172 
Captain Kater's description of 
the horizontal collimator should remain stationary, and that 
the usual course of observations should be referred to it, its 
inclination to the horizon having been previously determined, 
and its permanency, w^hen thought requisite, being examined 
by means of the vertical collimator. 
As it is not necessary for the telescope of the collimator to 
have a tube, the object glass and the cross wires in the hori- 
zontal construction may be fixed in two uprights cast in one 
piece with the float. The distance of the object glass from 
the cross wires must be capable of the nicest adjustment. 
This may be effected by a screw cut on the outside of the 
tube in which the object glass is set, and a collar, by means 
of which after it is adjusted, it may be firmly secured in its 
proper place. There should be short pieces of tube screwed 
on each side of the upright, to protect the cross wires from 
injury, and also to contain the eye glass, which is convenient, 
as well for illuminating the wires, as for placing the colli- 
mator in the proper direction. This construction appears to 
promise the most perfect invariability of relative position be- 
tween the line of collimation and the float. The box should 
be sufficiently deep to include the whole instrument, and 
should have apertures made in the ends, opposite to the ob- 
ject glass and to the cross wires. It is scarcely necessary to 
add, that it should also have a cover to exclude dust from 
the mercury, and a piece of ground glass or oiled paper 
should be placed between the cross wires and the lamp by • 
which they are illuminated. 
The accurate adjustment of the cross wires is a point of 
extreme importance. Upon whatever portion of an object 
glass parallel rays fall, they are converged precisely to the 
