MODELS FOR USE IN TEACHING ELEMENTARY CRYSTALLOGRAPHY. 75 



I --L.J I. I S 



'g- 7 - 



Tncltnic 



Pig. 4. Rhombic 



Fig. 5 — Model for monoclinic system showing effect of a single 

 plane of symmetry. 



Fig. 6 — The same with the rotating axis turned through L80°, 

 showing the effect of a single plane of symmetry and an 

 axis of binary symmetry at right angles to it. M — mirror- 

 representing plane of symmetry ; A — rotating axis ; F — 

 card to represent face of a crystal ; P — pins carrying F ; 

 A\ F\ P — reflections of A, F, and P in M. 



Fig. 7 — Model for triclinic system showing effect of a centre of 

 symmetry without any other element of symmetry. C — 

 cork carrying pins and representing centre of symmetry; 

 F — card to represent face of a crystal ; P — pins carrying F. 



