i|(J OKXKRAL CONSIDERATIONS 



.iirtniiiMjnt, and J*~-* a wire so that when the instrument was in its proper 

 pOMtion the image of the plumb-line formed by the lens exactly coincided with 

 the (bud wire. By moving the instrument till this coincidence was observed, 

 ^ ingtrumrnt wan mtd to its proper position. This contrivance was long 

 known a* "lUnuden's Ghost." It is, in fact, a simple form of the reading 

 niiiiiiOOtlpn; nd there is no better method of ascertaining that a delicately- 

 oliject w exactly in its proper place. 



17. COLLIMATINO TELESCOPE. 



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

 the ftflti M M0n 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 colliniating 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 collimating 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. 



18. 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. 



19. FUNCTIONS OF INSTRUMENTS. 



The foregoing remarks apply to the constituent parts of instruments, without 

 reference to the special department of science to which they belong. 



