ON T DRAWING THE FIGURES OF CRYSTALS. 403 



Fig. 366. 



If we look along a street, and imagine that we are 

 seeing it through a transparent skreen, the upper and 

 lower edges of the fronts of the houses, which we 

 know to be nearly if not accurately parallel, appear 

 to converge at the remote end of the street, forming 

 a series of lines on the skreen something like that 

 shewn in fig. 366* 



And it is obvious that if this mass of houses were 

 a single solid body, and even if it were very much 

 reduced in dimensions, it must still be represented 

 on a plane surface by lines some of which must con- 

 verge, as those representing the upper and lower 

 edges of the supposed fronts of the houses do in the 

 above figure. 



But a representation of the figures of crystals in 

 this manner would not convey a sufficiently precise 

 notion of their forms, and it would be extremely 

 difficult to understand the figures of complicated 

 secondary crystals, if they were thus traced. 



In order to retain in the drawings of crystals the 

 apparent symmetry of their forms, another kind of 

 perspective has been used, which is known by the 

 name of orthographic or geometrical projection, or 

 simply by that of projection. 



In this kind of perspective, the object to be repre- 

 sented is supposed to be removed to an infinite dis- 

 tance from the eye ; in consequence of which all the 

 lines which are parallel in the figure would appear 

 3 E2 



