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The Description of a Floating Collimator. By Captain Henry Kater, 

 F.R.S. Read January 13, 1825. [Phil. Trans. 1825,;;. 147.] 



The apparatus described in this paper (of which a drawing is now 

 laid on the table) is intended to determine the situation of the line 

 of collimation of a telescope attached to an astronomical circle, with 

 respect to the zenith or horizon in some one position of the instru- 

 ment ; in other words, to determine the zero point of the divisions on 

 the limb. This is at present usually performed by the use of the level 

 or the plumb line, or by the reflection of an object from the surface 

 of a fluid. The author describes the defects and inconveniencies of 

 each of these methods. Those of the plumb line, when applied to 

 small instruments (to the improvement of which he describes his at- 

 tention to have been particularly directed,) are referrible chiefly to 

 want of sufficient delicacy. Those of the level are referrible to a va- 

 riety of causes not under the command of the observer ; while ob- 

 servations, by reflection, the most perfect perhaps of any now prac- 

 tised, require an union of favourable circumstances rarely occurring. 

 Add to these when levels or plumb lines are used, the necessity of 

 reversing the instrument, and observing out of the meridian. And 

 when observations are made by reflection, that of deferring the cor- 

 responding observation to the following night, which has proved so 

 great an inconvenience at Greenwich, as to necessitate the erection 

 of a second circle for the purpose of simultaneous observation. 



The principles on which the floating collimator is constructed are 

 two : the first is the property of a telescope employed by Mr. Gauss, 

 and subsequently by Mr. Bessel, in virtue of which the cross wires 

 of a telescope adjusted to distinct vision on the stars, may be distinctly 

 seen by another telescope, also so adjusted, at whatever distance the 

 telescopes may be placed, provided their axes coincide ; the rays di- 

 verging from the cross wires of either telescope, emerging parallel 

 from its object-glass, and being therefore refracted by that of the 

 other telescope to its sidereal focus, as if they came from an infinite 

 distance. The author here translates an account by Professor Bessel, 

 of a method of using this principle to determine the horizontal or 

 zenith point of a circle by the use of a level, employed to place the 

 collimating or subsidiary telescope in a horizontal position, a method 

 which though characterized by him as the best mode of using a level 

 that has yet been devised, is still liable to the objections urged against 

 levels in general. 



The other principle which the author substitutes in the place of 

 the level, is the invariability with respect to the plane of the horizon 

 of a body of determinate figure and weight floating on the surface of 

 a fluid. In former inquiries he had satisfied himself that a body floating 

 on mercury might be so contrived as to have always, when at rest, the 

 same inclination to the horizon. He had thus a floating support to 

 which he could attach a telescope, a support requiring no adjust- 

 ment, offering the ready means of extreme accuracy, and precluding 

 all fear of those errors which might arise from the use of a level. 



