Index mirror adjustment screw (first adjustment). These are spring clips that hold the mirror to its frame. Its purpose is to reflect light to the horizon mirror. When properly adjusted it should be perpendicular to the plane of the instrument. The index mirror is rectangular in shape and set in a frame attached to the moveable index arm. It is operated by pressing the button(s) in the handle and is useful for taking twilight sights. The light bulb is contained in a special holder so arranged that the light illuminates the arc, micrometer drum and vernier. Seconds or decimals of a minute of arc are read where one of the vernier graduations lines up perfectly with one of the minute graduations on the micrometer drum. Minutes of arc are read off the micrometer drum opposite the vernier index mark. The micrometervernier has five or six graduations which correspond to 0.2’ or 10” of arc respectively. Turning the micrometer drum screws the worm along the rack and permits fine adjustment of an observed angle or altitude. While whole degrees are read directly from the arc, minutes of arc are read from the micrometer drum. Exerting pressure on the quick release clamp disengages the worm from the rack and allows the index arm to be moved. The worm is an endless tangent screw which engages with the rack or gearing which is cut into the lower limb of the sextant. On releasing pressure, the index arm is automatically clamped in position.

The index arm can then be moved to the required angle. By exerting finger pressure, the clamp disengages or unclamps the index arm from the gearing cut into the lower limb of the sextant. The arrow of the index arm is known as the index mark. The index arm is mounted on a circular base plate and is free to rotate on a central axis underneath the index mirror. It is “let in” flush with the “lower limb” of the sextant and is graduated from 0° to 120° “on” the arc and from 0° to -5° “off” the arc. If the sextant is of the metal variety, the arc will be constructed of a thin strip of metal which has a low co-efficient of expansion. This adjustment alters the area of the horizon mirror viewed through the telescope and hence alters the brilliance of the reflected image. By adjusting the rising piece the telescope is brought closer to or further from the frame. The telescope or monocle is attached to the frame by and adjustable slide or rising piece which is fitted with a milled head quick release screw. A wide field of view is important when trying to locate stars which are not easily found because of the apparent size. Higher magnification is desirable when observing vertical sextant angles of distant shore objects to obtain a clearer view of the sun’s lower limb when taking sights. The monocle on the other hand has a magnification of 6 X and a field of view of 8½°. The telescope usually has a magnification of 4 X and a field of view of 5°. This is used to enlarge the observed object and to make accurate observations easier.

The handle is fitted with a button switch and has a compartment for batteries to power the electric light on the index arm. The curved, lower part of the frame is referred to as the lower limb of the instrument. The upper side of the frame is referred to as the Plane of the instrument. It incorporates three legs upon which the sextant rests when in the horizontal position. This is either of rigid metal construction or moulded plastic. The horizon mirror (third) adjustment screw.The horizon mirror (second) adjustment screw.The index mirror (first) adjustment screw.The sextant is made up of the following parts:

Nautical telescope drawing