﻿J. 
  M. 
  Blake 
  — 
  Plotting 
  Crystal 
  Zones 
  on 
  the 
  Sphere. 
  239 
  

  

  conform 
  to 
  the 
  same 
  laws. 
  When 
  we 
  deviate 
  from 
  this 
  best 
  

   view-point, 
  the 
  rows 
  converge 
  to 
  some 
  point 
  in 
  the 
  distance. 
  

  

  The 
  planes 
  of 
  a 
  fully 
  developed 
  crystal 
  may 
  be 
  regarded 
  as 
  

   forming 
  a 
  mutually 
  dependent 
  whole. 
  We 
  may 
  find 
  certain 
  

   planes 
  undeveloped 
  on 
  one 
  crystal 
  which 
  may 
  be 
  present 
  on 
  

   another 
  crystal 
  identical 
  in 
  composition 
  ; 
  and 
  also, 
  a 
  crystal 
  

   may 
  present 
  a 
  perfect 
  microscopic 
  edge 
  where 
  there 
  should 
  be 
  

   a 
  truncation 
  by 
  a 
  frequently 
  occurring 
  plane. 
  We 
  will 
  assume 
  

   that 
  each 
  species 
  has 
  a 
  limited 
  number 
  of 
  planes, 
  all 
  of 
  which 
  

   may 
  be 
  developed 
  under 
  favoring 
  conditions. 
  

  

  By 
  growing 
  a 
  polished 
  sphere 
  of 
  a 
  salt 
  to 
  a 
  proper 
  degree, 
  

   reflections 
  may 
  be 
  obtained 
  from 
  its 
  surface 
  for 
  all 
  developable 
  

   planes. 
  Such 
  a 
  sphere, 
  when 
  grown, 
  has 
  distinctive 
  markings 
  

   brought 
  out 
  on 
  its 
  surface. 
  With 
  the 
  same 
  end 
  in 
  view, 
  a 
  

   corner, 
  or 
  an 
  edge 
  of 
  a 
  crystal, 
  can 
  be 
  rounded 
  and 
  then 
  grown. 
  

  

  O 
  

  

  o 
  

  

  o- 
  

   o- 
  

   o 
  

  

  o 
  

  

  o 
  o 
  

  

  o 
  

  

  3 
  

  

  In 
  the 
  latter 
  case, 
  we 
  have 
  some 
  of 
  the 
  original 
  planes 
  left 
  to 
  

   aid 
  in 
  locating 
  any 
  less 
  frequently 
  occurring 
  planes 
  that 
  may 
  

   be 
  developed 
  by 
  the 
  treatment. 
  

  

  The 
  system 
  of 
  measuring 
  crystals 
  by 
  complete 
  zones 
  was 
  

   originally 
  suggested 
  by 
  the 
  serious 
  need 
  of 
  such 
  a 
  plan 
  for 
  use 
  

   in 
  the 
  ready 
  plotting 
  of 
  the 
  planes 
  on 
  the 
  sphere. 
  The 
  expedi- 
  

   ent 
  of 
  attaching 
  the 
  crystal 
  by 
  a 
  short 
  piece 
  of 
  lead 
  wire, 
  while 
  

   measuring, 
  was 
  proposed 
  in 
  1866 
  by 
  the 
  writer. 
  This 
  wire 
  

   support 
  will 
  allow 
  of 
  making 
  a 
  complete 
  zone 
  reading 
  for 
  each 
  

   adjustment. 
  

  

  The 
  Gnomonio 
  Projection. 
  — 
  We 
  would 
  here 
  draw 
  attention 
  

   to 
  the 
  value 
  of 
  the 
  gnomonic 
  projection 
  of 
  crystal 
  planes, 
  as 
  an 
  

   important 
  first 
  step 
  in 
  the 
  study 
  of 
  crystals. 
  An 
  objection 
  has 
  

   been 
  made 
  that 
  some 
  of 
  the 
  tangents 
  are 
  liable 
  to 
  become 
  too 
  

   seriously 
  extended, 
  and 
  hence, 
  a 
  very 
  general 
  resort 
  has 
  been 
  

  

  Am. 
  Jour. 
  Sci. 
  

   17 
  

  

  -Fourth 
  Series, 
  Vol. 
  XLIII, 
  No. 
  255.— 
  March, 
  1917. 
  

  

  