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The portion of a graduated instrument, as a quadrant or astrolabe, carrying the sights or telescope, and showing the degrees cut off on the arc of the instrument Whewell.

y; as, fiducial power. Spelman. Fiducial edge (Astron. & Surv.), the straight edge of the alidade or ruler along which a straight line is to be drawn. -- Fiducial line or point (Math. & Physics.), a line or point of reference, as for setting a graduated circle or scale used for measurments.

rame in the form of an octant having a graduated scale upon its arc, and an index arm, or alidade pivoted at its apex. Mirrors, called the index glass and the horizon glass, are fixed one upon the index arm and the other upon one side of the frame, respectively. When the instrument is held upright, the index arm may be…