﻿J. 
  M. 
  Blake 
  — 
  Plotting 
  Crystal 
  Zones 
  on 
  the 
  Sphere. 
  241 
  

  

  intersection 
  of 
  the 
  two 
  zones. 
  Preliminary 
  zone 
  plots 
  on 
  paper 
  

   are 
  useful 
  as 
  a 
  guide 
  in 
  this 
  work. 
  The 
  cross 
  zones 
  can 
  be 
  

   added 
  in 
  turn, 
  and 
  we 
  eventually 
  secure 
  all 
  the 
  planes 
  by 
  this 
  

  

  Fig. 
  3. 
  

  

  • 
  • 
  

  

  process. 
  In 
  case 
  of 
  a 
  triclinic 
  crystal, 
  it 
  may 
  be 
  required 
  to 
  

   triangulate 
  with 
  the 
  dividers 
  to 
  attain 
  a 
  first 
  approximation 
  in 
  

   locating 
  the 
  second 
  zone. 
  The 
  balance 
  of 
  the 
  cross 
  zones 
  can 
  

   then 
  take 
  their 
  proper 
  places. 
  

  

  The 
  next 
  step 
  is 
  to 
  obtain 
  a 
  record 
  of 
  the 
  position 
  of 
  all 
  the 
  

   planes 
  that 
  occur 
  in 
  one 
  hemisphere. 
  The 
  reading 
  of 
  the 
  

   equatorial 
  circle 
  will 
  be 
  from 
  to 
  360°. 
  The 
  reading 
  of 
  the 
  

   vertical 
  leg 
  of 
  the 
  triangle 
  will 
  be 
  from 
  at 
  the 
  point 
  of 
  con- 
  

   tact 
  of 
  the 
  tangent 
  plane 
  at 
  the 
  top 
  of 
  the 
  sphere 
  down 
  to 
  90° 
  

   where 
  the 
  triangle 
  rests 
  on 
  the 
  equatorial 
  ring. 
  

  

  We 
  shall 
  require 
  a 
  record 
  of 
  the 
  horizontal 
  or 
  equatorial 
  

   reading, 
  and 
  of 
  the 
  corresponding 
  vertical 
  angle. 
  It 
  will 
  

  

  