A new system of Crystallographic Symbols. 253 
plane on which they incline. Thus suppose anintermediary to be sit- 
uated between the edge é, angle a and plane P’, the symbol will be 
z’,; if between the edge e angle a and plane P', the symbol will be 
ae, ; if between e, a and P the symbol is e'. ‘There is no dash be- 
low in this instance, as the plane on which it inclines (the base) has 
no dash to its P. 
There is certainly no taxing of the memory here, farther than as 
regards the use of the italic letters. We have only to notice wheth- 
er a dash inclines to the right or left, in order to determine the edge, 
angle and face between which a plane is situated. 
These symbols become more simple as we descend from this cli- 
max of irregularities among the primitive forms, to those whose sim- 
ilar parts are more numerous. In the oblique rhombic prism the 
lateral faces are similar to one another, and consequently: also the 
front superior basal edges and the front inferior basal edges. It is 
therefore unnecessary to distinguish them from one another. Hence 
instead of P’ and P', each of the lateral planes may be named P 
whose distinguishing mark is virtually a combination of the two dash- 
es used in the oblique rhomboidal prism. ‘So also we have e for é 
and é, e for é and é, and a for & and d. The lateral angles of this 
primitive form have the two front plane angles (the crystal being in 
position) equal to one another, but unequal to the two posterior plane 
angles which also are equal to one another. Consequently a decre- 
ment may take place on one pair, and not on the other. Suppose 
then a plane on a lateral superior solid angle to imcline on a front 
face. Itssymbol according to the above rule will be a. But if it 
inclines on the posterior face, the curve below must be inverted thus, 
a. Inthe rhombohedron the faces are equal, the superior edges 
similar, and also the lateral edges. ‘The planes are marked P, the 
superior edges e, the lateral edges e. So also the dominant solid an- 
gle a, and the lateral solid angles a. If this form were placed in 
the same position as the oblique prisms, these letters would result 
by merely dropping the dashes, which here become unnecessa- 
ry. ‘Two kinds of planes may exist on the lateral angles of the 
rhombohedron, owing to the two kinds of plane angles which com- 
pose it. If the plane cuts off the dominant* plane angle, (these 
* As Ihave called the superior solid angle the dominant or governing solid an- 
gle of the rhombohedron, so also its plane angles and their equals in this solid may 
be named the dominant plane angles. 
