﻿J. 
  M> 
  Blake 
  — 
  Plotting 
  Crystal 
  Zones 
  on 
  the 
  Sphere. 
  237 
  

  

  Art. 
  XXI. 
  — 
  Plotting 
  Crystal 
  Zones 
  on 
  the 
  Sphere', 
  by 
  

   John 
  M. 
  Blake. 
  (Article 
  4.) 
  

  

  In 
  the 
  early 
  sixties 
  the 
  writer 
  made 
  use 
  of 
  plotting 
  on 
  the 
  

   sphere 
  in 
  the 
  solution 
  of 
  certain 
  crystal 
  problems. 
  Some 
  pre- 
  

   liminary 
  trials 
  were 
  made 
  with 
  a 
  nine-inch 
  wooden 
  slate-coated 
  

   sphere, 
  and 
  the 
  results 
  were 
  so 
  encouraging 
  that 
  a 
  hollow 
  cast 
  

   zinc 
  sphere 
  was 
  made 
  of 
  about 
  twelve 
  inches 
  diameter. 
  This 
  

   sphere 
  was 
  accurately 
  turned 
  in 
  a 
  special 
  lathe, 
  and 
  was 
  then 
  

  

  Fig. 
  1. 
  

  

  Fig. 
  1. 
  J. 
  M. 
  Blake's 
  Plotting 
  Sphere. 
  

  

  coated 
  with 
  a 
  liquid 
  slating 
  compound, 
  and 
  its 
  spherical 
  figure 
  

   was 
  perfected 
  by 
  recoating 
  the 
  low 
  portions 
  and 
  then 
  grinding 
  

   the 
  surface 
  with 
  a 
  pumice 
  stone 
  mould 
  of 
  the 
  same 
  curvature. 
  

   A 
  hard, 
  smooth, 
  true 
  surface 
  was 
  thus 
  secured. 
  It 
  was 
  pro- 
  

   vided 
  with 
  a 
  graduated 
  equatorial 
  ring. 
  On 
  this 
  ring 
  a 
  right- 
  

   angled 
  spherical 
  triangle 
  could 
  be 
  rested 
  while 
  plotting. 
  

  

  The 
  base 
  of 
  the 
  sphere 
  rotates 
  on 
  rollers 
  set 
  with 
  the 
  axes 
  

   radial. 
  The 
  sphere 
  can 
  be 
  turned 
  independently 
  of 
  the 
  base 
  

   by 
  slackening 
  three 
  leather-tipped 
  centering 
  screws 
  ; 
  and 
  it 
  

  

  