50 



STRUCTURE OF MINERALS. 



L 



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 w. 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 adjuster 

 crystal. The wheel, n, i 

 connected with an axi 

 which {Kisses through th( 

 main axis, (which is hollox* 

 for the purpose,) and moves 

 merely the parts to which 

 the crystal is attached, in 

 order to assist in its adjust, 

 ment. The contrivances for 

 the adjustment of the crysta 1 

 are at p, q, 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. Explain the modf 

 >f using the instrument. 



