286 APPENDIX CALCULATION OF THE 



Second, To determine the angles at which the 

 secondary plane inclines upon the adjacent pri- 

 mary planes, the elements of the primary form, 

 and the law of decrement by which the secondary 

 plane is produced^ being known. 

 Or, to determine the particular values of the gene- 

 ral indices given in the tables at p. 254, the 

 inclination of the secondary planes to the pri- 

 mary being known ; and to determine those in- 

 clinations when the indices are known. 



The elements of the several classes of primary forms 

 consist of 



1st. The angles at which the primary planes in- 

 cline to each other. These may be ascertained by 

 means of the goniometer, if not already known. 



2d. The plane angles of the primary planes. When 

 these angles cannot be ascertained by other means, 

 they may be deduced by spherical trigonometry, from 

 the known inclination of the primary planes to each 

 other. 



3d. The comparative lengths of the primary edges, 

 and of such other lines upon or within any crystal 

 as maybe required for facilitating our calculations of 

 the laws of decrement, or for delineating its primary 

 or any of its secondary forms. The methods of de- 

 ducing: such of these elements as cannot be ascer- 



O 



tained by measurement of the crystal, will be des- 

 cribed where the elements of the several classes of 

 primary forms are described. 



In the tables at p. 254, &c. the letter p is used to re- 

 present any whole number ', or fraction. But it will be 

 more convenient for our present purpose to represent 

 simple and mixed decrements by the general fractional 



index fL; p expressing the decrements in breadth, and 



q 



q the decrements in height. 



