46 ON MAGNETISM. 



another lens on the telescope-object-glass; by which 

 process the horizontal reading for magnetic meridian 

 was found. 



By such methods as these, "Variation Charts" or 

 more properly "Declination Charts" have been pre- 

 pared for the use of mariners, exhibiting the Compass- 

 Declination on every part of our globe. And it is 

 mainly by the use of these, in connexion with the 

 compass, that ships are steered. 



23. More accurate method of determining the Local 

 Magnetic Meridian; Reversed Telescope carried by Mag- 

 net; elimination of corrections for position of Magnetic 

 Axis, and for torsion-power of suspension-thread. 



The most accurate apparatus for determining the 

 Declination is however that represented in Figure 18, 

 which is nearly copied from the instrument used at the 

 Royal Observatory, Greenwich. It is founded essen- 

 tially on the use of the " Reversed Telescope " or (as 

 it is frequently but inappropriately called) the u Colli- 

 mator." It is an ordinary optical theorem that, if 

 parallel rays fall upon an object-glass, they will be 

 made to converge to a point: and conversely, if rays 

 diverge from a point at the focus of an object-glass and 

 fall on that object-glass, they will emerge parallel. 

 This quality of parallelism of rays possesses two im- 

 portant advantages. First, that the small object at 

 the focus (which object will usually be a cross of wires) 

 can be distinctly seen by use of a telescope, adapted to 

 receive parallel rays, or rays coming from a distant 



