LAWS OF DECREMENT. 303 



It will be recollected that in framing the general 

 symbol of any secondary plane, we are generally to 

 consider p^> #> r> s. 



By carefully observing the position of the plane 

 we have measured, and whose law of decrement we 

 require to know, we shall feel no difficulty in adapt- 

 ing an appropriate symbol to it. And having found 

 our general symbol, we may readily find the particu~ 

 lar values of these indices by the methods already 

 described, or by such as will be detailed in the suc- 

 ceeding part of this appendix. 



From what has preceded, the method will be rea- 

 dily perceived by which we may determine the ratios 

 of the primary edges of crystals, if we assume some 

 observed secondary plane replacing an edge of those 

 forms whose terminal edges are equal, or replacing 

 an angle of those whose terminal edges are unequal, 

 to have been produced by some given law of decre- 

 ment. 



If we assume that a plane replacing an angle or 

 edge of any primary form, has resulted from a decre- 

 ment by one row of molecules, we determine the 

 ratio of the primary edges by discovering the r^tio of 

 the edges of the defect occasioned by that plane. And 

 if we assume any other law of decrement to have 

 produced the given plane, the ratios of the primary 

 edges may evidently be determined, by dividing the 

 ratios of the edges of the defect by the assumed law 

 of decrement. 



