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The difference of declination of two ftars, eVe, 

 may be obferved with great exactnefs, becaufe the 

 motion of the ftars is parallel to the threads; but in 

 taking any other diftance, the motion of the ftars 

 being oblique to them is a great impediment, be r 

 caufe if one ftar be brought to one thread before the 

 eye can be directed fo as to judge how the other 

 thread agrees to the other ftar, the former muft be 

 fomewhat removed from its thread, fo that in this 

 fort of obfervations the beft way of judging when 

 the threads are at the proper diftance is by fre- 

 quently moving the eye backwards and forwards 

 from one to the other : this method mud chiefly be 

 made ufe of when the diftance of the objects is pret- 

 ty large, and the motion or rowling of the eye 



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The micrometer is fo contrived that it may be ap- 

 plied to telefcopes of different lengths; but then, 

 there muft be a table for each telelcope, by which 

 the revolutions of the fcrew may be turned into minutes 

 and feconds of a degree. In order to this it is ne- 

 ceflary that the threads of the micrometer fhould be 

 placed exactly in the common focus of the object: « 

 glafs and eye-glafs, that is, where the images 

 of objects ken through the teleicope are diitiricliy 

 formed. The readied way of doing this: is 3 firft to 

 flide the micrometer into the grooves fixt to the fhort 

 brafs tube, which carries the whole apparatus of 

 eye-glafs, &c. and then to draw the eye-gia's out by 

 means of its Aiding work, till the threads of the mi- 

 crometer are in its focus, which is known by th< 

 appearing moft diftind, &c. Then thru ft the frioi 

 £ube before- mentioned into its proper plac'v as f i 



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