﻿J. 
  M. 
  Blake 
  — 
  Crystal 
  Drawing 
  and 
  Modeling. 
  399 
  

  

  its 
  correlated 
  drawing 
  in 
  plan, 
  and, 
  if 
  thought 
  best, 
  dispense 
  

   with 
  the 
  latter 
  after 
  it 
  has 
  served 
  its 
  purpose. 
  A 
  T-square 
  

   should 
  be 
  used 
  in 
  order 
  to 
  obtain 
  sufficient 
  span. 
  We 
  bring 
  

   the 
  two 
  selected 
  normal 
  points 
  in 
  line 
  under 
  the 
  square 
  when 
  

   this 
  is 
  placed 
  in 
  a 
  vertical 
  position. 
  We 
  next 
  apply 
  the 
  square 
  

   against 
  the 
  side 
  of 
  the 
  board, 
  and 
  the 
  edge 
  of 
  the 
  square 
  will 
  

   then 
  give 
  the 
  direction 
  of 
  the 
  crystal 
  edge. 
  The 
  usual 
  proce- 
  

   dure 
  can, 
  in 
  general, 
  be 
  adopted 
  in 
  regard 
  to 
  other 
  details. 
  

  

  Two 
  drawings 
  are 
  shown, 
  one 
  in 
  plan 
  and 
  one 
  in 
  perspective. 
  

   They 
  both 
  relate 
  to 
  Gay-Lussite 
  and 
  were 
  found 
  together 
  with 
  

   a 
  number 
  of 
  drawings 
  pertaining 
  to 
  that 
  species 
  that 
  were 
  made 
  

   by 
  the 
  writer 
  in 
  1866. 
  They 
  will 
  serve 
  to 
  illustrate 
  the 
  adapt- 
  

   ability 
  of 
  this 
  method 
  of 
  drawing 
  to 
  all 
  positions. 
  Plan 
  draw- 
  

   ings 
  are 
  as 
  a 
  rule 
  more 
  satisfactory 
  for 
  use 
  during 
  the 
  correct 
  

   proportioning 
  and 
  shaping 
  of 
  a 
  crystal 
  model 
  from 
  a 
  descrip- 
  

   tive 
  drawing. 
  The 
  fact 
  that 
  plan 
  drawings 
  are 
  better 
  for 
  

   shaping 
  models 
  also 
  implies 
  that 
  when 
  we 
  have 
  two 
  correlated 
  

   plan 
  drawings 
  from 
  well 
  selected 
  view-points, 
  we 
  are 
  provided 
  

   with 
  the 
  means 
  of 
  gaining 
  a 
  clear 
  conception 
  of 
  the 
  crystal 
  

   form. 
  Perspective 
  drawings 
  have 
  the 
  advantage 
  when 
  we 
  wish 
  

   to 
  show 
  the 
  back 
  planes 
  of 
  a 
  crystal. 
  

  

  The 
  making 
  of 
  both 
  kinds 
  of 
  drawings 
  will 
  be 
  greatly 
  facili- 
  

   tated 
  by 
  the 
  use 
  of 
  the 
  gnomonic 
  projection 
  method 
  based 
  on 
  

   the 
  plotting 
  of 
  the 
  plane 
  positions 
  on 
  the 
  sphere. 
  

  

  Modeling. 
  

  

  During 
  the 
  progress 
  of 
  a 
  crystal 
  investigation 
  it 
  will 
  be 
  

   found 
  a 
  great 
  help 
  to 
  be 
  able 
  to 
  model 
  any 
  of 
  the 
  forms. 
  The 
  

   writer 
  contrived 
  a 
  machine 
  for 
  this 
  express 
  purpose, 
  which 
  was 
  

   successfully 
  constructed 
  shortly 
  after 
  the 
  plotting 
  sphere 
  was 
  

   made, 
  the 
  idea 
  of 
  such 
  a 
  machine 
  naturally 
  following 
  the 
  suc- 
  

   cessful 
  use 
  of 
  the 
  sphere. 
  A 
  description 
  of 
  this 
  machine 
  will 
  

   be 
  given 
  farther 
  along. 
  

  

  Such 
  models 
  can 
  be 
  made 
  so 
  that 
  the 
  planes 
  have 
  the 
  correct 
  

   proportions 
  of 
  the 
  natural 
  crystal, 
  or 
  instead, 
  we 
  can 
  make 
  all 
  

   the 
  planes 
  tangent 
  to 
  a 
  sphere. 
  This 
  latter 
  plan 
  is 
  frequently 
  

   followed, 
  and 
  has 
  the 
  advantage 
  of 
  giving 
  each 
  plane 
  an 
  equal 
  

   chance 
  to 
  appear. 
  

  

  The 
  modeling 
  of 
  several 
  varieties 
  of 
  the 
  same 
  mineral 
  species 
  

   with 
  the 
  true 
  proportioning 
  of 
  the 
  planes, 
  when 
  carried 
  out 
  on 
  

   an 
  isomorphous 
  group 
  of 
  minerals, 
  sometimes 
  results 
  in 
  a 
  very 
  

   interesting 
  series 
  of 
  models, 
  as 
  for 
  instance 
  in 
  the 
  chrysolite 
  

   group, 
  which 
  includes 
  the 
  natural 
  crystals, 
  together 
  with 
  cer- 
  

   tain 
  furnace 
  products 
  in 
  w 
  T 
  hich 
  iron 
  predominates. 
  This 
  series 
  

   of 
  models 
  shows 
  very 
  wide 
  variations 
  in 
  the 
  development 
  and 
  

   appearance 
  of 
  the 
  members, 
  and 
  yet 
  the 
  angles 
  in 
  the 
  whole 
  

   series 
  are 
  practically 
  the 
  same. 
  

  

  