CHAP, i.] MIRRORS AND REFLECTING INSTRUMENTS. 207 



therefore called octant) : the one represented in figure 145 consists of 

 85. At the centre of the sector is fixed an arm furnished with a 

 movable index and a vernier V, which enables the fractions of a 

 degree on the arc to be read : I is a small microscope used for this 

 purpose. A silvered mirror M is fixed normally at the centre of the 

 sector and in the line of the zero of the movable index. It is 

 movable with this index. A second mirror M' is fixed on one of the 

 sides of the sector so that when the two mirrors are exactly parallel 

 the reading vernier is at the zero of the graduation : this second 

 mirror is only silvered on its lower half. 



FIG. 145. The sextant. 



Looking through a telescope L fixed on the opposite radius of the 

 sector to that on which the fixed mirror is placed, a point situated in 

 the direction LS', can be seen directly as half the mirror M', is trans- 

 parent, and by reflection, another luminous point reflected doubly 

 from I on the first mirror, and F on the second, can be seen when a 

 perfect coincidence of these two images has been brought about. It is 

 clear that the angle formed by the rays SI, ST, is double the angle 

 of the two. mirrors for the reason already stated. Xow, the angle of the 



