O' NEW YORK STATE MUSEUM 



A crystal is symmetric to a plane when it may be so divided 

 by that plane that every face, edge and angle on the one side 

 is repeated on the opposite side. In fig. 11, which represents a 

 crystal of pyroxene, the shaded space shows the intersection 



Fig. U 



Fig. 12 



of the crystal by a plane of symmetry, and it will be observed 

 that the portion of the crystal lying to the right of the plane 

 is related to the portion lying on the left in the same way that 

 the reflected or mirrored image of the half crystal is related 

 to the direct image. Fig. 12 shows fig. 11 as seen from above. 



b — 



Fig. 13 



Fig. 14 



A crystal is said to be symmetric to an axis of binary sym- 

 metry when it occupies the same position in space twice during 

 one revolution about the axis, the coinciding positions being 

 180° apart. A consideration of fig. 13 and 14 will make this 

 clearer; in fig. 13 b is an axis of binary symmetry and if the 

 crystal is revolved as shown in fig. 14 the point a will have to 

 traverse an arc of 180° and coincide with a' before the crvstal 



