177 
a floating collimator. 
larger than that of a sextant furnished with crossed wires,* 
and having its object glass next that of the zenith tube. 
To any convenient part, either of the shelf or of the 
wall, a support must be fixed, to which a circular screen 
of blackened tin may be attached by a joint, so as to be 
elevated to the vertical or lowered to the horizontal posi- 
tion at pleasure. In the centre of this screen a hole is to be 
made rather smaller than the telescope of the collimator. 
The screen iS' intended when in use to occupy a horizontal 
position, just above the crossed wires of the collimator, and to 
exclude false light from the object glass of the zenith tube. 
In order to illuminate the wires of the collimator, a small 
plane reflector, which may be of planished tin, is to be at- 
tached at a convenient angle to the upper side of the screen 
over its aperture. This may be made to turn stiffly upon a 
hinge to vary its inclination. 
Having put a sufficient quantity of mercury into the box to 
enable the float to act freely, the screen must be turned up 
and the micrometer adjusted, so that a star may pass along 
its moveable wire. The screen being then restored 'to its 
horizontal position, the crossed wires of the collimator will be 
distinctly seen when the arm carrying its telescope must be 
bent till they appear in the centre of the field of view, and 
the telescope of the collimator must be turned in its tube till 
the opposite angles of the crossed wires are bisected by the 
micrometer wire. These adjustments may be considered as 
permanent. 
To determine the zenith distance of a star, it must be 
A small black dot upon mother-of-pearl forms a very neat object instead of 
the crossed wires, but from some trials, I fear it cannot be made sufficiently small, 
MDCCCXXV. A a 
