208 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



two mirrors is that between the movable index and the zero of the 

 sextant. 



The way in which this instrument is used will be now easily under- 

 stood. 



The observer takes it by the handle with the left hand ; then, putting 

 the eye to the eye-piece of the telescope, he sights one of the objects, a 

 star for instance, through the unsilvered portion of the small mirror. 

 He then moves the sextant until the other star is in the plane of the 

 sector ; afterwards turning the index and the large mirror, he brings 

 the image of the second star, after two successive reflections, into 



FIG. 140. Naval officer observing with a sextant. 



coincidence with that of the first, in the centre of the field of the 

 telescope. 



When the angular distance from a star to the moon has to be 

 measured, the star is sighted directly and the image of the edge of 

 the luminous disc is brought in contact with the image of the star. 

 If the angular distance between the sun and the moon has to be deter- 

 mined, the image of the sun is brought in contact with the lunar 

 image ; but in this case coloured glasses are placed before the large 

 mirror to reduce the intensity of the sun's rays. 



