32 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 Ni e seven in number 
pags cede inl udes the sae: regular solachaine ae dodecahedron, the 
three crystallographic axes of which are of one kind, and intersect at right angles. 
the axes of which are rectangular and of two kinds, the vertical being unequal to 
the lateral, 
The Trimetric 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 kin 
‘The oclinate system includes the right rhomboidal prism and the oblique 
pany ss prism, in which, two of the intersections of the axes are rectangle and 
"The Diclinate system includes the oblique rectangular prism, in which one of 
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 Tetrazonal hangrt includes the rhombohedron and hexagonal pris, 
which contain four axes, viz. horizontal and one vertical. 
