IN TRUE PERSPECTIVE. 
507 
It would be superfluous to give here a greater number 
of examples for the process of representing complicated 
forms of crystals, as it is exactly similar, in every instance, 
also in the remaining systems of crystallisation. In many 
cases it may be useful, beside the actual representation of 
a crystal in perspective, according to the rules developed 
above, to sketch the horizontal projection, or the elevation 
of the crystal upon a certain plane, determined in respect 
to the axes of the combinations ; it will be particularly im- 
portant to make use of this method in hemi-prismatic and 
tetarto-prismatic forms, in which we are compelled to seek 
for symmetry, as referred to certain planes ; while, in the 
other systems, it possesses an absolute and persistent cha- 
racter. In the latter, indeed, as Haiiy observed, it would 
have the effect of monotony ; but it cannot be too much 
recommended to those, who already possess some know- 
ledge of forms, for studying certain classes of the more 
difficult crystals in nature. 
The result of applying the rules of projection to a parti- 
cular case, is the representation of a variety given, or ob- 
served in nature upon a plane, without the auxiliary lines, 
which it was necessary to make use of for arriving at this 
result. To this end, we copy the points determined by the 
intersection of the lines, by means of the fine point of a 
needle, and join the points transferred in this way from one 
sheet of paper upon another, by means of straight lines, 
according to the indications of the original drawing. One 
original may thus serve for a great number of projections, 
since it may be copied whenever a new form is added; and 
the degree of attention bestowed upon the first outhne is 
amply repaid by the exactness of the copies. 
It has been the custom of Haiiy and subsequent crystal- 
lographers, and it deserves to be retained, to draw the front 
edges in full lines, like AB, Fig, 16. ; the back edges, hke 
