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214 The Sextant #1
The Sextant sometimes called from
Plain its inventor "Hadley's Sextant" is a
graduated arc of a circle, with
description certain accessories, so arranged that it
can be Employed to measure angular
of distances, Especially of celestial
objects. It is an instrument of
the great practical importance to the
Navigator and traveller for determining
the time, the latitute and the longitude.
Though less directly an astronomers
Sextant instrument, it is often valuable for
for [struckthrough] purposes strictly astronomical,
such as ascertaining the time and
fixing the position of plan [struckthrough] comets,
from which Right Ascensions and
Declinations can be derived
The flat surface is called the plane
Names of the instrument. Then we have
the arc on of [struckthrough] limb, reading it by the
of Vernier Scale attached to the moveable
radius up to 10" and sometimes to 5"
the as the case may be. There are also
the Fore Shades on screens of
Various colored glass. The "Horizon glass"
the lower half of which is silvered
parts and which has also screws for its
of adjustment. There are also the
Black Shades or screens of colored
the glass, a "Telescope" which is
not always necessary. A moveable
Sextant radius carrying at one End the
Index glass, and at the other End
a Vernier Scale, which is read
by a microscope or its Equivalent.
Slow Motion is imparted to the
radius by a Tangent Screw.
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The Sextant #2 215
The principle of the Sextant depends
on the practical application of the
following theorem in optics;
When a ray of light, proceeding
in a plane at right angles, to Each
of 2 plane mirrors, which are
inclined to Each other at any angle
whatever, is successively reflected
at the planes of Each of the mirrors,
the total derivation of the ray, is
double the angle of inclination of
the mirrors.
The adjustments of the Sextants
are 5 in number
1. First.
That the Index glass should be
perpendicular to the plane of the
instrument.
2. Second
That the Horizon glass
should be perpendicular to the plane
of the instrument
3d. [Third] That the horizon glass should be
paralell to the index glass, when the
zero of the vernier coincides with
the zero of the arc.
4. The the line of collimation, or
the optical axis of the telescope
be paralell to the plane of the
sextant
5 That the index Error be Known
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