CODIFICATIONS. 



281 



Fig. 32 L 



There is, however, no real difference in the charac- 

 ter conferred on the plane by the two methods of 

 representing it. 



The only difference between them consists in this ; 

 the indices used in this volume simply give the cha- 

 racter of the compound molecule by whose continual 

 abstraction the new plane is produced, while the 

 Abbe Hatty's symbol supposes this molecule com- 

 pounded of several other compound molecules. 



This will be readily understood by a reference to 

 the above figure, which we shall suppose a doubly 

 oblique prism, with an intermediary decrement on 

 the solid angle at O. 



Let dec represent a compound molecule consisting 

 of three molecules in height, four in the direction a d, 

 and six in the direction b c, and let us suppose this 

 the molecule abstracted from the first plate super- 

 imposed on the terminal plane, and let us also sup- 

 pose that two of these would be abstracted from the 

 second plate, and so on, as explained in p. 22 and 23. 



The edge d a, in the above figure, corresponds 

 with the edge D of the primary form, b e with F, and 

 c/with H. 



The symbol we should use to represent the plane 

 produced by this decrement, would be (D4 H3 F6), 



2N 



