[ 52 ] 
as the fhoulders of the brafs work will admit, and 
place the obje£t glafs in its cell, and looking through 
the telefcope at fome very diftant objedt Hide the 
wooden tube in or out till you make the objedfc 
appear mod: diftindl, or till it has the leaf! motion 
upon the threads when the eye is moved to and 
fro ; for then the threads of the micrometer will 
be in the common focus of both glaffes, and that 
will be the proper diftance that the otjedt-glafs 
ought always to be at from the threads; and there 
fhould be made fome mark or ketch in the wooden 
tube in order to fet it always at the fame diftance. 
The proper diftance of the threads -from the ob- 
jedi-glafs being thus fettled, the table for turning 
the revolutions, &c. of the fcrew into angles or 
minutes and feconds of a degree may be made fe- 
veral ways ; but as good and eafy a method as any 
is carefully to meafure how many inches and parts 
of an inch the objedt-glafs is diftant from the threads, 
and with the fame lcale to find alfo how many 
inches and parts of an inchan hundred, &c. revolu- 
tions or threads of the fcrew of the micrometer are 
equal to : then, making the firft diftance radius, 
the laft will be the fine or tangent of an angle an- 
fwering to ico revolutions. And having the angle 
anfwering to i oo revolutions, the angle for any other 
number will be eafily known and let down in the 
Able, as alfo the parts of a revolution ; for in final! 
angles, luch as can be obferved with the microme- 
ter, their fines tangents or cords are nearly in the 
fame proportion with the angles themfelves. The 
diftance before- mentioned (to be ufed as radius) 
ought ftriftlv to be taken from the threads to a 
point 
