LV.-1 — ^ 



Hi. ._ _ 



ASTRONOMICAL PAPERS. 



95 



We 



r 



y- 



excepting our new catadioptric micrometer, which, I have 

 lately learned, is ofDollond's conRru£tion; not having any 

 author by us, from which wc could get the ufe of that 

 curious inftrument, we were obliged to have recourfe to 

 experiments. When the micrometer was fixed upon the 

 telelcope, it was found by trial, that objeds could not be 

 fcen with the flune focal diftance as when it was off, and 

 we were obliged to fcrew up the fmall fpeculum nearer to 

 the eye; for which there is an optical reafon. From 

 whence it was concluded, that objeds fliould always be ob- 

 ferved in the moft diflind point of view, the fame with 

 the micrometer on, as when it was off. The next thing 

 to be done was to find the apparent diameter of an objed 

 (ortlie angle fubtendcd at the eye by two objeds) by this 

 inftrument. In order to this, we ftretched a cord, as 

 ftraight as poffible, one thoufand feet In length; which 

 was meafured feveral times over, in order to avoid miftake. 

 At the end of the cord was fet two circular objeds, made 

 of wdaite paper, in a line perpendicular to the cord, and ex™ 



adly ten feet apart; {landing at the other end of the cord, 

 and by opening the micrometer, wc could bring the two 

 images into an exad coincidence, or could make one of 

 the images appear like two, and by bringing their limbs 

 into contad, the diflahce of their centers was fhewn on the 

 fcale, to the five hundredth part of an inch. Now from the 

 rules of trigonometry, the angular diflance of the two ob 



jeds 



was 



34 



22 



^^^8i from thence it was known, how 



jnany inches and parts of an inch were anfvverable to that 

 angle* Thefe experiments were repeated every fair day 

 (for no other was fuitablc for thclc obfcrvatlons) till we 

 could many times going find the diameter of a body to a 

 fecond of a degree. From tlicfe obfervations we were en- 

 abled to make a table for the micrometer, as far as the 

 fcale extended. Thefe experiments were carried yet far- 

 ther; for, by lookiiig at two bodies wliofe difiance from 

 each other was known, we could tell tlieir diflance from the 



place 



Vol. L 



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