The Collimator Apparatus. 



For adjusting the Line of Collimation, etc., in the Telescopes of Geodetic Instruments, 



In use since 1872. 



NOTE. A collimator is an optical instrument whose function is to furnish a pencil of rays of light the 

 axis of which is in a given direction. The collimator is commonly arranged so that the rays shall be parallel, 

 as if emanating from an object at an infinite distance ; or it may be arranged so that the rays shall be 

 divergent, as if emanating from an object at a given finite distance. These changes are made by moving th 

 object-slide of the collimator to previously determined positions, as in focusing a telescope. 



The pencil of rays emanating from the collimator is received by the telescope of the instrument to be 

 adjusted, precisely as if it had emanated from a fixed star, or from an object at a given finite distance, and the 

 telescope must be manipulated precisely as if it were pointing at natural objects. This renders the adjust- 

 ment of the telescope, its wires, level, etc., very simple. Even where the necessary long sights are con- 

 venient and the state of the atmosphere is favorable, the use of a collimator is to be recommended on account 

 of the greater ease, sharpness and accuracy with which the collimator objects can be scan as compared with 

 crdirary objects. 



C. L. B'i-u'i- <V Sons' Auxiliary Apparatus. 

 Used during the construction of their Instruments of Precision 



a more detailed description, see article in Zeitschri/t fiir InstntmentenkuntU. 



