154 
Mohs' System Crystallography 
to Cl, and reach the surface IB, while the point K describes 
1^1 — hm^ and during the time that K has completed the re- 
mainder I B, there will have been propagated from the point w, 
a spherical wave, whose semidiameter m n will have the same 
ratio to ZB as lA : KB. The waves mn and I A will there- 
fore have the same tangent AB, and the same is true of all the 
other spherical waves that are propagated out of the crystal by 
the impulsion of all the points of the wave IK against the sur- 
face of the ether IB. The tangent BA will therefore be the 
continuation of the wave IK out of the crystal, when the 
point K has come to B, and consequently I A, which is the 
perpendicular to BA, will be the refraction of the ray Cl in 
going out of the crystal. But lA is parallel to BC dnce 
IB ==: CK, and lA = KO and A and 0 right angles. 
Art. XXVIII. — Outline q/^ Professor Mohs's New System of 
Crystallography and Mineralogy 
1. Simple and Compound Forms, — Some crystalline forms 
consist of faces entirely equal and similar ; others are bounded 
by faces which are not all equal and similar : the former are called 
simple the latter compound forms, or combinations. 
2. Regularity q/* the Combinations. — Every combination is 
composed of simple forms, and the junction that takes place 
among those simple forms, is the most regular which their na- 
ture admits. 
3. Symmetry and General Problem of Crystallography . — 
The regularity of junction among the simple forms of which 
compound forms are composed, is called the symmetry of those 
combinations. Crystallography furnishes the mathematical ba- 
sis on which this symmetry is founded. 
4. Edges of Combination. — The edges formed by the meet- 
ing of the faces of two different simple forms, are named edges 
of combination. Of this species are the edges between P and 
f Fig. 4. Plate VIII. 
Communicaied by a Pupil of Professor Mohs. Ed, 
