336 POPULAR LECTURES AND ADDRESSES. 



the azimuth mirror the eye may be placed at any 

 distance, of from an inch or two to two or three 

 feet, from the compass, according to convenience, 

 and in any position, and may be moved about 

 freely through a considerable range on either side 

 of the line of direct vision through the lens, 

 without at all disturbing the accuracy of the 

 observation. This last condition is secured by 

 the lens being fixed in such a position of the 

 instrument that the divided circle of the compass- 

 card is in its principal focus. Thus the virtual 

 image of the divided circle is at an infinite 

 distance, and the images of distant objects seen 

 coincidently with it by reflection in the plane 

 mirror show no shifting on it, that is to say, no 

 parallax, when the eye is moved from the central 

 line to either side. From the geometrical and 

 optical principles explained previously, it follows 

 also that if the azimuth instrument be used for 

 taking the bearing of an object whose altitude 

 exceeds 27, then the effect of turning the frame 

 carrying the lens and mirror in either direction 

 will seem to carry the object in the same direction 

 relatively to the degrees of the card ; or in the 

 contrary direction if the altitude is less than 27. 

 But if the altitude of the object be just 27, then 

 the azimuth instrument may be turned through 

 many degrees on either- side of the compass -card, 

 without sensibly altering the apparent positions of 

 the objects on the degree-divisions. 



