406 APPENDIX OX DRAWING THE 



Fig-. 369. 



It will be more consistent with most of the follow- 

 ing explanations of the methods of drawing" the 

 figures of crystals, to suppose the position of the eye 

 fixed, and the back of the crystal to be elevated. 



In this new relative position of the crystal and the 

 eye, the figure traced on the interposed skreen, 

 would resemble that exhibited in fte. 369. 



O 



It is evident from the preceding explanation, that 

 the relative positions of the eye, the object, and the 

 skreen, may be varied at pleasure, so as to produce 

 in the drawing, such a representation of the object 

 as best suits the illustration it is intended to afford. 

 And although, as it has been stated in p. J02, an 

 advantage will generally attend the placing the 

 figures of crystals belonging to the same class of 

 primary forms always in the same position, there 

 may nevertheless be exceptions to this rule when the 

 position of the modifying planes on the secondary 

 crystal, is such, as to require some new position for 

 their more perfect exhibition. The position chosen 

 by the Abbe Haiiy for the crystal of felspar, is per- 

 haps the best that could be adopted for exhibiting 

 advantageously the secondary planes of the crystals 

 of that substance ; yet the front lateral planes of his 

 figure correspond to the back planes of the doubly 

 oblique prism as it is given in the tables of modi- 

 fications. 



