Simpler Methods in Grystcdlography. 325 



lines will be referred to as zones. Furthermore, it will soon 

 be evident that the poles of each zone are perpendicular to 

 one line, which will be referred to as the zone axis. It will 

 also be obvious, in the case of Barytes, that each pole passes 

 through a pair of faces, front and back, right and left, above 

 and below, as the case may be; and that several of these 

 pairs of faces are obviously symmetrical each to the other. 



6. Some further aid in understanding the geometry of 

 crystals, and the projection of the poles of the commoner 

 forms, is to make a set of crystal models from nets such as 

 Jordan's (Murby & Co.), and to place one simple form after 

 another, all with similar orientation, in the place of the 

 crystal, choosing the Orthorhombic system as being the most 

 suitable for the purpose. By this means we can combine 

 into one projection the whole of the forms represented in the 

 series. 



7. It is as well at first to turn the crystal about in several 

 directions within the glass shade, and to draw in the poles 

 and the various zones which include those poles, until the 

 broader features are quite clear. When this stage is reached, 

 the zones and poles may advantageously be studied in more 

 detail with the aid of any good table of indices, such as are 

 to be got from Dana. 



8. The next step is to do the work somewhat more exactly 

 by means of a fair-sized ball of soft wood, or even an india- 

 rubber ball : the wooden ball is better for some reasons. 

 Now, instead of the ink dots on the glass shade for the poles, 

 insert small pins, each pointing as accurately as possible to 

 the centre of the ball. Also pencil in the zones connecting 

 them as straight as possible by hand. ISTote that these zones 

 all form great circles on the sphere — that is to say, that 

 they all have a common centre with the sphere itself. It 

 will not be difficult to put on the poles of a few other forms 

 than one is likely to meet with on any single crystal, and in 

 order to do this, some temporary departure must be made 

 from the plan adopted in this paper, and we must, for once, 

 have recourse to angular measurements. First, two opposite 

 poles must be chosen to represent the front -and-back axis of 

 the crystal, and we mark these {a), and in like manner the 



