SCIENCE-GOSSIP. 



347 



CHAPTERS rOE YOUNG ASTEONOMEES. 



By Frank C. Dennett. 



CHOICE OF A TELESCOPE. 



(Continued from fage 315.) 



The next point to be considered is the stand. 

 Many smart-looking stands are very shaky, and 

 this is decidedly an instance where the old saying, 

 proves trixe, " handsome is as handsome does." A 

 little while since I was using a three-inch telescope 

 with a badly-made stand, which not only made the 

 instrument uncomfortable for observation, but 

 very diificult to truly focus ; besides being sus- 

 ceptible to the slightest tremor of the atmosphere. 

 The stand must be steady, and its motions not 

 loose, but smooth. If rack or screw motions are 

 fitted, all the better, especially when high powers 

 are being used. 



There are two kinds of stands, viz., alt-azimuth 

 and equatorial. The first-named has vertical and 

 horizontal motions, otherwise, in altitude and 

 azimtith, hence the convenient contraction — alt- 

 azimuth, which was first sug- 

 gested by Dr. WoUaston. The 

 ordinary pillar-and-claw stand 

 is the commonest form of this 

 class, but is, at the same time, 

 a very inconvenient pattern for 

 observing objects near the 

 zenith. The pattern shown of 

 a Cooke stand is a very great 

 improvement ('), especially for 

 over-head work. Celestial 

 objects, however, do not appar- 

 ently travel in circles parallel to 

 the horizon, but parallel to the 

 equator ; consequently their 

 altitude is constantly increasing 

 until they reach the meridian, 

 and then as constantly decreas- 

 ing. To keep an object in the 

 centre of the field of view, both 

 hands are kept employed — one 

 in turning the handle which 

 gives the horizontal motion, and 

 the other turning the handle to 

 raise or depress the telescope. '- 



For real work the equatoreal 

 is much the simplest and best. 

 A simple stand for a refiector, 

 having equatorial motion, is 

 illustrated. This has one motion parallel to the 

 equator, the axis of rotation being parallel to the 

 pole of oru' earth ; and another motion at right 

 angles to the first movement. Thus mounted, 

 the telescope may be pointed to an object in the 

 east, and, by the one motion, kept iipon the object 

 until it is well down in the west. The motion is 

 always in the same relative direction to a line in 

 the field of view, whilst ^vith the alt-azimuth it is 

 constantly changing. Consequently it is mvich 

 easier to know the exact points of the compass, 

 which is most important when making observations. 

 For work on the sun, moon, and planets, especially, 

 the instrument figured, with its 5 5 inch mirror, is 

 most serviceable, as I can personally testify from 

 much experience. 



As the motion is parallel to the equator, if a 



circle be added, having the hours I. to XXIV. 



marked upon it, and a circle on the other axis, 



showing how many degrees the telescope is 



(^) See figure, ante, p, 314. 



Calver b " Popular 



pointing above or below the equator, it is easy 

 at any time to turn the instrument on an object 

 above the horizon whose right ascension and 

 decKnation is known. A figure of a simple stand 

 of this class, by Banks, in size suitable for a 

 three-inch refractor, will appear in the next 

 nrunber of Science-GtOSSip. 



For the equatorial telescope to be of really 

 practical use, it must be accurately placed in 

 adjiistment, and, so far as possible, not moved 

 aboiit, otherwise the reading of the circles is not 

 to be depended ixpon. To get over this difficulty, 

 three stone or metal cups are very useful if fixed 

 in the ground in which the points of the tripod 

 fit, when the telescope is ready for operation ; but 

 such cups must be protected from injiuy. The 

 stand of the reflector above mentioned is conveni- 

 ently portable. It should have a stout square 

 box filled with earth to stand upon. The box 

 should be set "ndth its sides as nearly east, west, 

 north, and soirth as possible, and then made a 

 fixture. The telescope and stand can be set iipon 

 it, and got into adjustment. A 

 piece of beading may be laid 

 against the two toes of the stand, 

 and nailed to the box ; the stand 

 may then be easily removed and 

 replaced, with a little care, in 

 approximate adjustment. Of 

 course, the box should be well 

 protected with paint. 



As the motion to keep an object 

 in the field of view of an equato- 

 rially-mounted telescope is only 

 in one direction, that motion may 

 be communicated by clockwork, 

 wliich leaves both hands of the 

 observer free to make drawings 

 or measiu-ements. 



In advising the purchaser in 

 the choice of a telescope, there 

 are two important considera- 

 tions : how much can one afford 

 to spend ? and for what is the 

 instrument wanted? If the astro- 

 nomer desires an occasional peep 

 at the heavens, and then to take 

 the glass down to the sea-side to 

 use as an ordinary telescope, 

 a refractor will be most useful. 

 If, however, he means down- 

 right hard work in physical astronomy, my candid 

 advice is to have a reflector. I will give my 

 reasons. We will suppose that d£30 is the cash 

 to be expended ; for this sum a refractor of 3 in. or 

 34 inches aperture on an equatorial stand, may 

 be obtained, with say four eye-pieces and a solar 

 diagonal. On the other hand, a 5| inch reflector 

 may be purchased with the same number of eye- 

 pieces and the solar diagonal. The former would 

 divide double stars as close as about 1-5 ' or 

 1-4', also define the principal division on 

 Saturn's ring, and show the cluster 13 Messier in 

 Hercules on the finest dark nights as though 

 sparkling with mtnvite gold dust. The latter 

 instrument would divide double stars Q-Q inch 

 apart, show the "crape veil" of Saturn, with 

 possibly an occasional glimpse of a fine division 

 of the outer ring, and exhibit 13 Messier as a 

 sparkling group of stars. 



{To be continued.) 



Keflector. 



