CRYSTALLOGRAPHY. 



11 



The reflecting goniometer affords a more accurate method of 

 measuring crystals that have lustre, and may be used with those of 

 minute size. The principle on which this instrument is constructed 

 will be understood from the annexed figure, representing a crystal, 

 whose angle a b c is required. The eye, look* 

 ing at the face of the crystal b c, observes a 

 reflected image of m, in the direction P n. On 

 revolving the crystal till a b has the position of 

 b c, the same image will be seen again in the 

 same direction P n. As the crystal is turned, 

 in this revolution, till a b d has the present 

 position of b c, the angle d b c measures the number of degrees through 

 which it is revolved. But d b c subtracted from 180 c equals the angle 

 of the crystal a b c. The crystal is therefore passed, in its revolution, 

 through a number of degrees equal to the supplement of the required 

 angle. 



This angle, in the reflecting goniometer of Wollaston, is measured 

 by attaching the crystal to a graduated circle which revolves with it, 

 one form of which is here represented. 



C is the graduated circle. The wheel, m, is attached to the main 

 axis, and moves the graduated circle together with the adjusted crystal. 



