1 48 ] 
’■when the index {lands at the beginning of the divi- 
fions : for, if they do not, there mud be an allow- 
ance made in every obfervation ; to avoid which, it is 
always bed to adjud the threads to the beginning of 
the divifions when they are firft put on ; lor 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 fcrews which faden it to the 
moveable arm, through which the long fcrew pades. 
The other parallel thread, which I call the fixed 
one, muft be fird adjuded by fetting its edge exadtly 
over the two marks made on each fide the ffiort 
diameter of the aperture in the broad plates, and the 
•crofs thread mud be likewife fet to agree with the 
drokes made on each fide the longed diameter, and 
then the interfedlion 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 fadened) by the 
worm, by turning of which the fixt parallel thread 
may eafily be made to lie parallel to the apparent 
motion of any objedt 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 dar, 
r$xc. move true along the fixt parallel thread, which 
is abfolutely necefiary 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 dar move exadlly 
^ipon -the thread, and it -can only be done 
by re- 
peated 
