IN TIlUE PERSPECTIVE. 
487 
all those lines parallel which may be drawn from the eye to 
any of its points whatever. Suppose, now, a plane to cut 
all these lines, or the visual ray, at right angles ; draw per- 
pendicular lines from every solid angle of the solid upon 
that plane, the intersections with it will be the projections 
of the solid angles ; join these points by straight lines with 
each other, the result will be a figure representing the crys- 
tal itself. 
In the art of drawing in perspective, this method is called 
the Orthographic Projection, on account of the right angle, 
which the visual ray includes with the plane upon which the 
solid is represented. Herein it differs from the method fol- 
lowed by several modern authors, in which it is supposed that 
the eye of the observer is at once in two different places ; but 
it agrees with the method employed in the works of Haiiy. 
If the relations of the simple forms among each other be 
known, according to the cry stall ographic methods of Haiiy, 
of Weiss, and others, but, particularly, if attention be given 
to the derivation of the simple forms from one another, 
and the laws of their combination, as proposed in the me- 
thod of Professor Mohs, it will not be attended with the 
slightest difficulty to draw the figures of the most compli- 
cated crystals which may occur, if only the projection of 
one of the simple forms contained in them, have been pre- 
viously completed ; because the situation of the edges in the 
compound forms depends solely upon the intersection of 
the faces of the simple ones, and upon the ratios of certain 
lines similarly disposed in the single forms, which enter in- 
to the combination. 
The first object to be attained, will therefore consist in 
projecting the figure of a form that may answer this pur- 
pose .; and as the most appropriate, we may select the hexa- 
hedron, or, in the purely geometrical mode of considering 
it, the right rectangular four-sided prism, whose sides are 
