System of Graduation:

The object vane has a vertical hair thin wire bisecting the object under observation. The observation vane (or eye vane) consists of a reflecting prism. Both the vanes are collapsible to be folded to lie on compass cover when not in use. A plane mirror is hinged to object vane to sight the object which is too high or too low to be sighted directly. The devotion of mirror could be adjusted to facilitate this process. In case of sun glare, while making the measurements become hard, sun screen of tinted glasses can be used through placing them in the line of sight among prism and object vane.

To dampen the oscillation of magnetic needle and providing stability to measurement procedure, a brake pin is provided on the side of the compass box. A lifting pin is also provided to lift the needle and to keep it pressed against glass cover when the object vane is folded and the compass is not in use. This prevents the pivot from excessive wear and tear.

(a) Prismatic Compass        (b) Surveyor's Compass

Figure: System of Graduation