^ Weiss, Mohs, and Hauy. '^1 
even make it necessary or opportune to alter entirely the lan- 
guage of Haiiy. As the object in contemplation is so simplcj 
it might have been expected that many different methods would 
have arisen ; for the easier a thing is to do^ the greater number 
of ways generally are proposed to do itj the differences of which 
are insignificant, but to which, however, great importance often 
is attached. To be better able to appreciate the comparative 
advantages of the new method of designation^ I shall briefly state 
in what essentially consist the two principal ones, I mean those 
of Professors Weiss and Mohs. 
In the first, all the planes of crystals are referred to three 
rectangular axes, in all the substances, the primitive form of which 
is either a rhomboid, or an oblique rhombic prism, or a doubly 
oblique prism. These axes correspond, then, to those of some 
octohedron with a rhombic or square base, which may always 
be assumed as the primitive. The position of any secondary 
plane is then determined, by giving the lengths of the parts of 
two of these axes, cut by a parallel plane passing through the 
extremity of the other ; and the notation by which, according to 
this method, a secondary plane is represented, is ma, nh : c ^ 
n, h, c, being the lengths of the axes, and ma^ nh, the parts of 
the two axes a and h cut off by the parallel plane drawn through 
the extremity of the axis c. If the secondary plane be parallel 
to one of the axes, to 5, for instance, its corresponding sign will 
be I ma : oo5 r c | ; if parallel to two of the axes, to a and h, it 
will be Goa : go 6 : c For substances derived from a rhom- 
boid, the method is analogous, one of the axes to which secon- 
dary planes are referred, is that of the rhomboid, and there are, 
for the sake of symmetry, three others perpendicular to it, in- 
clined to one another at an angle of 120°, and corresponding to 
the perpendiculars drawn from the extremities of the superior 
edges of the rhomboid upon its axis. For substances whose 
primitive form is an oblique rhombic prism, or a doubly oblique 
prism, this method will require some modification, which has 
not hitherto been made. It would, however,' apply to forms 
derived from an oblique rhombic piism, if the property of these 
forms assumed by Haiiy was found generally to obtain ; for then 
the secondary forms miglit be conceived to derive from an oetx?.. 
