26 



OF THE GONIOMETER. 



If therefore we suppose the lines gf, dh, to be very 

 thin and narrow plates, and to be attached together 

 by a pin at e, serving as an axis to permit the point 

 / to be brought nearer either to d, or to h; and that 

 we were to apply the edges e d, ef, of those plates, to 

 the planes of the crystal fig. 50, so as to rest upon the 

 lines ed, ef, it is obvious that the angle g e h, of 

 the moveable plates fig. 51, would be exactly equal to 

 the angle def of the crystal fig. 50. 



Fig. 52. Fig. 53. 



The common goniometer is a small instrument cal- 

 culated for measuring this angle geh, of the move- 

 able plates. It consists of a semi-circle, fig. 53, 

 whose edge is divided into 360 equal parts, those 

 parts being half degrees, and a pair of moveable 

 arms dh, gf> fig. 52. The semicircle having a pin at 

 i, which fits into a hole in the moveable arms at e. 



The method of using this instrument is, to apply the 

 edges de, ef, of the moveable arms, fig. 52, to the 

 two adjacent planes of any crystal, so that they shall 

 accurately touch or rest upon those planes in directions 

 perpendicular to the edge at which they meet. The 

 arm dh, is then to be laid on the plate m n of the 

 semicircle fig. 53, the hole at e, being suffered to drop 

 on the pin at /, and the edge nearest to h of the arm 

 ge, will then indicate on the semicircle, as in fig. 54, 

 the number of degrees which the measured angle 

 contains. 



