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The difference of deciination of two ftars, &rc. 
may be obferved with great exadtnefs, 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- 
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 obiervations 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 muft chiefly be 
made ufe of when the diftance of the objedts is pret- 
ty large, and the motion or rowling of the eye 
great. 
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 telefcope, by which 
the revolutions of the fcrew may be turned into minutes 
and leconds of a degree. In order to this it is ne- 
ceflary that the threads of the micrometer ftiould be 
placed exadtly in the common focus of the objedt- 
glafs and eye-glafs, that is, where the images 
of objects feen through the telefcope are diftinbtly 
Formed. The readied way of doing this is, firft to 
Aide the micrometer into the grooves nxt to the fliort 
brafs tube, which cariies the whole apparatus of 
eye-glafs, &c. and then to draw the eye-glafs out by 
means of its Aiding work, till thethieads of the mi- 
crometer are in its focus, which is known by their 
appearing moft diftindt, &c. Then thruft the fliort 
tube before-mentioned into its proper plac°, as fir 
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