1 48 J 



••vc-hcn the index flands at the beginning of the div't- 

 -fions : for, if they do not, there muft be an allow- 

 ance made in every obfervationi to avoid which, it is 

 always btft to adjuft the threads to the beginning of 

 "the divifions when they are firfr. put on 5 for which 

 purpofe the holes in the little plate which carries 

 "the moveable thread are made oblong to give room 

 to move it as occafion requires, before it is pinched 

 hard by the fmall ferews which faften it to the 

 moveable arm, through which the long fcrew pafTes 

 The other parallel thread, which I call the fixed 

 one, mull: be firft adjufted by fetting its edge exactly 

 over the two marks made on each fide the fhort 

 diameter of the aperture in the broad plates, and the 

 crofs thread muft be likewife fet to agree with the 

 'ilrokes made on each fide the longeft diameter, and 

 then the interferon of the crofs thread and the fixt 

 .parallel one will be the center of the motion given, 

 to the outer plate of the micrometer (to which the 

 great fcrew index and threads are faftened) by the 

 worm, by turning of which the fixt parallel thread 

 may eafily be made to lie parallel to the apparent 

 ^motion of any object in order to take the differ- 

 ence of declination and right afcenfion from any 

 • other that follows through the aperture of the te- 

 lefcope. 



This contrivance is of very great ufe to make a ftar, 

 s&c. move true along the fixt parallel thread, which 

 is abfolutely neceffary in order to 'take the true dif- 

 ference of right afcenfion and declination between 

 it and any other that follows. Without this contri- 

 vance it is very difficult to make a ftar move exactly 

 mpon the thread, and it can only be done by re- 

 peated 



