MODELS FOR USE IN TEACHING ELEMENTARY CRYSTALLOGRAPHY. 71 



a mirror. The use of a mirror therefore explains the effect 

 of a plane of symmetry exactly, and the combination of 

 several mirrors reproduces the effect of the highest orders 

 of symmetry. 



I have constructed models representing the symmetry 

 of the normal group of each of the six systems of crystals. 

 Those for the cubic, tetragonal, hexagonal and rhombic 

 systems consist of three mirrors each ; that for the mono- 

 clinic system of a single mirror and a rotating axis ; that 

 for the triclinic system simply of a cork. 



The mirrors should be of the thinnest gauge of glass 

 obtainable, and must be cut very accurately to shape, and 

 very carefully assembled, or else the multiple reflections 

 give a distorted figure. I found it possible to get the 

 glass cut much more accurately if ordered in rectangular 

 shapes, than if triangles of given angles are specified. I 

 made six inches my unit of length and then the following 

 glasses were needed : — 



5 squares, 6 inches x 6 inches (3 for rhombic, one each 

 for tetragonal and hexagonal). 



1 square, 6 inches x 6 inches, cut across diagonally (one 



part for cubic, one for tetragonal). 



2 rectangles, 6 inches x 8*48 inches — 6v2 inches — (one for 



tetragonal, one for monoclinic). 

 1 rectangle, 6 inches x 8*48 inches, cut across diagonally, 



(both parts for cubic). 

 1 rectangle, 6 inches x 6*93 inches — 4v3 — (for hexagonal). 

 1 rectangle, 6 inches x 3*46 inches — 2v3 — (one part for 



hexagonal, one part wasted). 



The pieces are fixed together by means of strips of paper 

 gummed across the edges of adjacent pieces at the back. 

 The figures (fig. 1) are of the nature of "nets" to indicate 

 the construction of the models for the first four systems. 



