Astronomical Circles and other Instruments. 425 
the third microscope and cutting frame, which microscope I 
shall in future distinguish by calling it the fixed microscope, is 
then to be firmly screwed to the arc, as near to its right ex- 
tremity as conveniently may be, and so that the intersection 
of the cross wires of the microscope may be a little without 
the exterior circle described on the face of the instrument, and 
yet that both the circles that limit the divisions may pass 
througli the field of view. The support of the fixed micro- 
scope must also be so placed, as that the cutting point may 
movQ in the direction of the diameter of the circle to be di- 
vided. The tnoveable microscopes are to be attached to the 
arc by their clamps, and to be always so adjusted that the inter- 
sections of their wires may appear half way between the 
circles intended to limit the divisions. The microscope which 
is placed next to the fixed microscope I shall distinguish by 
the letter A, and the other by the letter B. The micrometer 
head of A must be placed to the right, and that of B 10 the 
left, of tiieir respective microscopes. 
It will, perhaps, be found most convenient to commence by 
dividing the circle into five equal parts of seventy-two degrees 
each. For this purpose, having fixed the circle by means of the 
clamp attached to its tangent screw (with which every instru- 
ment is provided), draw a fine line with the cutting point from 
the exterior circle described on the face of the instrument to 
the edge of its limb: this line I shall call the line of reference. 
Adjust the fixed microscope to distinct vision, by sliding it in its 
support, and cause the line of reference accurately to bisect the 
vertical angles formed by the cross wires of the microscope, by 
moving the circle, and by turning the microscope in the tube 
of its support. Move the eye to the right and left, and observe 
