MEASUREMENT OF CRYSTALS. 



47 



ON MEASURING ANGLES OF CRYSTALS. 



A.s the angles of crystals are constant, minerals, as has 

 been stated, may often he distinguished by measuring these 

 angles. This is done by means of instruments called goni- 

 ometers, a term meaning, literally, angle-measurers.* These 

 are of two kinds ; one is called the common goniometer, the 

 other the reflecting goniometer. 



The common goniometer depends on the 87 



very simple principle that when two straight a 

 lines cross one another, as A E, C D in the 

 annexed figure 87, the parts will diverge 

 equally on opposite sides of the point of in- ' 

 tersection (O) ; that is, in mathematical language, the angh 

 A O D is equal to the angle C O E, and A O C is equal to D O E 



The instrument in common use is here represented. 



It consists of two arms, ab,cd, moving on a pivot at o : the arms 

 open and shut, and their divergence, or the angle they make 

 with one another, is read off on the graduated arc attached. 

 In using it, press up between them, the edge of the crystal 

 whosa angle is to be measured, and continue opening the arms 

 thus till the inner edges lie evenly against the faces that include 



How are the angles of crystals measured? Explain the principle of 

 the common goniometer from the figure. Explain the common goni- 

 ometer and its ase. 



* From the Greek gonu, angle, and metron, measure. 



