82 On the Drawing of Figures of Crystals. 
oblique projections. ‘The rays of light may fall perpendicularly on 
the plane of projection, or may be obliquely inclined to it; in the 
former case the projection is termed orthographic, in the second 
clinographic. In the horizontal position of the plane of projection, 
the projection is always orthographic. In the other positions, it 
may be either orthographic or clinographic. It has been usual to 
give the plane of projection an oblique position, and to use the or- 
thographic mode of projection. It is however preferable to employ 
the vertical position and clinographic projection, and this method 
will be elucidated in the following remarks. 
PROJECTION OF THE PRIMARY FORMS. 
4. The projection of the axes of a crystal, is the first step pre- 
liminary to the projection of the crystal itself. It will be more con- 
venient, to illustrate first the projection of the axes in the monometric 
primaries, which are equal and intersect at right angles. ‘The projec- 
tion of the axes in the other classes, may be obtained by varying the 
lengths of the projected monometric axes, and also, when oblique, 
their inclinations. ! 
5. 1. Monometric system.*—When the eye is directly in front 
of a face of the cube, neither the sides nor top of the crystal are 
visible, nor the secondary planes that may be situated on the inter- 
mediate edges. On turning the crystal a few degrees from right to 
left, a side lateral plane is brought in view, and by elevating the eye 
slightly, the terminal plane becomes apparent. Half the planes on 
the crystal are now visible, and consequently this is a convenient 
* The systems of crystallization at present recognized, are seven in number. 
The Monometric includes the cube, regular octahedron and dodecahedron, the 
three crystallographic axes of which are of one kind, and intersect at right angles. 
The Dimetric system includes the right square prism and square octahedron, 
the axes of which are rectangular and of wo kinds, the vertical being unequal to 
the lateral. 
The Tyimetric system includes the right rectangular and rhombic prisms, and. 
the rectangular and rhombic octahedrons, the axes of which primaries are rect- 
angular, and wnequal, or of three kinds. 
The Monoclinate system includes the right rhomboidal prism and the oblique 
rhombic prism, in which, two of the intersections of the axes are rectangular and 
one oblique. ~ * 
The Diclinate system includes the oblique rectangular prism, in which one of 
the intersections of the axes is rectangular and two oblique. 
The Triclinate system includes the oblique rhomboidal prism, in which the 
three intersections of the axes are oblique. 
_ The Tetravonal system includes the rhombohedron and hexagonal prism, 
which contain four axes, viz. three horizontal and one vertical. 
