C 147 3 
VII. The description of a floating Collimator. By Captain Henry 
Kater, F.R. S. 
Read January 13, 1825. 
X^HE line of collimation of a telescope is a line which passes 
through the centre of the object glass and the intersection of 
the cross wires placed in its focus. The apparatus which I 
am about to describe is intended to determine the situation 
of this line with respect to the horizon or the zenith, in some 
one position of an astronomical circle to which the telescope 
is attached. 
A plumb line, a level, and an image reflected from the sur- 
face of a fluid, are the means which have hitherto been em- 
ployed for this purpose. The defects and inconveniences of 
each of these have been felt, and the subject has for some 
years past engaged much of my attention, principally with a 
view of bringing instruments of portable dimensions into com- 
petition with those of a larger size. 
Since a plumb line even of six feet in length would be 
subject to a deviation of only about three ten thousandths of 
an inch with an angular variation of one second, the applica- 
tion of the plumb line to small instruments becomes useless 
when great precision is required. 
The difficulty of procuring a good and sensible level is 
well known, and though a very valuable instrument when 
