[ 50 ] 
the middle of the aperture of the eye-glafs ; and fo? 
this reafon the whole micrometer is contrived to 
fide to arid fro^ as the cafe requires. The fame 
motion is alfo of gfe in taking the difference of right 
afcenfion and declination, by Hiding the fixt parallel 
thread (on which the preceding flar is brought to 
move) towards one fide of the eye-glals ; for by that 
means a greater angle may be taken in between the 
parallel threads, if need be. And it muff always 
be remembered that the moveable parallel thread 
fhould be fet either north or fouth of the other, 
according as the following flar is expedted to be 
really fouth 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 objecls; but then, it mull be 
remembered to allow fomething for the thicknefs of 
the wire, in cafe the obfervation be not made from 
that part to which the index is adjufted. In ob~ 
ier/ing the diameters of the fun moon or planets, 
it may perhaps be mofl convenient to make ufe of 
the outer edges of the threads, becaufe they will ap- 
pear mofl diflindt when quite within the limb of 
the planet ; &c. but if there fhould be any fenfible 
infledtion of the rays of light in paffing by the wires, 
this would be befl avoided by ufing the inner edge 
of one wire and the outer edge of the other. And 
in taking the dillance or difference of declination 
between two flars, £cc. the middle of the threads 
may perhaps be mofl convenient: but, however the 
obfervation is made, due corredtion mufl be allowed 
for the thicknefs of the wire, if requifite. 
The 
