On Crystallography. 459 



the no;ht or to the left of the letter, according as they shall 

 take place \i\ one direction or another. 



Thus D will express a decrement by two ranges, going 



from D towards C ; C a rlecrenicnt by three ranges, going 

 from C towards D; D a decrement by two rangt-s, descend- 

 ing on the face M ; ^H a decrement by three ranges, going 

 from H towards G ; -G a decrement bv four ranges, going 

 from G towards the ridge opposite to H, Sec. 



In the case in which weshoidd be obliged to desigriateby 

 means of a small letter such as d, a decrement on the ridge 

 «r(fiir. 50), opposite to that which bears the large later 

 D (fi*'-~^ 48), we should suppose the crystal to be turned 

 upside down. Thus d would express a decrement by twd 



ranees ascending above the inferior base p, as D would 

 'expR'ss one v\hich is ascending on the upper base P. For 

 the same reason c would express a dtcremcnt by three ranges 



3 



proceeding from sp (fig. 30) towards KO. 



If the same solid angle or the same ridge undergoes se- 

 veral successive decrements on the san)e side, or several 

 decrements which take place on different sides, we sliall 

 repeal the same number of times the index letter, varying 

 the cyphers conformably to the diversity of the decrements. 



Thus D D will designate two decrements on the ridge D, 



the one by two ranees ascending on the base P, the other 

 by three rano;es descendmg on the face M. 'H, -H will 

 deslffiiale two decrements, the one by two ranges, the other 

 by tour, to the kit of the ridge 11, &c. 



' If there are mixed decrements, we shall indicate tliem on 

 the same principles, by employing the fractions -«-. -5, &c., 

 which represent them, and the numerator of which refers 

 to the decrement iu breadth, and the denominator to the 

 decrement in height. 



It now remains to find a method of representing the in- 

 termediate decrements. An example will show that which 

 we have adopted. Let AE O I (fig. 51) be the same face 

 with fiff. }8. Let us suppose a decrement by one range of 

 double'molecnles, following parallel lines xy, so that O;/ 

 measures double lines bv one ridge of molecules, and O x 

 lines simply equal to this ridge." We shall thus indicate 



this decrement (OD'F'). The parenthesis shows in tht5 



first place that the decrement is iuiermcdialc ; O indicates 

 ' that 



