[ So 1 



the middle of the aperture of the eye-glafs ; and for 

 this reafon the whole micrometer is contrived to 

 ilide to arid fro, as the cafe requires. The fame 

 motion is alfo of ufe in taking the difference of right 

 afcenfion and declination, by Hiding the flxt parallel 

 thread (on which the preceding flat* is brought to 

 move) towards one fide of the eye-glafs; for by that 

 means a greater angle may be taken in between the 

 parallel threads, if need be. And it mull always 

 be remembered that the moveable parallel thread 

 mould be fet either north or ibuth of the other, 

 according as the following flar is expected to be 

 really foutb or north of the preceding. 



In making an obfervation, either the inner or the 

 outer edges or the middle of the wires may be 

 brought to touch the objects; but then, it muff, be 

 remembered to allow fomething for the thicknefs of 

 the wire, in cdfe the obfervation be not made from 

 that part to which the index is adjusted. In ob- 

 Jcr/ing the diameters of the fun moon or planets, 

 it may perhaps be mod -convenient to make ufe of 

 the outer edges of the threads, becaufe they will ap- 

 pear moft diftinct when quite within the limb of 

 die planet ; &c. but if there fhould be any feniible 

 inflection of the rays of light in paffing by the wires, 

 this would be bed avoided by uling the inner edgQ 

 of one wire and the outer edge of the other. And 

 in taking; the diftance or difference of declination 

 between two ftars, 6cc. the middle of the threads 

 may perhaps be moif convenient : but, however the 

 obfervation is made, due correction mud be allowed 

 for the thicknefs of the wire, if .requifite. 



The 



