TELKSCOPE. 



Thefe obfervations of Mars were niadt> near the meridian, 

 when he was a little paft oppofition, and confequcntly when his 

 diameter was near a niaximiini.whiolicircunillance we mention, 

 becaufe aftronomers have given very difeoidant accounts of 

 the apparent diameter of this planet ; and perhaps no deter- 

 mination has beL'n more accurate than we have here given. 



4. The iun's diameter was mealured at noon on the 24th 

 of September 18 16, by Trougliton's micrometer adapted 

 to the 30.15 inch telefcope, iji the following manner ; viz. 



II I II 



29.21 X 65.7 = 31 59.1 



tions, unconncded with each other, k. i".24 for i-acli unit 

 read on the vernier, and this datcrminaliun was afurwards 

 confirmed by an obfcrvaiion of the fun taken on the zcth 



1919" 



September 1816, viz. - - = I ".24 very nearly. After 



having given a value to DoUond's micrometer thus fitted up, 

 on the 2 ill of Auguft 1816, the diameter of Saturn's riim 

 was meafured when its longer diainetcr was very nearly hori- 

 'O'ltal, both to the right and left of zero, and was found 



By the celeftial power - 29.21 x 65.7 = 31' ^g.'l •^l"*' to one fubdivifion and 7.7 on the vernier, or 25 + 7.7 



By theterreftrial, with theeye-l ^ _ = 32.7' parts of the vernier ; then 32.7 x l".24 = 40". J48 



tube Hided to dot 30" I "3-92 X 3° - 3' 57-6 is the meafure of the greatefl length of Saturn's ring taken 



By the terrelhial, at dot 28" 68.50 X 28 =31 58. 



By the terreftrial, at dot 25" 76.74 x 25 =31 58.5 



4)127 53-2 



Average meafure ... 



By the Nautical Almanac 15' 59".! X 2 



= 3« 

 = 3» 



58-3 

 58.2 



On the Aiding tube of this telefcope, the dots on the fcale 

 run from 31" to 19", at which dots the powers are to eacii 

 other inverfely as thefe numbers ; but the whole diameter of 

 the fun cannot be taken on the fcale of the micrometer when 

 a greater power is ufed, than when the pofitioii is at 

 dot 25", or middle dot of the Aiding tube, where the power 

 is about 82. 



In thefe four examples, the diameters meafured were the 

 vertical diameters, for taking which Troughton's microme- 

 ter is peculiarly adapted ; but the horizontal diameter of a 

 body in motion cannot be taken with the fame accuracy with 

 this inftrument, on account of the difficulty of keeping the 

 extreme edges of the objeft in contaft with the fpidcr's 

 lines, while the final .idjuftment of the meafure is making. 

 For this ptirpofe, DoUond's divided objeft-glafs micrometer 

 IS more convenient, and may have its fcale appreciated, and 

 tl»e values thereof tabulated in the way we have already ex- 

 plained. For inftance, we obtained a divided obje£t-glafs, 

 with the requifite adjuftmcnts both for circular motion and 

 for the feparation of the centre of the femi-lenfes, of three 

 inches and a half diameter, and fitted it over the objeft-end 

 of the 45.75 inch achromatic of TuUey, while the original 

 objeft-glafs, of the fame dimenfions, remained in its place. 

 The focus of this divided objeft-glafs was fo long, that it 

 fhortened the original focus only to 40.3 inches. The fcale 

 of the objeft-glafs is divided into inches and twentieth parts 

 of an inch, one of which parts or fubdivifions is again re- 

 duced by a vernier into twenty-five fubordinate parts, fo 

 that -j'j- of -jJj, or -sTroth of an inch, is the fmallefl quantity 

 appreciable bv the vernier. On the gih of AuguR i8i6, 

 when the fun's diameter was 31' 37", or 1897", the oppofite 

 limbs of the two apparent images of the fun coincided 

 when the fcale indicated three inclies, one-twentieth part, 

 and eleven towards 25 on the vernier, after an allowance 

 was made for the index error by a crofTed obfervation of a 

 very fmall angle. Now thefe numbers reduced into the 

 loweft denomination, give 1536 parts of the vernier, and 



1897" 



— 2__ =; i".25r is the value of one of thefe parts ; but by 

 1536 



a terreftrial meafureraent, to be explained hereafter, the 

 value taken at 700 feet diftancc, with a correftion for want 

 of paralleUfm of the rays at this diftance, the value came 



888" „ , r u- ^ J • 



out- = i".245; the average 01 which two dettrmioa- 



713 



near tlie meridian, when the patfage was at nearly 28 minutes 

 pall eleven P.M. ; and, confequently, when the planet was at 

 no greater dillance than eight days from oppofition. On 

 the 4th of Auguft 1815, tile gieateft diameter of Saturn':, 

 ring however, meafured with Troughton's micrometer at- 

 tached to the 63.5 inch telefcope, had been found by 

 careful meafurement = 1.50 x 3i"-3 = 46".95, the planet 

 being then only three days from oppofition. By the 

 fame apparatus the ring had been made 48".2 on the loth 

 of September 1815, and on the 25lh of the fame month 

 only 43". Thefe difcrcpancies (hew that no dependence can 

 be placed in horizontal meafures made with Troughton's 

 micrometer when the olijeft is in apparent motion, but for all 

 other meafures of fm.iU angles, it is no doubt the bell that 

 has been yet invented. 



Dr. Brevvfter's micrometer has the fame adv.int.igc as 

 DoUond's, when the divided lens is ufed as the Aiding lens 

 within the tube; but the power of the patent telelcopes 

 hitherto conftrui'.ted is fo fmall, that an angle can feldom be 

 meafured with it nearer than to 10", and frequently not fo 

 near. The principle, however, is apphcable to telefcopcs 

 of larger dimenfiDns. 



When DoUond's and Troughton's micrometers are both 

 apphed to the telefcope of 45.75 inches, the value of the 

 fcale of Troughton's becomes ."Utered from 43". 5 to 49".4, 

 namely, in the invcrfe ratio of the diminifhed focus ; and 

 they may both be ufed with great convenience at the fame 

 time, in which cafe, one may meafure the angular length 

 and the other the angular breadth of tiie fame body ; or, 

 if the body 'oe celeftial, one may give the horizontal and 

 the other the vertical dimenfions at the fame inftant. This 

 mode of applying two micrometers, one optical and the 

 other mechanical, at the fame time, affords a mnuial check 

 on the meafures of eacli, when the body is round, like one 

 of the heavenly bodies, and gives a very falisfadlory refult, 

 when it can be adopted. When Troughton's micrometer 

 is ufed as a celeftial eye-piece, along with DoUond's micro- 

 meter attached to the telefcope 45.75, ftiortened to 40.3, 

 the double images are formed beyond both eye-glaffes, 

 reckoning from the eye, and gives there I ".24 as the value 

 of one uroke on the vernier ; whereas when a common 

 celeftial eye-piece is ufed with DoUond's, the fecond glafs 

 of the compound piece (hortens the focus of the objed- 

 glafs a little, and the images arc feen between the two 

 glafl'es of the eye-piece ; confequently the value of Dol- 

 lond's micrometer varies a trifle with every difte-rent eye- 

 piece, which is not the cafe with Troughtoii.'s, where the 

 image is always in the unaltered focus ol the objett-glafs. 

 On the 26th of September 1816, a careful lines ol ob- 

 fervations W.1S made of the fun's diameter with both Trough- 

 ton's and DoUond's micrometers ufed at the fame time, 

 when the former gave 38.85 X 49".4 = S''59"-2, and the 

 other 15.47 (3 ii. I div. 22 on vcm.) x i".2i := 31' 

 58".28, the diameter of the fun by the Nautical Almanac 



being 



