I 52 ] 



as the fhoulders of the brafs work will admit, and 

 place the object glafs in its cell, and looking through 

 the telefcope at ibme very diftant object Hide the 

 wooden tube in or out till you make the object 

 appear moil diftincl, or till it has the leaft m6tiori 

 upon the threads when the eye is moved to and 

 fro 3 for then the threads of the micrometer wilt 

 be in the common focus of both glafies, and that 

 will be the proper diftance that the objedt-glafs 

 ought always to be at from the threads j and there 

 mould 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 oh* 

 ject-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 objecli-glafs is diftant from the threads, 

 and with the fame icale to find alfo how many 

 inches and parts of an inch an hundred, &c. revolu- 

 tions or threads of the fcrew of the micrometer are 

 equal to : then, making the hrft diftance radius, 

 the laft will be the fine or tangent of an ajnpje 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 fet down in the 

 table, as alio the parts of a revolution ; for in fmall 

 angles, fuch as can be observed with. the microme- 

 ter, their lines tangents or cords are nearly in the 

 : proportion with the angles themfelves. The 

 . nee before- mentioned (to be ufed as radius) 

 ought ftncllv to be taken from the threads to a 



point 



