302 Analysis of ScAentific Books, 



observation to the following niglit, 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 ad- 

 justed, at whatever distance the telescopes may be placed, pro- 

 vided their axes coincide, — the rays diverging 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 ze- 

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

 collimating or subsidiary telescope in a horizontal position — a me- 

 thod which, though characterized by him as the best mode of 

 using a level that has been yet devised, is still liable to the ob- 

 jections 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 him- 

 self practically that a body floating on mercury might be so con- 

 trived 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 adjustment, offering the ready 

 means of extreme accuracy, and precluding all fear of those errors 

 which might arise from the use of a level. 



The collimator, in its perfect state, consists of a piece of cast 

 iron about 8 inches long, 4 wide, and from a quarter to half an 

 inch thick, having two uprights in the form of Y's, in which the 

 collimating t«lescope is firmly fastened. The support is then floated 

 on mercury in a deal box, somewhat larger than the flat portion 

 of the iron, and having its bottom just covered with mercury. 

 The float is kept in its situation in the middle of the box, and 

 prevented from moving horizontally by two smooth iron pins pro- 

 jecting from its sides, and moving freely in vertical polished 

 grooves of metal let into the sides of the box. The whole of the 

 telescope projects above the edges of the box, and a screen of 

 black pasteboard, with an aperture equal to that of its object- 

 glass, is fixed to the end of the box to keep off false light. The 

 instrument was placed on a table attached to the wall of the ob- 

 servatory, and directed (by looking through its telescope) to the 

 wires of a fine achromatic, furnished with a wire micrometer. 

 The cross-wires of the collimator were then illuminated by a 



