FIGURES OF CRYSTALS. 433 



If we recollect that the planes p result from a decre- 

 ment by 2 rows in breadth, it will be apparent that 

 the directing plane/? ppf of No. 1, corresponds to p 

 of No. 2, and p" p" p" f of No. 1, top" of No. 2. 

 But the intersection of 'p p pf, and p" p" p" f, is the 

 directing line//'. 



If therefore we take any point f in the vertical 

 edge of No. 2, and draw//', parallel to//' No. 1, 

 /' No. 2 being the intersection of the new edge with 

 an oblique diagonal, and if from the points / and 

 /' we draw lines parallel to the inferior primary 

 edges, we shall obtain a representation of the planes 

 p and p". 



The intersection ofp and m, No. 2, is shewn by the 

 directing line p p, No. 1 ; and that of p 1 and m, No. 2, 

 by p' h, No. 1 ; and the intersection ofp andp', No. 2, 

 is parallel to the line pf, No. 1.* 



It now remains only to add the planes g to -our 

 figure. For this purpose we may trace in pencil, as 

 at No. 4, fig. 389, an accurate copy of No. 2, fig. 388 ; 

 and above it draw the primary form, and the direct- 

 ing planes shewn by No. 3. 



We observe here that g g' g", No. 3, corresponds 

 to plane g 9 No. 4. The intersection of this plane 

 with m is parallel to the horizontal diagonal of P", 

 and its intersections with P and P' are parallel to 

 g g', and g g", No, 3. Its intersections with p and 

 ^/, are parallel to the directing lines i I and i V, No. 3; 

 the points /, and /', being the only ones at which the 

 edges of the planes p, p', and g, intersect each other, 

 and the point i being common to the three planes. 



The intersections of g with o and o' are the lines 

 i k, i k'. For the planes o, and o', might evidently 



* It may he observed that there are three dotted lines terminating at 

 , No. 1, fig. 388; one of these has p at its other extremity, another 

 has/, and the reader is requested to add/i' to the third. 



3i 



