J. M. Blake — Plotting Crystal Zones on Paper. 489 



many circles. This is also a fault with certain more recent 

 stereographic drawings. The gnomonic projection is given in 

 fig. 2, and the general relation of the zone heing plotted to the 

 other planes on the crystal can he hettcr traced out upon this 

 projection. The direction of the two tangent lines shown in 

 fig. 1 is indicated on fig. 2. 



In the zone represented in fig. 1, the clarifying process has, 



Fig. 1. 



of course, been already applied, and we can only follow, and 

 have to accept that which remains of the preliminary work, as 

 we find it. If we could refer to an original measurement of 

 this zone, we would have our choice in selecting the developed 

 tangent spaces we regarded most suitable for estimating the 

 axial ratio. In this selection we would discriminate against 

 imperfect reflecting surfaces, and also spaces too far removed 

 from the zero point, because far out from this point the tan- 

 gent changes rapidly with small change of angle. "We would 

 take the average of the most suitable available spaces. In this 



