Astronomical Circles and other Instruments, 
4<^7 
accurately through their intersection, cause it to do so by 
means of the screw of the moveable support of the microscope 
B. The intersections of the cross wires of both microscopes 
will now be equidistant from the centre of the circle. A ^jiist 
both micrv .scopes to distinct vision, in the manner described 
for the fixed microscone, using the adjustable d j 1 instead of 
the line of reference. Return the line of reference to the fixed 
microscope, clamp the circle, and, by means of the tangent 
screw, bring the line of reference again accurately under the 
intersection of the cross wires. Place the second adjustable 
dot precisely under the intersection of the wires of the micro- 
scope B. When all has been thus arranged, the line of 
reference, and the two adjustable dots, should appear in the 
intersection of the wires of their respective microscopes, and 
the dot of the cutting frame, if moved through the field of view, 
should pass through the intersection of the wires of the fixed 
microscope. This arrangement of the microscopes is repre- 
sented in plate XVI. from which the manner of placing them in 
the subsequent parts of the operation will also be readily 
understood. 
Turn the circle on its axis till the dot which was under A, 
arrives under B. Clamp the circle, and, by means of the 
tangent screw, bring the centre of this dot under the inter- 
section of the wires of the microscope B. Take off the adjust- 
table dot which was under B, and which will now be found to 
the left of that microscope, place it beneath A,^ and, by means 
of its adjusting screws, bring its centre under the intersection 
of the cross wires, carefully remarking whether the circle has 
* I'he artist will perceive, that five dots in this stage of the process will be more 
convenient than two, and obviate the necessity cf removing the dots. 
