;4 SCIENTIFIC APPARATUS. 



moving the instrument till this coincidence was observed, the 

 instrument was adjusted to its proper position. This contrivance 

 was long known as " Ramsden's Ghost." It is, in fact, a simple 

 form of the reading microscope ; and there is no better method of 

 ascertaining that a delicately-suspended object is exactly in its 

 proper place. 



17. COLLIMATING TELESCOPE. 



If two telescopes are made to face one another, and if the cross- 

 wires of the first, as seen through the second, appear to coincide 

 with the cross-wires of the second, the optic axes of the two 

 instruments are parallel. This mode of ascertaining parallelism is 

 used in practical astronomy, and is called the method of collimat- 

 ing telescopes, or of collimators. 



It is also used in the Kew portable magnetometer. The magnet 

 is hollow, and carries a lens at one end and a scale at the other, 

 at the principal focus of the lens. The magnet is thus a colli- 

 mating telescope, and is observed by means of a telescope mounted 

 on a divided circle. The disadvantage of this method is, that 

 when the magnet is deflected, the scale soon passes out of the field 

 of view, and the observer has to shift his telescope, in order to get 

 a new reading. 



1 8. THREE CLASSES OF READINGS. 



We may, in fact, arrange instruments in three classes, according 

 to the method of reading them. 



In the self-recording class the observer leaves the instrument to 

 itself, and examines the record at his own convenience. 



In those which depend on eye observations alone, the observer 

 must be there to look at the indicator of the instrument, but he 

 does not touch it. 



In the third class, which depend on eye and hand, the observer, 

 before taking the reading, must make some adjustment of the 

 instrument. 



