The figure of this curve is a polar curve, whose
equation is _r_ = _a_ +- _b_ sin. 2 [theta], where _a_ is the distance
from the zero graduation to the axis of the mirror, and _b_ is the
length of the scale from zero to 2, and [theta] is the inclination of
the mirror. In the perspective view, Fig. 1, the curved edge cuts the
scale at 1. The instrument being thus set, the following elements may
be read either directly on the scales or by simple arithmetical
calculation:
[Illustration: FIG. 3]
The radius = 1.
AB, the chord, read direct on the straight scale.
AFB, the length of the arc, read direct on the back or under
surface of the plate.
FH, the versed sine, read direct on the curved scale.
ACB, the angle in the segment, read direct on the graduated
edge.
EAB, the angle between the chord and the tangent, read direct
on the graduated edge.
GAB, the tangential angle = 180 deg. - ACB.
AOB, the angle at the center = 2GAB.
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