to designate the Planes of Crystals. 431 
1. Mohs designates the fundamental rhombohedron by R, and 
the principal series of rhombohedrons by 
...R-3, R-2, R-1, R, R+1, R4+2... 
In the same manner, in the rhombic and square-pyramidal 
systems, he designates the fundamental pyramid by P, and the 
principal series of derived pyramids by 
POP, Vn POP PRES Cig 
This notation violates the principle that the signs + and — cannot 
properly be used to connect signs of form (R or P) with signs 
of quantity (2, 3, &c.). Such ideas are altogether heterogeneous, 
and should not be combined as if they were capable of addition 
or subtraction. 
It may be said that in this notation -—2, -1, 1, &c. are not 
quantities added, but indices. If so, they should be so written 
and the principal series would be then 
ee sri eae 2k, &e. 
which would be free from the above objection. 
This designation of the principal series is somewhat simpler 
than ours, which is 
bd 278 By 27h Ry 2) ety 2 7R; ¢Bee.s 
where the exponent of the root 2, in this case, is the index in the 
system previously mentioned. And this simplicity would be a 
reason for preference, if the series here referred to contained the 
only forms which are to be taken into consideration. But, in 
using this notation, Mohs is compelled, when forms occur which 
are not in this series (subordinate series), to adopt a new convention 
for expressing them; whereas in our method, the principal, and 
the subordinate series, are cases of the same general notation. 
And the law of the principal series is sufficiently evident in our 
symbols. 
Vol. II. Part IL. 3K 
