12 DEFINITIONS, 



tig. 25. 



A line, as 06, or c tf, fig. 25, drawn through two 

 opposite angles of any parallelogram, and dividing the 

 plane into two equal parts, is called a diagonal of 

 that plane. 



In the oblique rhombic prism, the doubly oblique 

 prism, and the rhomboid, fig. 21, 22, and 23, the line 

 a c, which appears to lean from the spectator, will be 

 termed the oblique diagonal, and the line fg of the 

 oblique rhombic prism, and df of the rhomboid, the 

 horizontal diagonal. 



The line dfof the doubly oblique prism, may also 

 for the sake of distinction be termed its horizontal 

 diagonal; although from the nature of the figure, 

 that line must be oblique when the lateral edges are 

 perpendicular. 



The diagonal plane of a solid, as a b c d, fig. 25, 

 is an imaginary plane passing through the diagonal 

 lines of two exterior parallel planes, dividing the 

 solid into two equal parts. 



The axis of a crystal, generally, is an imaginary 

 line passing through the solid, and through two oppo- 

 site solid angles. 



In prisms, this may be termed an oblique axis, to 

 distinguish it from another line which passes through 

 the centres of their terminal planes, and may be 

 termed a prismatic axis. 



The axis of a pyramid, passes through its terminal 

 point and through the centre of the base. 



