DniETEIO, OR TETKAGOiSAX SYSTEM. 



31 



bein« exactly alike in inclination to the basal plane as well aa 

 \like in lustre. 



There are two distinct square prisms. In one (fig. 10) the 



10. 



11. 



^^: , t» ^*| 



j < 



: 





i " 





a 



A n 



12. 



! IF 



Ui 



axes connect the centres of the lateral faces. In the other 

 (tig. 12) they connect the centres of the lateral edges. In fig. 

 1 1 the two prisms are combined ; the figure shows that the 

 planes of one truncate the lateral edges of the other, the inter- 

 facial angle between adjoining planes being 135°. Figs. 2, 3, 

 4, 7, are of others having planes of both prisms. In fig. 13 one 

 prism is represented within the other. 



Fig. 14 represents an eight-sided prism, and fig. 15 a combi- 

 nation of a square prism (i-i) with an eight-sided prism (i-2) 



15. 

 -0- 



illl 



Another example of this is shown in fig. 4, and also in fig. 9, 

 the planes i-2 in one, and i-3 in the other. 



The basal plane in these prisms is an independent plane, be- 

 cause the vertical axis is not equal to the lateral, and hence it 

 almost always differs in lustre and smoothness from the lateral. 



Like the square prisms, the square octahedrons are in two 

 series, one set (fig. 16) having the lateral or basal edges parallel 

 to the lateral axes, and these axes connecting the centres of 

 opposite basal edges, and the other (tig. 17) having them diago- 

 nal to the axes, these axes connecting the apices of the opposite 



