166 ASTRONOMY. 
employed, which, by their constructions, compensate forthe want ofa 
fixed station. Among these belong first the reflecting sextant. This con- 
sists of a circular sector, amounting to from 60-65 degrees, and is an 
instrument of great value on land, but absolutely indispensable at sea. As, 
however, from the theory of the instrument, the angle indicated on the 
sextant is exactly half the true angular distance, every degree of the sector 
indicates two degrees of angular distance. The instrument, therefore, 
measures angles of 120° to 130°, on which account every half degree of 
graduation is marked as a whole degree. About the centre of the sector 
rotates an alidade, which carries a large plane mirror, passing through the 
centre of the sector; another somewhat smaller plane mirror is fixed per- 
pendicularly to the plane of the sextant, and so adjusted, that when the 
alidade is brought to the zero point of the graduation, the planes of the two 
mirrors are parallel. This plane mirror is uncovered in its upper half, and 
in practice, the signal of one leg of the angle to be measured is seen by 
direct light, that of the other immediately under it by reflection. At the 
back part of the sextant, a handle of wood is attached, by which it is held 
during observation. Between the two mirrors are hinged variously colored 
glasses for the protection of the eyes when observing in a bright light. 
The astronomical telescope is screwed in such a manner into the frame 
that the objective end lies next to the mirror. The alidade carries a ver- 
nier with a lens for reading off the degrees. The sextant must not be too 
heavy, as it is to be held in the hand when in use. Sextants of greater 
dimensions, as of 8 inches radius and more, have stands specially adapted 
to them. 
The errors of such an instrument must be ascertained before using it. 
The first of these is the error of collimation ; the second, a want of parallel- 
ism of the axis of the telescope with the plane of the instrument; the third 
consists in an unequal distinctness of the direct and reflected image of the 
same object. The fourth error is when the sides of the mirrors are not 
accurately plane and parallel to each other; and the fifth has reference to 
the same circumstance in the colored glasses. All these errors must there- 
fore be rectified before the instrument can be used. 
The first application of the sextant is in measuring the angle between two 
objects at any direction with respect to the horizon. Here the least illumi- 
nated object is selected as the one to be seen by direct light. The second 
application is to the measurement of altitudes. To determine the altitude 
of an object by means of the sextant, look directly through the telescope at 
the image of the object in the horizon, which may either be a natural or an 
artificial one; bring the plane of the sextant into the vertical position, and 
move the alidade until the reflected image of the object covers its direct 
image: the angular position of the alidade will indicate double the altitude 
desired. 
Pl. 15, fig. 23, gives a perspective view of another form of sextant with 
a glass prism, of simpler form and less expensive construction. ABB is the 
body, BB the graduated limb, C the movable alidade with the vernier, D the 
lens for reading off the graduation, GF the telescope, E the box containing 
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