330 POPULAR LECTURES AND ADDRESSES. 



correctors, even if their highest points rise as high 

 as five inches above the glass of the compass-bowl. 

 The instrument may be described as follows : A 

 tube, so placed that an observer looking down cen- 

 trally through it sees the divisions on the compass- 

 card beneath, is supported on a frame resting on the 



FIG. 43. 



cover of the bowl, and moveable round a vertical axis. 

 In the tube is fixed a lens at such a distance from 

 the compass-card that the degree divisions of its rim 

 are in the principal focus. At the top of the tube 

 a prismatic mirror is mounted on a horizontal 

 axis, round which it can be turned into different 



