TERRESTRIAL MAGNETISM. (APP. B.} 331 



positions when in use. In the two methods of 

 observation which I am going to describe, the mirror 

 covers about one half of the top of the tube ; the 

 upper half in the first method and the lower half in 

 the second. 



(i) When taking a bearing by the first method, 

 the principle of which corresponds with the ordinary 

 camera lucida, the observer turns the instrument 

 round its vertical axis until the mirror and lens 

 are fairly opposite to the object. He then looks 

 through the lens at the degree divisions of the 

 compass-card, and turns the mirror round its hori- 

 zontal axis till he brings the image of the object to 

 fall on the card. He then reads directly on the 

 card the compass bearing of the object. Besides 

 fulfilling the purpose for which it was originally 

 designed, to allow bearings to be taken without 

 impediment from the quadrantal correctors, the 

 azimuth mirror has a great advantage in not 

 requiring any adjustment of the instrument, such 

 as that by which in the prism compass the hair 

 is brought to exactly cover the object. The 

 focal length of the lens is about 12 per cent, 

 longer than the radius of the circle of the com- 

 pass-card, and thus, by an elementary optical 

 principle, it follows that two objects a degree 

 asunder on the horizon will, by their images seen 

 in the azimuth mirror, cover a space of ri2 of the 

 divided circle of the compass-card seen through 

 the lens. Hence, turning the azimuth instrument 



