NAVIGATION. 



Pepper's ghost, in the transparent plate-glass 

 coincidently with A seen through the same. From 

 the law of reflection, that the incident and reflected 

 rays make equal angles with the mirror, it is clear 

 that when the silvered mirror is turned through 



FIG. i. Sextant. 



any angle, the ray reflected from it turns through 

 twice as large an angle, and after its second re- 

 flection (as the second mirror is fixed) it turns 

 through still the same angle, that is to say, 

 through twice the angle turned by the silvered 

 mirror. The angles through which the silvered 



