FIGURES OF CRYSTALS. 



405 



Fig. 368. 



t. . J_ I 



If we now suppose the eye and the skreen to be 

 moved horizontally toward the right of the spectator, 

 the skreen retaining its parallelism to the plane F', 

 the rays proceeding from the edges of that plane, 

 may be conceived to pass obliquely towards the 

 skreen in its new position, and the edges of the plane 

 P" will now be visible, and may be traced on the 

 skreen as in iig. 368. If we suppose the eye and the 

 interposed skreen to move round the crystal, the 

 skreen retaining its perpendicular position, but ceas- 

 ing to be parallel to any plane of the cube, excepting 

 at some particular points of its progress, it will be 

 obvious, that while the eye and the skreen continue 

 to move in the same horizontal plane^ the vertical 

 planes of the crystal, and those only, will become 

 visible in succession ; but the terminal plane will 

 not be perceived. To see the terminal plane we 

 must suppose the eye and the skreen to be raised ; 

 or, if the eye retain its position, the back of the crys- 

 tal must be elevated. 



