17 
AbJi'MMEKTS of The sextant. 
the horizon glass, and must be perpendicular to the plane of the instru¬ 
ment, in such a position that its plane shall be parallel to the plane of the 
index glass when the index points to zero (0°) on the arc; it is adjusted 
by means of the screw K*. 
L and M are coloured glasses of different depths of shade, any one or 
more of which can be turned down in front of either the index or horizon 
glass to moderate the intensity of the light before reaching the eye* 
when a bright object, such as the sun* is observed. N is a microscope 
which is carried on a moveable arm 0, and can be adjusted to read the 
divisions on the graduated arc and vernier. T is the telescope , at the 
eye end of which coloured shades can be attached which should always 
be used when observing the sun in an artificial horizon in preference to 
the shades L, M. It is carried by a double ring, R, so constructed that 
it furnishes means of adjusting the line of collimation: this ring is 
attached to a stem S, which can be raised or lowered until objects seen by 
reflection, and directly, appear of*the same brightness. U is the handle 
which is often fitted with a brass centre, having a hole in it, to admit 
of its being fastened to a stand. 
Adjustments of the Sextant. 
The principal are the following 
1. To make the index glass perpendicular to the plane of the 
instrument. 
2. To make the horizon glass perpendicular to the plane of the 
instrument, and parallel to the index glass when the index points 
to zero (0°) on the arc. 
3. To make the axis of the telescope parallel to the plane of the 
instrument, in which the index moves. 
ls£ Adjustment .—This adjustment rests with the maker; and being 
once made cannot be deranged, except by a fall or blow, against which 
every precaution must be taken. The instrument should, however, be 
occasionally verified by the observer in the following manner:—Set the 
* The form and position of this screw differs very much in different sextants ; 
in many, the adjustment is made by two small screws bearing on the back cf the 
glass. 
VOL. I. 
c 
