[ 540 '] 
ties of the wire, E W the adjuftment of the wires 
and micrometer may be more readily performed, and 
the obfervation may be nude on two ifars, although 
their iriCridian diftance from one another fliould be 
almoft equal to E W tlie diameter of the field of the 
telefcope. It is evident, that if two ftars be thus 
obferved wliofe difference of declination is well fettled, 
the value of the fcale of the micrometer may be 
thereby determined. 
In the foregoing diredlions It has been fuppofed that 
the images of the two ftars can be brought to appear 
within the field of the telefcope on the wire E W 
at the fame time ; but this is not abfolutely neceffary. 
For if the micrometer be fet to the difierence of de- 
clination nearly, and then the ffar w^hich palfcs firft 
through the telefcope be made to run along the wire 
E W by touching one of the handles of the rack- 
work of the telefcope, and afterwards the other ffar, 
when it comes into the telefcope, be brought to the 
wire E W by altering the opening of the glafles of 
the micrometer, the difference of the declination 
will be had, by taking h^lf the fum of the numbers 
fiiewn by the micrometer, at the two feparate ob- 
fervations of the two ftars on the wire E W. This 
will be true, In cafe it can be depended upon that the 
two femicircular glaffes recede equally in contrary 
diredfions; which may indeed be doubted, the work 
on which the motion of the glaffes depends not being 
defigned for fuch a purpofe, and therefore probably 
not made fufficiently accurate for it. 
The manner In which Mr. Dollond has contrived 
the motion of the glafles in his new improvement of 
the objed-glafs micrometer intirely obviates this 
diflicultv, 
0 * 
