. Annual report. New York State Museum; Science; Science. 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 an}- 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 gnomo
. Annual report. New York State Museum; Science; Science. 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 an}- 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. Platel[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 descri^^tion and illustration it is necessary to repre- sent the crystallographic combination by 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. The}' 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 crvstal observed from above, is closely related to the stereographic spherical pro- jection and can be readily constructed from it.'. Fig. 27 * Story-Maskelyne, N. Treatise on the Morphology of Crystals. Oxford 1895. p. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original w
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