PARALLELISM OF EDGES. 401 



The first, second, and third of these formulae are 

 complicated from the generality which has been 

 given to them. They are all remarkable too, for not 

 containing the values of the primary edges, which 

 were used in the preliminary equations. 



The linear dimensions of the primary form are not 

 therefore necessary to determine the laws of decre- 

 ment of such planes as those we have been consider- 

 ing. Nor can the dimensions of the primary form be 

 deduced from any observation of parallelism between 

 the edges of a secondary crystal. 



It is necessary to add that if any of the planes 1 to 

 5, should cut one of the co-ordinate axes on the nega- 

 tive side, its index referring to that axis must be 

 taken negatively in the preceding formulae. 



As this short abstract is given merely to introduce 

 the formulae, the reader will more thoroughly com- 

 prehend the author's views, by consulting the paper 

 itself. 



