TELESCOPE. 



DroancinE Wi/ontal motion, will be bed underftood 

 Ln, an cxumination of /^ 7. --h.ch t"e parts^e en 

 larged, and in wl.icl. the fame letters denote the fame 

 thines. The thumb-fcrew V, concealed m /^. 6, when 

 turned round, prttTcs on the fixed meta3 under .t and 

 draws up the frame Y, of wh.ch one fide is feen, that holds 

 the aNis of the fcrew U, and that turns on two pivots at its 

 remote end X ; and in this manner the fcre^v is brought into 

 contaa with the notches of the racked femi<ircle, or is de- 

 tachcd from it by a contrary motion. In the former cafe 

 there is a now motion in altitude, and in the latter a quick 

 one Again, the axis S, of another horizontal and parallel 

 fcrew, receives the handle, like/^. 5, for giving the motion 

 in azimuth. This fcrew is alio prclTed into contad with a 

 horizontal racked wheel, that lies fixed between S and W, 

 and gives the flow motion. The fcrew of prefTiire is leen 

 above" S, and when the axis of the handle has its fcrew de- 

 tached from theracked circle, the telefcopeis at liberty to have 

 a quick motion in azimuth. The chamfered plate, on which 

 the racked circle rcfts, is graduated, and the vernier at W 

 reads the hours and minutes of time : but in order that this 

 circle may be parallel to the equator in the heavens, fo as to 

 indicate time truly, it is ncceffary to turn the upper half, C, 

 of the block half round, and to turn the whole ftand fo 

 that the plane of the graduated circle may be parallel to 

 the plane of the equator, which it will be when the upper 

 point of the axis of motion is direfted exaftly to the north 

 pole, in which fituation the hour-circles will coincide with 

 the horary circles of the heavens, or miift be made fo by 

 an adjullmcnt, which the graduated circle is capable of re- 

 ceiving, by means of an elongated hole, into which the fcrew 

 enters that fixes it to the block. This block, vve have faid 

 before, is called Smeaton's block, and anfwers the purpofe 

 of giving an equatorial motion to the telefcope, when fol- 

 lowing a heavenly body by night, and is ufeful for finding 

 one by day, from a knowledge of its right afcenfion and 

 declination ; for what was altitude in the horizontal pofi- 

 tion, becomes declination in the equatorial elevation. This 

 telefcope is one of the four of which we propofe to ex- 

 emphfy the ufes in conjunftion with Troughton's micro- 

 meter ; and therefore we have had the micrometer put into 

 its place at K, when the tcrreftrial tube 1 K is ufed ; but 

 it might have been at H in place of the celeftial eye-piece, 

 where the value of the revolutions of the micrometer fcrew 

 would have been as we have tabulated it in our laft feftion. 

 The two terreflrial eye-pieces in j?f . 8, and one not given 

 there, are made to fcrew into the terreftrial tube at K, 

 in addition to all the four celeftial eye-pieces, w-.ich have 

 an adapter to firt them to the fame place fucceffively ; which 

 variety affords the choice of feven terreftrial powers with 

 one pair of field-glaffes ; and as there are three pairs of ficld- 

 glaftes, the variety becomes 3x7 = 21 different powers 

 with a fixed eye-tube ; but as the eye-tube alfo Hides, the 

 pov.-crs may be varied in fmall quantities at pleafure between 

 the two extremes. In this inftrumcnt, the end I of the ter- 

 reftrial tube does not fcrew into the celeftial tube that bears 

 the rack, but into an intermediate fliding-tube, which is 

 here marked H, the ufe of which is to allow an adjuft- 

 ment for vifion at very ftiort diftances, which adjuftment re- 

 quires a great length of tube to be drawn out. For the 

 conftruftion of Troughton's and other micrometers, we 

 miift refer to our article Micrometer. 



The fcrew G of adjuftment for diftin£V vifion, is concealed 

 in our drawing by thejiniicr or fmall telefcope attached, over 

 A, to the ir.ain tube, the ufe of whicli is to bring the ob- 

 jeft readily into the field of view of the large telefcope ; 

 which is not an eafy matter, when tbe power is great ; for 



a 



as the field of view increafes with a diminution of the power 

 of the telefcope, and i)/V.» vcr/il, the fmall telefcope faves much 

 time in fearching for any objetl that is vifible in it. But 

 the micrometer would be of little ufe in the night for mea- 

 furing fmall angles, except when the moon is the objed, 

 unlefs fome mode of illuminating the wires, or fpider's 

 lines, ufed in making the meafurement could be adopted. 

 Formerly this objeft was effefted by a piece of brafs, faced 

 with card paper, attached to the objeft-end of the telefcope, 

 as mjig. 15, and turning on a pivot, A, to any angle of in- 

 clination or reclination that the pofitfon of a lamp, or 

 candle, might require, of which the light was to be re- 

 fledlcd into the tube ; and an oval hole, in the middle of 

 the refle£ling plate, admitted the incident rays coming from 

 the objeft to be viewed : this plan, however, is attended by 

 the inconvenience, that cither the lamp muft be fufpended 

 by the objeft-end of the telefcope, fo as to rife and fall 

 with it, or otherwife the angle of inclination or reclination 

 of the reflefting piece muft be altered in every new pofition.. 

 The firft perfon who, we believe, laid afide this apparatus, 

 and introduced a diagonal refleftor into the body of the 

 tube, was the Rev. Dr. Ufhcr of Dublin, who brought 

 the light within the axis of motion of his tranfit telefcope, 

 as is now commonly done ; which method has ihe advantage 

 of giving light irt the fame manner at all elevatiohs, while the 

 place, where the lamp is placed, never varies. (See the 

 Tranfaftions of the Royal Irifli Academy, 1788, vol. ii. 

 p. 13.) This method, however, was not confidered as appli- 

 cable to an ordinaiv telefcope, where the axis of motion is 

 below the tube ; but Mr. Troughton has vei-y lately applied 

 this principle with fucccfs in the telefcope before us, and in 

 others of the fame conftruftion. At Z, over the centre of 

 motion, a hole is made in the tube, of about one half the 

 diameter of the tube, into which a covering cap of bvafs 

 fcrews, when extraneous light is not wanted ; then an ellip- 

 tical plate of brafs, rough gilt, with an elliptical hole in 

 the middle, is reclined in an angle of 45°, within the main 

 tube, in fuch a way as to rec^vc the lateral light of a lamp 

 or candle, which it reflefts along the tube-tothc eye-piece 

 of the micrometer ; and the light thus reflefted is not only 

 mild and pleafant to the eye, but may be modified, as to 

 quantity, by the pofition of the lamp, and will remain the 

 fame in all altitudes, if the lamp be in the fame horizojital 

 line vs'ith the reflefto% and ftand at a proper angle with the 

 plane of the refleflor. In ufing the telefcope before us, 

 we found that fome of the rays falling on the extreme parts 

 of the objeft-glafs were loft in the tube, and that confc- 

 quently either the diapiiragm was too fmall, or that the 

 oval aperture of the inclined refleftor was not fufficiently 

 large ; but the diftinfinefs of the image is no doubt pro- 

 moted by fuch exclufion : and, indeed, it is the praftice oi 

 fome opticians, when they find the longitudinal aberration 

 arifing from the fpherlcal figure of the convex glafs not 

 well compenfated by the concave one, to fiiut out the ex- 

 treme rays on purpofe, by the ufe of a fmall diaphragm, 

 for which aflertion we have Mr. DoUond's authority. In 

 the inftance before us, we have afcertained by a dynameter, 

 that the reduftion of the aperture is in the ratio 30 : 25.6, 

 namely, from 4.5 to 3.84 inches: but our intention is to 

 have the original aperture reftored. 



When a refrafting telefcope exceeds five or fix feet iu 

 length, it requires to be fupported at both ends, and then 

 the nearer fupport muft have adjuftments for both altitude 

 and azimuth, while the remote one may be a point of reft. 

 Fig. 8. Plate XXX. fhews a fupport for the eye-end of a 

 long telefcope, which we believe was contrived by Snieaton, 

 and v.'hich anfwers its purpofe fufficiently ; A B C D is a 



mahogany 



