TELESCOPE. 



tor ii! ihis liiuauon the illuminated portion of the moon is 

 always apparently larger than the dark portion. 



4. The fun's diameter was again taken at nine o clock 

 A M. on the 15th of Oaober, with the telefcope of 63.5 

 inches, and was found equal to 61.56 turns, vvhen the alti- 

 tude was fuch as to require an addition of 2 '.6 for the va- 

 riation of ihc rcfrattion ip half a degree of altitude ; and 

 here we have from Table I. 60 4- I = 31' 18" + 3i".3 - 

 ,,'4g".3, and from Table II. .56 = if .5, making, to- 

 gether with + 2".6 the correftion, the fum 32' g".^, the 

 diameter given in the Nautical Almanac being i6'4".8 X z 



= 32' 9"-6- r T • 1 



In all tiiefe examples, as well as in the data from which 

 the preceding tables were computed, Troughton's microme- 

 ter was ufed as a celeftial eye-piece, where the objed was 

 confequently inverted, which is the manner in which tins 

 micrometer was intended to be ufed ; but according to the 

 conftruaion of the modern teiTeftrial eye-tube, this micro- 

 meter may be fubftituted, by help of an adapter, for the 

 two glaffes at the eye-end of this tube, in which fituation 

 the magnifying power is very confiderably increafed, and 

 confequently the fcale rendered capable of meafuring fmaller 

 portions of a fecond, than in the ufual way, particularly 

 when there is light enough in the field of view, :. e. when 

 the objeft-glafs has a large diameter. When the microme- 

 ter in queition is applied to the eye-end of the terreftrial 

 tube of the telefcope of 45.75 inches, one turn is equal 

 to only i6".03, which (hews the power to be fomewhat 

 greater than when the 118.8 inches telefcope was ufed as a 

 celeftial telefcope with the fame micrometer ; and with the 

 telefcope of 63.5 inches, which has three pair of feparate 

 field-glafTes, the terreftrial powers with the laid micrometer 

 gives the refpcclive values of one turn of the fcrew 9".97, 

 "/".gS, and 5". 15 ; fo that this telefcope has four various va- 

 lues of the micrpmetrical fcale, which may be ufed in fuc- 

 ceffion for meafuring the fame angle, according to circum- 

 ftances, and each variety may have a feparate table computed 

 for its particular ufe. This application of Troughton's 

 micrometer to the terreftrial tube, and the additional pairs 

 of field-glafTes, were contrived by the author of this article, 

 and led to another addition, wlach in itfelf admits of ftill 

 greater varieties. On obferving that the modern terreftrial 

 eye-tube is in faft a compound micro/cope, it occurred to him, 

 that there are three modes of increafing the power of this 

 inftrument ; firft, by (hortening the compound focus of the 

 eye-glaffes ; fecondly, by fhortening the compound focus of 

 the pair of field-glaffes ; and thirdly, by lengthening the dif- 

 tance between the compound eye-glafs and compound field- 

 glafs (or objeft-glafs of the microfcope). The two former 

 modes had now been tried, and afforded the varieties in the 

 meafures which we have fpecified : the laft one was therefore 

 reforted to thus ; a tube was made to Aide within the teiTef- 

 trial tube, after its own eye-piece was withdrawn, and the 

 micrometer was made to fcrevir into this moveable tube, fo 

 as to vary the diftance of the micrometer glafles from the 

 field-glaffes of the telefcope at pleafure. The refult proved 

 as was expefted ; every new pofition for diftance gave a new 

 value to the fcale of the micrometer, and the two extremes 

 of thefe values, with the 63.5 inch telefcope, were 10" and 

 5* refpeftively p&r revolution of the fcrew ; at leaft the 

 points were found by experiment on the Aiding tube, where 

 tliefe values, and alfo the intermediate ones 9", 8", 7", and 

 6" per revolution, were marked with a graver. The dif- 

 tances of thefe points depended on the field-glaffes ufed 

 with the Hiding tube ; and three fets of points were inferted, 

 to correfpond to the three pairs of field-glaffes, any one of 

 which admitted the fcale to be fubdivided into tenths of a 

 10 



fecond. This mode of applying a (liding micrometer in the 

 terreftrial tube is as ufeful as novel ; for when the pofition is 

 made for an exaft number of feconds p^r tiu-n of the fcrew, 

 the tables are difpenfed with ; the only operation being to 

 multiply the number of turns by the number of feconds be- 

 longing to the pofition of the Aiding tube, and then to re- 

 duce them to minutes by 60 as a divifor. A few examples 

 will render thefe new methods of ufing the micrometer per- 

 fcftly intelligible^ and will at the fame time Aiew that they 

 contribute greatly to accuracy, by a fpecies of repetition of 

 the meafurc, of which they are capable. We will firft ex- 

 emplify the method without the Aiding tube. 



I. 'The meafures of Jupiter's diameter, taken by the 63.5 

 inch telefcope on the 19th of April 181 6, were as follow ; 



Bv Troucrhton's micrometer ufed 7 , _ " " o 



' fn- 1 ■ f 1-43 X 31.3 — 44.8 



as a celeftial eye-piece - 3 ^j j j -r-r 



By N° I. 6f the field-glaffes withl ^ 



the terreftrial tube - J 



By N° 2. of ditto - - 5.53x7.98 = 44.13 



By N° 3. of ditto - - 8.32x5.15=42.85 



4)173-98 



Average of the four meafures 



43-495 



The values of the tliree field-glaffes had been taken by 

 ten-eftrial meafurement at 700 feet, on the 31ft of March 

 1816, and may require farther correction. 



2. On the 30th of April 1816, Jupiter being very nearly 

 in oppofition, his diameter was meafured with the 45.75 inch 

 telefcope, wliich has only two varieties, a celeftial and a ter- 

 reftrial application of the micrometer, and the refult was 

 thus : viz. 



By the celeitial meafure, taken to the! 

 • . , r y 1.04 turns, 



right ot zero " ' ' J 



By the fame, taken to the left of zero - 1.05 ditto. 



2)2.09 



By the terreftrial meafure 2.72 x 16". 03 



1.045 



II 

 = 43-52 

 = 43-50 



2)87.02 



The average of the celeftial and terreftrial meafures = 43.5 1 



3. On the 25th of Oftober 1815, the following meafures 

 were taken of the diameter of Mars with Troughton's mi- 

 crometer attached to the Aiding tube of the telefcope 63.5 

 inches ; •vix,. 



Firft pofition at the dot of 7"! '^'""'- " _ " 



with third field-glafs - ]" 3-43 X 7 -24.01 



Second pofition with ditto - 2.96 x 8 ^ 23. 6S 



Third pofition with ditto - 2.68 x 9 = 24.12 



Fourth pofition with ditto - 2.40 x 10 = 24.00 



With N° 1. field-glafs and its'? , 



dot 4" - - . I 0-0 X 

 With N° 2. field-glafs and itsi 



dots" - - - i 3-0 X 



With the micrometer ufed as a^j 



celeftial eye-piece - j '' ' 3 -3 — 4- 



4 = 24.00 

 8 = 24.00 



7)167.91 

 23.987 



Thefe 



