J.D. Dana on Lettering figures of Crystals. 401 
to infinity, while the ratio between the lateral axis is constant 
(2:1). And with the crystal before the student, he might at 
once perceive the truth of the general law that in such a series, 
that is, with the 2nd ratio constant, the mutnal intersections of 
the planes in the series are parallel and horizontal. The zone 
2-2, 3-3, 4-4, exhibits the relations of planes in that oblique series, 
the general formula being m-m, or mPm in Naumann’s system, 
The zone 0, 0-2, 0-3, 0-0, is a horizontal zone, with the vertical 
axis infinite, the planes being ‘parallel to this axis, 4, 4-2, 4-4, 
4-4, 4-2, 4 lie in another series, the mutual intersections being 
parallel, since in each the vertical axis equals 4a, (4 being the first 
figure in the lettering of each plane.) In this system and all the 
others, the terminal plane is conveniently designated by P. It 
is UP of Naumann, the vertical axis being cousidered as zero. 
In figure 3, of the trimetric system, as the lateral edges are 
unequal, the figures that refer to the shorter axis have a short 
1, 
Ber 
lg or aE : 
\\ 
Zi 
Sy 
Arragonite. : Glauber Salt. 
TrimeTRic S¥sTEM.  ---,—«, >», Monocninte System. 
mark (~) over them, and those referring to the longer axis, would 
in like manner have a long mark (—). The series 3-0, 1-0, 2-0, 0-0, 
Szvonp Seruss, Vol. XIII, No. 39.—May, 1852. 51 
