Astronomkal Circles and other Instniments, 429 
the line of reference and the two dots are under tlie inter- 
sections of the wires of their respective microscopes, and each 
of the other divisions, successively after the dots have been 
made to change their places ; and thus the circle will be 
divided into five equal parts, which seem to be subject to no 
other error than what may be attributed to the imperfection 
of vision, or inaccurate execution. 
Each division of seventy-two degrees is next to be sub- 
divided into three parts. To effect this, bring the line of 
reference to the intersection of the wires of the fixed micro- 
scope, and verify the adjustment of the cutting frame. Place 
the microscope B, above the first division from the fixed 
microscope, and the microscope A, at the distance of one third 
of a division, or twenty-four degrees by estimation from B. 
Cause the division which is under B, to bisect the vertical 
angles of the cross wires, by means of the micrometer screw, 
and by turning the microscope in its tube, and adjust both 
microscopes to distinct vision. Place an adjustable dot under 
the intersection of the wires of A ; unclamp the circle, and 
turn it till the adjustable dot is under B; when, if it does not 
pass precisely through the intersection of the cross wires, it 
must be made to do so, as before described, by means of the 
screw attached to the support of the microscope. Bring back 
the line of reference to the wires of the fixed microscope, 
when the adjustable dot should be precisely in the intersection 
of the wires of A, and the first division (seventy-two degrees 
from zero) accurately bisecting the vertical angles of the cross 
wires of B. Turn the circle round till the adjustable dot arrives 
under B, and by means of the tangent screw bring it accurately 
to the intersection of the cross wires. Place another adjustable 
MDCCCXIV. 3 K 
