CHAP. i.l MIRRORS AND REFLECTING INSTRUMENTS. 209 



If the distance of a star above the horizon, called its ' altitude, 

 is to be measured, the sextant is held vertically, so that the star is 

 in its plane, and the horizon formed by the surface of the sea is 

 sighted directly with the telescope. If this horizon is undefined, an 

 artificial one is used, either a mercury bath, or a polished glass 

 lendered horizontal by three levelling screws and a level. 



III. GONIOMETERS. 



It is known that in nature there exists a great numbei 

 which have a definite geometrical form, most often marked by plane 

 and polished sides or faces grouped in various ways. Such are crystals. 

 ]\J ineralogists who find crystals ready formed in the rocks, and chemists 

 who obtain them by various methods, both take equal care to define 

 their shapes and to mark with precision the angles of each of their faces. 

 They accomplish this by the help of instruments called goniometers 

 (from 7ow'a, angle and ^erpov, measure), which are also based on the 

 principles of reflection. Very often, indeed, the faces of the crystals 

 are so far reflecting that each of them may be considered and 

 employed as a plane mirror. 



Eeflection goniometers are of various forms. We will confine 

 ourselves to the description of the two most used, Wollaston's 

 and Babinet's goniometers. 



Wollaston's goniometer is composed of the two following parts : 



1. D is a vertical disk divided into degrees on its edge and movable 

 on a horizontal axis, which may be turned by means of a milled head, 

 G. A vernier Y fixed to the support of the instrument serves to 

 indicate the angle through which the limb has been turned. 



2. The axis of the limb is hollow ; a rod passes through it, which 

 by using another milled head, A, can be rotated independently of the 

 graduated circle. This rod supports a jointed arm and this again has a 

 metal plate, capable of turning in different directions by means of a 

 handle and joints. The crystal is placed on this plate and its angle 

 measured, as follows. 



Choice is made of two horizontal parallel sights, for instance, the 

 edge of a roof of a house and a bar of a ground floor window ; or 

 again, an upper sight is secured by the top of an open \vindow, the dark 



P 



