CALCITES OF NEW YORK 57 
Gnomonic projection. The gnomonic projection, which has come into 
considerable use in recent years, usually assumes the plane of projection 
tangent to the circumscribed sphere at the upper vertical pole, figure 27. 
The projection of the pole of any face is situated at the mtersection of 
the extended normal of the face with this tangent plane. All zones are 
projected in gnomonic projection as 
Plane of Gnomonic. Projection 
straight lines. The poles of the 
Psmaniemzonerdo MOU appeakaiml | 78-4) ea 
projection since their normals are 
parallel to the gnomonic projection 
plane. Plate l[see pocket] is a gno- 
monic projection of the established 
~ Plane of Stereograiohic E 
Projection } i 
forms of calcite constructed on the 
basis of a sphere of 7 centimeters 
radius. 
LINEAR PROJECTIONS 
For purposes of description and 
illustration it is necessary to repre- Bese 
sent the crystallographic combination bya drawing of its intersecting edges. 
This is accomplished by viewing the crystal in various positions the point 
of vision in every case being assumed at an infinite distance. Two such 
parallel projections are in general employed for this purpose. They assume: 
1 The plane of projection perpendicular to the vertical axis. This is 
known as the orthographic projection. 
2 The vertical plane of projection as revolved to the right with 
respect to the horizontal axis II [fig. 15] and slightly inclined toward the 
upper prolongation of the vertical axis. This is known as the clinographic 
projection. 
The orthographic projection which is essentially a plan of the crystal 
observed from above, is closely related to the stereographic spherical pro- 
jection and can be readily constructed from it.! 
1 Story-Maskelyne, N. Treatise on the Morphology of Crystals. Oxford 1895. p. 476. 
