C. Palache — Crystal Measurement^ etc. 



285 



r 



the stereographic projection may be derived from it with an 

 accuracy not attainable by direct construction. The ortho- 

 graphic and clinographic projections may also be directly 

 obtained from the gnomonic projection,* whereby the con- 

 struction of the linear projection and the projection of axes 

 is dispensed with. 



Twin crystals are treated exactly as are simple ones both in 

 measurement and projection, the position of the twinning 

 plane, if not already known, being determinable from the pro- 

 jection and the perspective figure being derived as in the case 

 of sinfifle individuals. 



This principle of crystal measurement has also been applied 

 in a contact goniometer. This instrument is shown in fig. 5. 

 The crystal is borne on the horizontal circle which takes the 

 place of the vertical circle in the reflecting goniometer. The 

 horizontal circle H revolves with the crystal and its position is 

 read from the graduated edge. 



The vertical circle Y is fixed at right angles to H and in the 

 plane of its center. It is graduated from the top in either 

 direction from 0° to 100°. On Y slides a closely-fitting carrier 

 which holds a steel rod, S, arranged to slide in and out in a 

 direction radial to Y. On its end is a plate ^, normal to its 

 extension, with which contact is made with the surface to be 

 measured. 



The crystal is mounted on a support which is attached to H 



* Idem, p. 82, and Ueber Krystallzeichnen, Zeitschr. f. Kryst., xix, 1891, p. 352. 



