436 Mr. WHEWELL on the Selection of a Notation 
to indicate the half of a form xnRm, it will be better to write h 
nRm 
before it, than to write it , as is done in this part of Mohs’ 
notation. 
The notation = = 
(right or left) in which hemihedral faces follow each other round 
the figure, both in the upper and under halves of the form, is 
convenient and significant, and seems not open to any sufficient 
objection. 
, &c. by which he designates the direction 
As it is only in cases of hemihedral’ combinations that these 
symbols occur, it does not appear necessary to indicate this by 
the denominator 2. I have, therefore, written simply - nRm, 
nRm 
r 
r— hnRm. 
Fethgte 
r 
and not z 
If one of the hemihedral portions arise from the fundamental 
form in a transverse position, this circumstance is indicated by 
giving an accent to the corresponding letter of the symbols -, &e. 
Thus, =, nRm indicates that half the faces of Rm, at one apex, 
are combined with half the faces of nR’m at the other, both 
portions being turned to the right. 
11. In the octahedral or tessular form, Mohs uses a different 
conventional letter for almost every different species of form. 
To this, perhaps, there is no great objection, as, from the regu- 
larity of these forms, the facility of reasoning does not depend 
much upon the generality of the symbols. It may be observed, 
however, that, according to our system, all these forms are pro- 
vided with symbols, according to the analogy of the other systems 
of crystallization. 
12. The designation of twin crystals by Mohs seems also 
