254 



HISTORY OF SCIENCE, 



the horizon remains in view through the unsilvered part of L, and the 

 movable arm, carrying with it the mirror c, is turned until the image 

 of the star appears to coincide with the horizon. Thus c D may re- 

 present the new position; the rays from the star will be reflected along 

 the path s c L/, and s will appear to the observer to coincide with H. 

 It may easily be shown that the angle BCD must be exactly half of 

 the angle K c s ; and the graduations on the limb A B being arranged 

 accordingly, we have only to read off the angle BCD. The great 



FIG. 124. H ABLE Y'S SEXTANT. 



advantage of the instrument is that both the objects whose angular 

 distance is to be taken are visible at the same instant, and that the 

 instrument requires no levelling, etc., being simply held in the hand. 

 Fig. 124 exhibits the actual construction of the sextant, c and L are 

 the mirrors as before, c E is the movable arm provided with a vernier 

 (page 212), and with a lens G, for reading off the small divisions of the 

 scale, j K is the telescope, o is a handle for holding the instrument, 

 c is a milled head for accurately adjusting the position of the movable 

 arm. M and N are screens of coloured glasses, which can be turned 

 in the path of the rays of the sun when he is the object of observation. 

 The use of the instrument we have just described furnishes a striking 

 illustration of the powers of astronomical calculation, and of the cer- 

 tainty of those principles upon which they rest. " A page of lunar 



