14 TERMINOLOGY. 



. 24. SOLID ANGLE. 



-The meeting of three or more plane angles forms a solid 

 angle. 



The solid angle is denominated according to the particular form 

 in which it is found, and also agreeably to its situation and quality. 

 Thus we say, solid angles of the Octahedron, prismatic solid angles. 

 &c. 



. 25. SIMILAR FACES. 



Those faces of a regular form are said to be similar to 

 one another, whose corresponding edges are proportional, 

 and whose corresponding angles are equal. 



In crystals, similar faces are not always equal to each other in their 

 dimensions. Sometimes a single face is more enlarged than the rest. 

 In such a case, its similarity is inferred from its situation. Crystal- 

 lography, in developing the relations of crystals to each other, takes 

 no notice of such irregularities, inasmuch as they are accidental ; 

 but investigates their forms as they are presented in their highest 

 regularity and perfection. 



. 2. SIMILAR EDGES. 



Edges are said to be similar when formed by the meet- 

 ing of faces equally inclined to each other. 



In crystals, the length of the edges, produced by the meeting of 

 faces respectively similar, is liable to variation from the same cause 

 which produces irregularities among similar faces. Disparity in 

 length, therefore, does not affect the idea of their similarity. 



. 27. SIMILAR PLANE ANGLES. 



Plane angles are similar when they are equal, and con- 

 tained within similar edges, respectively. 



