The projection of the pole of any face is situated at the intersection of
the extended normal of the face with this tangent plane. All zones are
projected in gnomonic projection as
straight lines. The poles of the
prismatic zone do not appear in
projection since their normals are
parallel to the gnomonic projection
plane. Plate 1 [see pocket] is a gno-
monic projection of the established
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-
sent the crystallographic combination b\' a 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. 1.5] 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.'

' Story-Maskelyne, N. Treatise on the Morphology of Crystals. Oxford 1895. p. 476.
Fig. 27



58 NEW YORK STATE MUSEUM

The clinograpliic projection represents the crystal in parallel perspective
and is specially valuable as presenting the aspect which shows it to the
best advantage.
