Crystallography : a treatise on the morphology of crystals / by N. Story-Maskelyne.

  • Story-Maskelyne, Nevil, 1823-1911.
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: Crystallography : a treatise on the morphology of crystals / by N. Story-Maskelyne. Source: Wellcome Collection.

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    Orthogonal projection. 4 77 ffaces enclosed by the various edges; and in fact, when the stereo- igraphic projection of the crystal is once correctly figured, the ^ systematic projection is produced by a simple process of evolution ffrom it. In this way the systematic projection of a crystal of ibournonite given in Fig. 375 (6) can be immediately obtained from tthe stereographic projection of the poles given in Fig. 375 (a). 468. Orthogonal projection : the plane of projection not a syste- 1 malic plane. In order, however, to obtain a more complete view of a 1 crystal than is given by the systematic projection, it is necessary to ibe able to represent the various forms more as they would be seen iin a perspective drawing, as well in order to give an aspect of : solidity to the figure as to indicate the relative importance of the 1 different forms. The systematic projection, for instance, indicates only by its I boundary-lines, and therefore in fact obliterates as faces, the planes 1 in the zone of which the edges are parallel to the line of sight; ; and these frequently include important pinakoid-, dome-, or prism- faces, to which the crystal may owe its most characteristic aspect. The drawings are therefore generally made in such a way as that the plane of projection is not in parallelism with any systematic plane, nor, intentionally, with any face. The crystal is in fact placed in such a position that while its orientation gives to the axes nearly the positions in space usually assigned them in geometry, certain rotations through small angles, forward and from right to left respectively, bring into view, and as it were open out, faces that in the systematic projection are represented by lines only; while the idea of solidity is further imparted to the figure by the difference in dimensions that faces of the same form assume when thus moved into positions in which they are differently inclined to the line of sight. 469. Cubic system. Let Ox, Oy, Oz be three rectangular lines of reference fixed in space, Ox and Oy being horizontal and Oz vertical; let xOz be the plane of orthogonal projection, and the visual rays have directions parallel to yO. Taking as the simplest case a cubic crystal, we may suppose the initial position of the crystal to be such that the crystal-axes OX, OY, OZ are coincident with the fixed lines Ox, Oy, Oz, respectively : in the corresponding