COLLIMATOR

Captain Katergave the name of floating collimator to an instrument invented by him for ascertaining the horizontal point. It consists of a small telescope, furnished with cro -wires in its focus, and fastened horizontally on a flat iron float, which is made to swim on mercury, and which of course, when left to itself, will always a ume one and the same invariable inclination to the horizon. If the cro -wires of the collimator be illuminated by a lamp placed in the focus of its object gla , the rays from them will i ue parallel, and consequently be in a fit state to be brought to a focus by the object gla of any other telescope, in which they will form an image as if they came from a celestial object in their direction, »'. e. at an altitude equal to their inclination. By transferring, then, the collimator still floating on a ve el of mercury from one side of a circular instrument to the other, we are furnished with two quasi-celestial objects, at precisely equal altitudes, or opposite sides of the centre; and if these be observed in succe ion with the telescope of a circle, bringiiiix its cro to bisect the image of the cro of the collimator, the difference of the two readings on the limb will be twice the zenith distance of either; whence the horizontal or zenith point is immediately determined. Another form of the collimator has the telescope placed vertically, whereby the zenith point is directly ascertained. (Phil. Trans. 1828; Pearson's Practical Astronomy, vol. ii.) [s. 262]