DAVIS Mark 15, 25 Sextant - Manual (Page 8)

Page 6
To adjust the horizon mirror (Mark 15 only):
1. Adjust the horizon mirror (the small, half-silvered mirror) for “side error”
by making it perpendicular to the frame.
2. Holding the sextant in your right hand, raise the instrument to your eye.
3. Look at any horizontal straight edge (the sea horizon or the roof of a
building, for example) and move the index arm back and forth.
The real horizon will remain still while the mirror horizon will appear only when
the body and drum scales read close to zero.
4. Line up the mirror horizon and the real horizon so that both appear as a
single straight line (Fig. 6).
Figure 6
Figure 7
5. Without changing the setting, look through the sextant at any vertical
line (a flagpole or the edge of a building, for example) and slowly swing
the instrument back and forth across the vertical line.
If the horizon mirror is not perpendicular to the frame, the line will seem to
jump to one side as the mirror passes it. To correct this, slowly tighten or
loosen the screw closest to the frame at the back of the horizon mirror until
the vertical line no longer appears to jump (Fig. 7).