a floating collimator. 151 
the star is formed upon the cross wires. Conversely, if when 
the telescope is so adjusted, rays be supposed to pass from 
the cross wires through the object glass, they will emerge 
parallel, as if they had come from an object at an infinite 
distance. The cross wires are therefore similarly circum- 
stanced with respect to vision, as a fixed star ; and if ano- 
ther telescope, adjusted by a star, be employed to view these 
cross wires through the object glass of the former instru- 
ment, they will be seen with perfect distinctness, however 
near to each other the telescopes may be placed. 
Professor Gauss first published an account of this beau- 
tiful property, and he availed himself of it to measure the 
angular distance of the wires of a transit instrument by 
means of a theodolite placed near the object glass. 
In the Astronomische IN'ach'icten, J\To. 61, Professor 
Bessel has given a “ new method of determining the flex- 
ure of the telescope of astronomical instruments,'' which for 
elegance of invention can scarcely be surpassed. This he 
effected by means of the property described by Professor 
Gauss ; and at the end of his communication, M. Bessel 
proposes a method of finding the zenith point of an instru- 
ment, of which the following is a translation. 
“ I may here be permitted further to remark, that the 
zenith point of an instrument may also be found without 
turning it in azimuth by a similar contrivance. For this 
purpose nothing more is required than a telescope, furnished 
with a sensible level, which may be placed on either side 
with respect to the axis. If this be placed alternately towards 
the north and the south, so that the bubble of the level may 
be similarly situated in both positions, then the mean of the 
