20 



DEFINITIONS. 



Fig. 42. 



Decrements have been divided by the Abbe Haiiy 

 into three principal classes simple, mixed, and inter- 

 mediary. The simple and mixed may however, in 

 strictness, be regarded as varieties of the same class. 



Simple decrements are those which consist in the 

 abstraction of any number of rows, in breadth of single 

 molecules, or of single rows, belonging to plates of 

 two or more molecules in thickness. 



Fig. 42 exhibits a simple decrement by one row in 

 breadth on the edge c d of the primary form. 



Fig. 43. 



Fig. 43 exhibits a simple decrement by one row in 

 breadth, on the angle c of the primary form. 



For the sake of rendering the expression rows 

 of molecules generally applicable to decrements both 

 on the angles and edges of a primary form, the term 

 row is applied to express the single molecule first 

 abstracted from the angle of any plate. 



Fig. 44. 



In fig. 44, the single molecule a, b, is regarded as 

 the first row to be abstracted from the angle of the 



