FIGURES OF CRYSTALS. 415 



upon it from one of the lateral solid angles of the 

 particular rhomboid we are about to delineate, is 

 supposed to have been ascertained.* 



We should then determine the height our proposed 

 figure is to be, which height will be the length of 

 its axis. Our next step is to find a line which 

 bears the same ratio to that which we have fixed on 

 for the axis of our figure, as the perpendicular upon 

 the axis of the crystal, does to the axis itself. This 

 may generally be done with sufficient precision, by 

 dividing the line we have assumed for our axis into 

 such a number of equal parts, as will give the length 

 of the required line in some other number of those 

 parts. If, for example, we have found, that the per- 

 pendicular upon the axis, is to the axis itself, in the 

 ratio of 7 to 10, and if we determine that our figure 

 shall be an inch high, the required line will be evi- 

 dently & of an inch. 



If, however, we are desirous of still greater accu- 

 racy, we may draw a perpendicular line equal to that 

 which we have fixed on for the height of our rhomb- 

 oid, and from the upper extremity of this line draw 

 a second, inclining to it at the same angle that the 

 axis does to a superior edge of the rhomboid we are 

 about to represent; and if we now divide our first 

 line into three equal parts, and from the upper point 

 of section, draw a perpendicular to it which shall 

 pass through the second line, the portion intercepted 

 by the second line will be the required length of the 

 perpendicular upon the axis. 



With a radius equal to this line, which is, in the 

 case we are supposing, ^ of an inch, describe the 

 circle abed ef> fig. 376. 



* The method of ascertaining this ratio has been already pointed out 

 in p. 363. 



