ON THE TABLES OF MODIFICATIONS. 103 



i 



are applicable in describing the secondary forms of 

 crystals. 



Some, or all, of the vowels A E 1 O, are used to 

 designate the solid angles ; some of the consonants, 

 B C D F G H, to designate the primary edges ; and 

 P M T to designate the primary planes of crystals. 



The same letter is repeated where the angles, edges., 

 or planes are similar; and different letters are used 

 where those angles, edges, or planes are dissimilar, 

 'according to the definitions of similar angles, #c. given 

 in p. 3. 



Thus the letter A is repeated on all the angles of 

 the cube, these being all similar; while A and E are 

 placed on the alternate angles of the right rhombic 

 prism, to shew that there, the opposite angles only 

 are similar. 



So in the cube, the letter P is repeated on all the 

 planes because they are all similar. 



In the right rhombic prism, the letter P stands only 

 on the terminal plane, the lateral planes having the 

 letter M placed upon them. This implies that the 

 lateral planes are not similar to the terminal plane; 

 but the letter M being repeated on both the lateral 

 planes, denotes that these are similar to each other. 



In the right oblique-angled prism, the lateral planes 

 are distinguished from each other by the letters M, 

 and T, implying that they are dissimilar to each other, 

 as both are to the terminal plane which is designated 

 by P. 



The ' and " added to some of these letters, serve 

 merely to distinguish two or more similar planes from 

 each other. 



Thus, by carefully observing the position of the 

 edges of the base of any figure, as it is explained in 

 p. 101 and 102, and the letters used to designate the 



