to designate the Planes of Crystals. 435 
8. Analogous to this process is the deduction, in the rhombo- 
hedral system, of the isosceles six-sided pyramid (Rr) from the 
rhombohedron. This deduction, however, is, by Mohs, desig- 
nated in a different manner, namely, by the change of R to 
P. And it may be again observed that this change from R to 
P is different from the same change made, by Mohs, in the 
manner mentioned in Observation 4, where both are in brackets, 
and indicates a process altogether different. 
9. We may observe, also, that Mohs has several other signs, 
which offend against the rule of having the symbols as homo- 
geneous, and the conventions, from which they derive their 
meaning, as few and as general as is possible. Thus, the pro- 
’ sodical marks - and © are introduced over the P and Pr to in- 
dicate the derivation being made from the extremity of the long 
ov short diagonal. And it may be observed also here, that the 
comparative length of these diagonals has nothing to do with 
the derivation; but only the consideration of one as principal, 
and the other as transverse to it. 
Again, the crotchets [ ] are used to mark that the limiting 
prism is in the diagonal position, which notation has no analogy 
with the other parts of the system, (Sect. 102.) 
Again, the negative sign, Sect. 128, is used to indicate a second 
half of a derived form. This also is suggested by no analogy, 
except in the case where the half form is the same as —1.R 
according to the definitions, and may, therefore, be properly re- 
presented by —R. I have employed an accent (as in R’) to 
indicate this change, which mark is also employed in an analogous 
manner in the next paragraph. 
10. In such cases, where we subject a form to some con- 
ditions, after we have established the way in which it is deduced ; 
it seems better that the part of the symbol which represents these 
conditions should be separate or separable from the rest. Thus, 
