4 5S OnCnjstullography. 



Pryslal^ with those wliich will have a relation to the angles or 

 eilt^cs on uhich the decrements will act. 



Let lis suppose in the first place, for greater simplicity, 

 that one of the solid angles, such as O, is intercepted by a 

 single additional facet. The decrement to which we refer 

 ihe'produclion of this facet may take place cither on the 

 base P, or on the pane T which is to the right of the ob- 

 server, or upon the pane M situated on his left.' 



In the first case, we shall place the index cypher above 

 the letter; in the second we shall give to the cypher the 

 place of a common exponent to the right and towards the 

 top of the letter ; and we shall indicate the third case by 

 placing the cypher on the left, and even towards the top of 

 the letter. 



2 



Thus O will express the effectof a dccrenitnt by two ranges 

 in breadth, jjarallel to the diagonal of the base P, which passes 

 by the angle E; 0> the effect of adecrenient by three ranges 

 in breadth, parallel to the diagonal of the face T, which 

 passes by the angle I, and '*0 the effect of a decrement by 

 four ranges, parallel to the diagonal of the face M, which 

 passes by the angle E. 



When the decrement has a reference to any of the three 

 solid angles I, A, E, the observer is considered as turning 

 round the crystal until he finds himself placed opposite this 

 angle, as it was naturally opposite the angle 0, in the case 

 which we have described ; or, vjhat comes to the same thing, 

 he is considered as turning the crystal, until the solid angle 

 under consideration faces him, anJ it is with respect to this 

 position that such a decrement is said to be towards the 

 right or the left. 



^For example : if the solid angle A is in question, the 

 sign A^ will represent the eflect of a decrement by two 

 ranges on the face AE 5r (fig. 50), or upon that which is 

 opposite to T (fig. 48), and *A will represent the effect of 

 a decrement by three ranges on the face A I w r (fig. 5Q), 

 or upon that which is opposite to M (fig. 46). We shall 

 subsequently see the advantage of this manner of proceed- 

 ing, relative to the uniformily of the method. 



As to the decrements on the ridges, we shall express 

 those which are furujcd towards the contour J> CFD of the 

 base, by a number placed above or below the letter, ac- ■ 

 cording as their effect shall take place in ascending or de- 

 scending, setting out from the ridge to which they will be 

 vefened ; and those which are relative to the longitudinal 

 ridges G, H, will be indicated by a sign placed cither to 



the 



