50 



STRUCTURE OF MINERALS. 



* 



preceding, which is only a simpler case of the same, are of 

 great importance in the measurements of crystals. 



Reflecting Goniometer. The reflecting goniometer affords 

 a more accurate method of measuring crystals that have 

 luster, and may be used with those of minute size. The 

 principle on which this instrument is constructed will be un- 

 derstood from the annexed figure (fig. 90) representing a 

 crystal, whose angle a b c is required . 

 ^ The eye, looking 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 abd 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°, equals the angle of the crystal a b c. The crystal is 

 therefore passed in its revolution through a number of de- 

 grees, which, subtracted from 180°, give 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, as here represented (fig. 91.) 



A B is the graduated cir- 

 cle. The wheel, m, is at- 

 tached to the main axis, and 

 moves the graduated circle 

 together with the adjusted 

 crystal. The wheel, w, it 

 connected with an axfe 

 which passes through the 

 main axis, (which is hollow 

 for the purpose,) and moves 

 merely the parts to which 

 the crystal is attached, in 

 order to assist in its adjust. 

 k ment. The contrivances for 

 the adjustment of the crysta* 

 are at j>, g, r, s. To use the instrument, it must be placed on 

 a small stand or a table, and so elevated as to allow the ob- 

 server to rest his elbows on the table. The whole, thus 



Explain the principle of the reflecting goniometer, 

 of using the instrument. 



Explain the mode 





