404 



APPENDIX- 



>N DRAWING THE 



parallel upon our supposed transparent skreen, and 

 not converging as they do in the above diagram.* 



The method of representing crystals in projection 

 may be thus explained. Let us for a moment forget 

 the abstract notion of the object being removed to an 

 infinite distance from the eye, and let us imagine it 

 distinctly within our view. 



Fig. 367. 



Let the figure to be represented, be a cube ; and 

 let us imagine this cube to be resting upon a hori- 

 zontal surface, and the eye to be placed opposite one 

 of its planes, and in the direction of a line drawn 

 perpendicularly through the centre of that plane. 



In these relative positions of the eye and the crys- 

 tal, only that plane opposite to the eye will be visi- 

 ble; and if a transparent skreen were interposed 

 between the eye and the crystal, and held parallel to 

 the plane which is seen, the only linear traces which 

 could be marked on the skreen would be the edges of 

 the observed plane, as represented in fig. 367. 



* This theoretical notion of the infinite distance of the object, is bor- 

 rowed from mathematical considerations of the nature of infinite lines ; 

 and may be taken here merely to imply what is stated in the text, that 

 the edges, or other lines, which are parallel on the crystals, are to be 

 represented by parallel lines in the drawing. 



