J AS. 6, 1882.] 



KNOWLEDGE ♦ 



201 



NIGHTS WITH A THREE-INCH TELESCOPE. 



TTXDER the above heading we propose to famish the beginner 

 \J in astronomy with such directions as shall enable him to 

 imploy, to the greatest pos.sible advantage, the kind of instrument 

 «-ith which he will, in all probability, at first provide himself. But, 

 be it noted at the outset, that this series of papers is not intended 

 for the possessors of telescopes of considerable aperture, equato- 

 rially mounted • or furnished with elaborate rackwork movements in 

 altitade and azimuth.* For theo^vnersof such, an abundant Utera- 

 ture is already in existence ; and they, at present, have such 

 admirable works as Webb's '" Celestial Objects for Common Tele- 

 scopes," Crossley, Gledhill & Wilson's " Handbook of Double 

 Stars," Chambers* one volume edition of Smyth's '' Celestial 

 Cycle," &c. We shall presuppose nothing on the part of our 

 readers, then, beyond an ardent desire to become familiar with the 

 beauties and glories of the celestial vault, and trust, if we can 

 secure their attention, to put them fairly in the way of gratifj'ing 

 such a high and laudable aspiration. To this end, we shall use as 

 oar text the maps of the face of the sky which appear monthly in 

 Knowledge, although we should strongly recommend the student 

 to possess himself of the smaller " Star Atlas " by the editor of 

 this journal, as well. 



As it is of the first importance that the workman should be 

 familiar with the tools he has to use, we shall devote this intro- 

 ductory essay to a description of the telescope itself, which wo will 

 imagine to be a 3-inch achromatic one, of about 42 in. focal length, 

 mounted apon an ordinary " pillar and claw " stand. Such an 

 instrument, as ordinarily sold, is shown in Fig. 1, which, however, 

 represents it as famished with a valuable little subsidiary contri- 

 vance (to bo immediately described), which the observer will have 

 to make, or get made, himself. 



Fi^. 1. 



And here, albeit wo are earnestly anxious to eliminate the com- 

 mercial element altogether from our considerations, we are com- 

 pelled to caution the student against supposing that a first-class 

 3-inch telescope for astronomical purposes can be made for £5, or, 

 in fact, for any sum approaching it. The object-glass alone must 

 cost the maker himself something like this amount. Hence, 

 as we propose to deal ivith and describe celestial objects, as seen in 

 an instrument of the highest class, we give this preliminary 

 warning, lest the young observer should spend his money in a 

 cheap glass, and then wonder at the discrepancy between our 

 delineations of stars and planets and his own views of them. 

 There is a vast amount of rubbish vended in the form of (so-called) 

 cheap telescopes, and no tvro should ever purchase such a one 

 mthout its previous examination and testing by a skilled expert. 

 Makers like Cooke, Dallmeyer, and Wray will not imperil their 

 great and deserved reputation by selling an inferior object glass, 

 even to a total stranger ; but instruments of unknown opticians 

 require the most rigid trial before they can be safely bought. We 

 shall give further on, a few tests by which the student himself may 

 judge somewhat of the quality of his purchase. It is time, how- 

 ever, to turn to our figure. Here we see the brass tube T, into 

 •>ne end of which screws the cell containing the object glass 0. 

 Through a tube projecting from the brass disc which covers the 

 other end of T, the smaller tube S is worked in and out by the 

 milled head F, acting in a rack and pinion. This is for the purpose 

 of focussing the telescope, and making the image of the object 



• These terms will be explained as we proceed. 



observed sharp and distinct. Into the tube S screw the eye-piece 

 E, consisting of two lenses mounted in a short piece of tubing. 

 Shortly, the action of the instrument is this. The object-glass 

 forms in its focus an image of the object to which it is directed, and 

 the eye-piece — which is really a microscope — magnifies this imago 

 before it enters the obser^-er's eye. So much for the telescope itself. 

 It is bolted, as will be seen, by two screws and nuts to a brass 

 plate, which has a vertical motion, by means of the knuckle-joint 

 at A, at the top of the stout brass pillar AB ; and a horizontal one, 

 furnished by the rotation of the whole of this top-fitting, inside the 

 pillar. Three massive feet form its support. The arm BM 

 shown in our drawing forms no part of the ordinary fitting of the 

 instrument ; it constitutes the subsidiary contrivance of which we 

 spoke above, and we shall explain its use presently. L in the 

 figure represents a terrestrial or four-lens eye-piece, which shows 

 objects erect, and hence is nsed for land-purposes. It screws in at 

 the extremity of S, just as E does. The ordinary astronomical, or 

 so-called " Hayghenian" eye-piece contains, as we have previously 

 said, only two lenses, and inverts, or turns objects upside-down. 

 This, however, is obviously immaterial in a star, and this construc- 

 tion of the eye-piece enables us to obtain high power with com- 

 paratively small loss of light. X is another astronomical eye-piece, 

 and P a dark cap or shade, screwing on to every eye-piece, for the 

 purpose of observing the sun. The student is earnestly warned 

 never to look at the sun through a telescope without first covering 

 the eye-piece with one of these shades. When, however, we come, 

 in a future paper, to speak of the sun, we shall describe how the 

 solar details may be telescopically shown >vithont looking through 

 the instrument at all. The powers usually supplied with a telescope 

 of the size we are describing are one terrestrial one, magnifying, 

 perhaps. 45, and three astronomical ones, giving powers of some- 

 thing like 50, 100, and ISO. If, however, the observer intends to 

 devote his instrument wholly to the sky, we should advise him to 

 replace the terrestrial eye-piece by two Huyghenian ones, magnify- 

 ing 25 (for comets, nebulae, and clusters), and 250 (for close doable 

 stars) respectively. For night use, too, a '" Dew-cap " will be 

 found indispensable. This may be made of a tin tube, bright 

 outside and blackened within, about 8 inches long, and fitting over 

 the object end of the telescope at 0. This prevents direct radiation 

 from the object-glass, and the consequent deposition of dew upon 

 it. Never u-ipe your object-glass if yoii can possibly help it. Expose 

 it to the heat of a fire (not too near) or of the son, should it become 

 heavily dewed. 



A word may now be said as to the use of the bar BlI shown in 

 our sketch. It is a fact famOiar to nearly everyone who has ever 

 opened an astronomical primer (and, at any rate, to be estab- 

 lished by a single winter night's observation of the sky from dusk 

 to dawn), that the stars all seem to describe circles round a centre 

 in the Northern sky, called the Pole, very close to which is situated 

 the star we call the Pole-star. The farther we go from this centre, 

 the larger these circles become, up to a distance of 90°, beyond 

 which they begin to diminish again. Moreover, the point round 

 which they turn is something over 50° above the Northern horizon 

 (depending on the observer's latitude) , so that they are all described 

 obliquely to the horizon. Obviously, were the apparent axis of the 

 concave celestial vault vertical, the Pole would be overhead, and the 

 stars, seeming to describe circles parallel to the horizon, would 

 neither rise nor set. In this imaginary condition of things 

 (imaginary in England, for it really exists at the Poles), the 

 mounting of the telescope shown in our figure above would enable 

 the observer to follow a star by merely turning the telescope round 

 the vertical axis, AB, when once that star was in the field ; but a 

 moment's thought will show that a simple movement round a 

 vertical axis will by no means accomplish this when the star's 

 path is described round an inclined one. The vertical move- 

 ment of the telescope, we may here say, is spoken of as its 

 motion in altitude ; its horizontal motion as that in azimuth. 

 It may require a little more attention to see that if we so tilted 

 the axis AB that it became parallel to (or practically coin- 

 cided) with the apparent axis of the sky ; that then the 

 simple motion round it would follow any star to which it was 

 directed, from its rising to its setting. A telescope thus placed is 

 said to be equatorially mounted. Now, the little device in our cut, 

 for which, in its existing form, we sire indebted to the Earl of 

 Crawford and Balcarres, is intended to communicate an approxi- 

 mately equatorial motion to the ordinary altazimuth mounting of 

 the instrument. It takes the form of a bar BM, extending from 

 the base of the pillar AB. In it, at such a distance from the point 

 B vertically under A that the angle ACB shall be = the latitude of 

 the place, a hole is bored, and a thumbscrew (shown at C) inserted 

 through the bar, so as to nip a light chain or thin wire tight when 

 it is passed through the hole. The other end of this chain is 

 fastened anywhere towards the end of the telescope at C, and 

 sufficient weight is put on to the eye end of the telescope to keep 



