154 
Captain Kater's description of 
focus, and two inches and three-quarters aperture, furnished 
with a wire micrometer, was placed upon a deal support 
with three legs. This support rested upon a flat stone laid 
upon sand which filled a pit of seven feet in depth, and 
afforded a perfectly steady foundation. The moveable wire 
of the micrometer was carefully placed in a horizontal position. 
The floating collimator was put upon a table attached to 
the wall of the observatory, and was placed in the proper 
direction by looking through its telescope, and moving the 
box till the cross wires appeared upon the wire of the micro- 
meter. The cross wires,* which formed an angle of about 
15 degrees, were illuminated by a small lanthorn placed 
upon the table with a piece of oiled paper interposed. 
My first trials were made with the telescope of a sextant, 
but as the object glass was not sufficiently perfect, -f I did not 
conceive it worth while to register them, and I shall merely 
remark, that after deranging the float by moving it in a 
variety of ways, the cross wires returned, as nearly as the 
imperfection of the image would permit me to judge, to the 
same situation. 
The telescope subsequently employed had an achromatic 
object glass of one inch and a quarter aperture, and seven 
inches and a half focus, and this gave a sufficiently good 
image. 
In the experiments I am about to detail, every method I 
could think of was used that could fairly introduce error. 
• The material which after numerous trials I found to answer best for cross 
wires, was the steel spring used in the balance of a watch. 
f I may here observe, that I^find this mode of examination to be a most severe 
test of the. goodness of an object glass employed as a collimator. 
