any quarter of the heavens. Such houses are now much more easily obtained than they once were, and anyone who has tried both ways can testify how much handier it is to have nothing to do but unlock the little observatory, and find the telescope ready for work, than to have to carry a heavy instrument out into the open. Plate IV. illustrates such a shelter, which has done duty for more than twelve years, covering an 8½-inch With, whose tube and mounting are almost entirely the work of a local smith; and in the Journal of the British Astronomical Association, vol. xiv., p. 283, Mr. Edwin Holmes gives a simple description of a small observatory which was put up at a cost of about £3, and has proved efficient and durable. The telescope-house has also the advantage of protecting the observer and his instrument from the wind, so that observation may often be carried on on nights which would be quite too windy for work in the open.

PLATE IV.

Telescope House and 8½-inch 'with' Reflector.

Should it not be possible to obtain such a luxury, however, undoubtedly the next best is fairly outside. No one who has garden room should ever think of observing from within doors. If the telescope be used at an open window its definition will be impaired by air-currents. The floor of the room will communicate tremors to the instrument, and every movement of the observer will be accompanied by a corresponding movement of the object in the field, with results that are anything but satisfactory. In some cases no other position is available. If this be so, Webb's advice must be followed, the window opened as widely and as long beforehand as possible, and the telescope thrust out as far as is convenient. But these precautions only palliate the evils of indoor observation. The open air is the best, and with a little care in wrapping up the observer need run no risk.

Provide the telescope, if a refractor, with a dew-cap. Without this precaution dew is certain to gather upon the object-glass, with the result of stopping all observation until it is removed, and the accompanying risk of damage to the objective itself. Some instruments are provided by their makers with dew-caps, and all ought to be; but in the absence of this provision a cap may be easily contrived. A tube of tin three or four times as long as the diameter of the object-glass, made so as to slide fairly stiffly over the object end of the tube where the ordinary cap fits, and blackened inside to a dead black, will remove practically all risk. The blackening may be done with lamp-black mixed with spirit varnish. Some makers—Messrs. Cooke, of York, for instance—line both tube and dew-cap with black velvet. This ought to be ideal, and might be tried in the case of the dew-cap by the observer. Finders are rarely fitted with dew-caps, but certainly should be; the addition will often save trouble and inconvenience.

Be careful to cover up the objective or mirror with its proper cap before removing it into the house. If this is not done, dewing at once results, the very proper punishment for carelessness. This may seem a caution so elementary as scarcely to be worth giving; but it is easier to read and remember a hint than to have to learn by experience, which in the case of a reflector will almost certainly mean a deteriorated mirror film. Should the mirror, if you are using a reflector, become dewed in spite of all precautions, do not attempt to touch the film while it is moist, or you will have the pleasure of seeing it scale off under your touch. Bring it into a room of moderate temperature, or stand it in a through draught of dry air until the moisture evaporates; and should any stain be left, make sure that the mirror is absolutely dry before attempting to polish it off. With regard to this matter of polishing, touch the mirror as seldom as possible with the polishing-pad. Frequent polishing does far more harm than good, and the mirror, if kept carefully covered when not in use, does not need it. A fold of cotton-wool between the cap and the mirror will, if occasionally taken out and dried, help greatly to preserve the film.

Next comes a caution which beginners specially need. Almost everyone on getting his first telescope wants to see everything as big as possible, and consequently uses the highest powers. This is an entire mistake. For a telescope of 2½ inches aperture two eye-pieces, or at most three, are amply sufficient. Of these, one may be low in power, say 25 to 40, to take in large fields, and, if necessary, to serve in place of a finder. Such an eye-piece will give many star pictures of surprising beauty. Another may be of medium power, say 80, for general work; and a third may be as high as 120 for exceptionally fine nights and for work on double stars. Nominally a 2½ inch, if of very fine quality, should bear on the finest nights and on stars a power of 100 to the inch, or 250. Practically it will do nothing of the sort, and on most nights the half of this power will be found rather too high. Indeed, the use of high powers is for many reasons undesirable. A certain proportion of light to size must be preserved in the image, or it will appear faint and 'clothy.' Further, increased magnifying power means also increased magnification of every tremor of the atmosphere; and with high powers the object viewed passes through the field so rapidly that constant shifting of the telescope is required, and only a brief glimpse can be obtained before the instrument has to be moved again. It is infinitely more satisfactory to see your object of a moderate size and steady than to see it much larger, but hazy, tremulous, and in rapid motion. 'In inquiring about the quality of some particular instrument,' remarks Sir Howard Grubb, 'a tyro almost invariably asks, "What is the highest power you can use?" An experienced observer will ask, "What is the lowest power with which you can do so and so?"'

Do not be disappointed if your first views of celestial objects do not come up to your expectations. They seldom do, particularly in respect of the size which the planets present in the field. A good deal of the discouragement so often experienced is due to the idea that the illustrations in text-books represent what ought to be seen by anyone who looks through a telescope. It has to be remembered that these pictures are, for one thing, drawn to a large scale, in order to insure clearness in detail, that they are in general the results of observation with the very finest telescopes, and the work of skilled observers making the most of picked nights. No one would expect to rival a trained craftsman in a first attempt at his trade; yet most people seem to think that they ought to be able at their first essay in telescopic work to see and depict as much as men who have spent half a lifetime in an apprenticeship to the delicate art of observation. Given time, patience, and perseverance, and the skill will come. The finest work shown in good drawings represents, not what the beginner may expect to see at his first view, but a standard towards which he must try to work by steady practice both of eye and hand. In this connection it may be suggested that the observer should take advantage of every opportunity of seeing through larger and finer instruments than his own. This will teach him two things at least. First, to respect his own small telescope, as he sees how bravely it stands up to the larger instrument so far as regards the prominent features of the celestial bodies; and, second, to notice how the superior power of the large glass brings out nothing startlingly different from that which is shown by his own small one, but a wealth of delicate detail which must be looked for (compare Plate XV. with Fig. 22). A little occasional practice with a large instrument will be found a great encouragement and a great help to working with a small one, and most possessors of large glasses are more than willing to assist the owners of small ones.

Do not be ashamed to draw what you see, whether it be little or much, and whether you can draw well or ill. At the worst the result will have an interest to yourself which no representation by another hand can ever possess; at the best your drawings may in course of time come to be of real scientific value. There are few observers who cannot make some shape at a representation of what they see, and steady practice often effects an astonishing improvement. But draw only what you see with certainty. Some observers are gifted with abnormal powers of vision, others with abnormal powers of imagination. Strange to say, the results attained by these two classes differ widely in appearance and in value. You may not be endowed with faculties which will enable you to take rank in the former class; but at least you need not descend to the latter. It is after all a matter of conscience.

Do not be too hasty in supposing that everybody is endowed with a zeal for astronomy equal to your own. The average man or woman does not enjoy being called out from a warm fireside on a winter's night, no matter how beautiful the celestial sight to be seen. Your friend may politely express interest, but to tempt him to this is merely to encourage a habit of untruthfulness. The cause of astronomy is not likely to be furthered by being associated in any person's mind with discomfort and a boredom which is not less real because it is veiled under quite inadequate forms of speech. It is better to wait until the other man's own curiosity suggests a visit to the telescope, if you wish to gain a convert to the science.

When observing in the open be sure to wrap up well. A heavy ulster or its equivalent, and some form of covering for the feet which will keep them warm, are absolute essentials. See that you are thoroughly warm before you go out. In all probability you will be cold enough before work is over; but there is no reason why you should make yourself miserable from the beginning, and so spoil your enjoyment of a fine evening.

Having satisfied his craving for a general survey of everything in the heavens that comes within the range of his glass, the beginner is strongly advised to specialize. This is a big word to apply to the using of a 2½- or 3-inch telescope, but it represents the only way in which interest can be kept up. It does no good, either to the observer or to the science of astronomy, for him to take out his glass, have a glance at Jupiter and another at the Orion nebula, satisfy himself that the two stars of Castor are still two, wander over a few bright clusters, and then turn in, to repeat the same dreary process the next fine night. Let him make up his mind to stick to one, or at most two, objects. Lunar work presents an attractive field for a small instrument, and may be followed on useful lines, as will be pointed out later. A spell of steady work upon Jupiter will at least prepare the way and whet the appetite for a glass more adequate to deal with the great planet. Should star work be preferred, a fine field is opened up in connection with the variable stars, the chief requirement of work in this department being patience and regularity, a small telescope being quite competent to deal with a very large number of interesting objects.

The following comments in Smyth's usual pungent style are worth remembering: 'The furor of a green astronomer is to possess himself of all sorts of instruments—to make observations upon everything—and attempt the determination of quantities which have been again and again determined by competent persons, with better means, and more practical acquaintance with the subject. He starts with an enthusiastic admiration of the science, and the anticipation of new discoveries therein; and all the errors consequent upon the momentary impulses of what Bacon terms "affected dispatch" crowd upon him. Under this course—as soon as the more hacknied objects are "seen up"—and he can decide whether some are greenish-blue or bluish-green—the excitement flags, the study palls, and the zeal evaporates in hyper-criticism on the instruments and their manufacturers.'

This is a true sketch of the natural history, or rather, of the decline and fall, of many an amateur observer. But there is no reason why so ignominious an end should ever overtake any man's pursuit of the study if he will only choose one particular line and make it his own, and be thorough in it. Half-study inevitably ends in weariness and disgust; but the man who will persevere never needs to complain of sameness in any branch of astronomical work.