6

ever, of what is often important, an adaptation of the power to the brightness of the object. Ordinary astronomical eye-pieces are shorter in proportion to their power. It is a better plan to change them by means of a short tube, or adapter,' than by a screw; in which case they are more liable to be dropped and injured. The power may be much increased by unscrewing and taking away the 'field-lens,'—that furthest from the eye; but the field will be very imperfect, excepting near its centre. The highest powers of large telescopes are often made thus, with single lenses, but the convex face of the lens is then turned towards the eye, as it gives sharper vision. Sir W. Herschel used the double convex form, having shallower The common kind, with two lenses, having the flat side of each next the eye, is called the Huygenian or negative eye-piece: the positive or Ramsden eye-piece has a flatter field, but is not, like the other, achromatic. The interposition of a combination called a Barlow lens raises the power with little loss of light; and as one will suit all the eye-pieces, it doubles the set at a small expense. The silvered specula mounted by Browning are provided with achromatic eye-pieces of peculiar construction and great excellence.

curves.

If the power of our oculars has not been engraved upon them,* we may get a fair approximation to it by viewing an equally divided scale at a distance (for low powers, a brick wall will answer) with one eye through the telescope, and with the other alongside of it, and noting how many unmagnified divi

These figures are not, however, always to be depended upon, and never, if the eye-piece was made for an instrument of a different focal length. The celebrated Short exaggerated the powers of his reflectors: and those of the great achromatics of Dorpat and Berlin were found by Struve and Encke to be overrated.

sions are covered by a single magnified image. Or, better still, we may have recourse to the Berthon Power-gauge, a little apparatus, the simple, efficient, and inexpensive character of which entitles it to very warm commendation.*

The test of excellence in separating power has been fixed by Dawes at the quotient, expressed in seconds, of 4.56 divided by aperture in inches. Thus a 10-inch object-glass or speculum ought to separate double stars at o"456 of distance between their centres. This value practically concurs with those given by Dallmeyer and Alvan Clark. Reflectors somewhat surpass achromatics in this respect, as theoretically they ought to do but they are apt to be more troubled by rings and scattered light. The best telescopes of either kind will bear a power of 100 per inch of aperture on stars for planets, half that power will usually more than suffice.

An object-glass of inferior defining power may sometimes be improved by stopping out defects, or contracting the aperture. Streaks or specks of unequal density are very injurious: they may be detected by turning the telescope to a bright light, taking out the eye-piece, and placing the eye in the focus; every irregularity will then be visible in the illumination which overspreads the object-glass; and, if of small extent, may be stopped out by a bit of sticking-plaster. If the performance is not thus improved, try a contracted aperture: make a cap of pasteboard fitting over the object-glass like the usual brass cap, but with a circular opening a little less than the clear aperture;-if the indistinctness is thus diminished but not removed, try several discs of pasteboard placed successively within this cap, with progressively contracted openings, till distinct vision is obtained; there we must stop, or valuable * It may be purchased for 7s. 6d. of Mr. Tuck, watchmaker, Romsey.

light will be lost. An excentric opening in the pasteboard cap may sometimes be serviceable, being turned round the axis so as to conceal different parts of the glass or mirror, till the best effect is produced: in other cases, a central pasteboard disc, supported by narrow arms from the sides, and leaving an open ring of light all round, may be tried. But for comets or nebulæ, it will be best to restore the original aperture, as with faint and ill-defined objects light is more essential than distinctness. To see whether the smaller

speculum which stands in the mouth of a reflector is rightly centered, take out the eye-piece, and look at the small speculum; in it will be seen an image of the large mirror, with the small speculum reflected again upon it: these images should be concentric with the small speculum; if not, a cautious alternate loosening and tightening of the three little screws at the back of the small speculum will bring all right.

A good stand is essential: if unsteady, it will spoil the most distinct performance; if awkward, it will annoy the observer; if limited in range, it may disappoint him at some interesting juncture. It may be well left to a respectable optician; but where expense is a serious consideration, a little mechanical ingenuity and knowledge of such contrivances will devise one which will answer sufficiently. The old arrangement, with a vertical and horizontal, or 'altitude and azimuth' motion, is simple and manageable: the equatorial form, which makes the telescope revolve on an axis parallel to that of the earth, has some great advantages, in following the object by a single motion, and where the expense of divided circles and spiritlevels is admissible, in finding planets and bright stars by day, and identifying minute objects by night: but, to do its work, it must be placed accurately in the meridian, and out of that

position has little advantage. In any case, if the stand is to be moveable, let it be strong enough for steadiness without being too heavy for portability.*

A sidereal clock is often considered a necessary adjunct to an equatorial mounting, in order to find objects invisible to the naked eye. But it may be dispensed with by the following method of 'differentiation' in all cases, excepting during the brief season of twilight, when neither sun nor stars can be employed. Write down the difference of Right Ascension (taking particular notice whether additive or subtractive) between the required and some known object-the sun by day, a neighbouring bright star by night. Seek the known object by the finder, and place it in the centre of your largest field: clamp the R. A. circle: set the telescope to the declination of the object sought, and clamp it there: unclamp in R. A. and move the telescope E. or W. as the case requires, to the value of the ascertained difference in R. A. and the object will be found in the field, somewhat W. of the centre, by a distance dependent on the duration of the process.

An observatory is by no means essential, but it would be difficult to over-estimate its advantage in point of comfort as well as economy of time. It used to be an expensive luxury; but a very simple and cheap 'telescope-house,' combining shelter with open-air freedom, to the great merit of which I can bear full testimony, has been devised by the Rev. E. L. Berthon, and is described in the English Mechanic, October 13 and 20, 1871.

We will close this section with the encouraging words of the Council of the Royal Astronomical Society, in their

* A very cheap equatorial stand is described in 'Astron. Register,' xiv. 35.

Report for 1828. Everyone who possesses an instrument, whose claims rise even not above a humble mediocrity, has it in his power to chalk out for himself a useful and honourable line of occupation for leisure hours, in which his labour shall be really valuable, if duly registered; those who possess good instruments, have a field absolutely boundless for their exertions.'

THE MODE OF OBSERVATION.

AN ordinary telescope may be easily prepared for use: to fix it on its stand; to point it by means of the finder; to adjust the focus to the eye (remembering that different eyes require different adjustments), are processes scarcely requiring instruction. But many mistakes may be made in detail; and in this, as in everything else, there are various methods of doing the thing the wrong way. The present section will, therefore, consist of negative rather than positive directions, pointing out rather what should be avoided than what should be done.

I. Do not begin by fixing the telescope in a warm room and opening the window. A boarded floor is bad, as every movement of the observer is liable to produce a tremor; but the mixture of warm and cool currents at the window is worse; it is an artificial production of the fluttering and wavering which, as naturally existing in the atmosphere, are such an annoyance to astronomers. If a window must be used, let it be opened as long beforehand as may be, and let the objectglass be pushed as far as possible outside; there should be no fire in the room; and any other windows, as well as the door, should be shut before beginning to observe: the nuisance may