﻿398 
  J. 
  M. 
  Blake 
  — 
  Crystal 
  Drawing 
  and 
  Modeling. 
  

  

  grouping 
  of 
  the 
  planes 
  in 
  a 
  tangent 
  plane 
  projection 
  ; 
  and, 
  in 
  

   case 
  of 
  a 
  perspective 
  drawing, 
  we 
  would 
  make 
  our 
  view-point 
  

   as 
  near 
  this 
  first 
  point 
  as 
  the 
  case 
  will 
  allow. 
  A 
  second 
  next 
  

   best 
  position 
  should 
  be 
  found 
  at 
  or 
  nearly 
  at 
  right 
  angles 
  to 
  

   this 
  first 
  best 
  position 
  that 
  would 
  be 
  suitable 
  for 
  making 
  a 
  cor- 
  

   related 
  drawing 
  to 
  supplement 
  the 
  first. 
  

  

  When 
  the 
  view-point 
  in 
  any 
  case 
  tends, 
  as 
  sometimes 
  hap- 
  

   pens, 
  to 
  give 
  too 
  long 
  a 
  tangent 
  line 
  on 
  the 
  projection, 
  we 
  

   must 
  change 
  our 
  view-point 
  to 
  avoid 
  this 
  difficulty 
  and 
  allow 
  

   the 
  troublesome 
  plane 
  to 
  come 
  exactly 
  edge 
  on 
  ; 
  or 
  else 
  make 
  

   it 
  overstep 
  a 
  sufficient 
  distance 
  into 
  the 
  other 
  hemisphere. 
  

  

  Fig. 
  1. 
  

  

  Fig. 
  2. 
  

  

  This 
  just 
  mentioned 
  trouble 
  exists 
  whatever 
  the 
  mode 
  of 
  draw- 
  

   ing 
  we 
  employ, 
  but 
  we 
  have, 
  by 
  the 
  use 
  of 
  the 
  sphere, 
  a 
  means 
  

   provided 
  to 
  trace 
  out 
  the 
  cause 
  of 
  the 
  difficulty. 
  

  

  We 
  begin 
  our 
  drawing 
  by 
  attaching 
  the 
  paper 
  to 
  the 
  projec- 
  

   tion 
  sheet. 
  If 
  we 
  choose, 
  we 
  can 
  carry 
  on 
  one, 
  or 
  even 
  two, 
  

   auxiliary 
  or 
  correlated 
  drawings 
  in 
  connection 
  with 
  the 
  main 
  

   drawing, 
  and 
  by 
  the 
  methods 
  of 
  descriptive 
  geometry, 
  one 
  

   drawing 
  will 
  supply 
  dimensions 
  which 
  may 
  be 
  lacking 
  in 
  the 
  

   other. 
  In 
  a 
  case 
  where 
  we 
  would 
  need 
  to 
  utilize 
  two 
  or 
  three 
  

   corresponding 
  projections 
  at 
  one 
  time 
  for 
  this 
  purpose, 
  it 
  

   would 
  be 
  desirable, 
  in 
  order 
  to 
  secure 
  compactness, 
  either 
  to 
  

   use 
  a 
  reduced 
  scale 
  of 
  tangents, 
  or 
  to 
  reduce 
  the 
  projections 
  by 
  

   a 
  pantograph. 
  Such 
  reductions 
  have 
  been 
  made 
  in 
  which 
  an 
  

   inked 
  type 
  was 
  used. 
  This 
  type 
  had 
  a 
  central 
  dot 
  inside 
  a 
  

   ring, 
  the 
  central 
  dot 
  making 
  it 
  easier 
  to 
  bring 
  into 
  alignment 
  

   the 
  two 
  normal 
  points 
  that 
  are 
  used 
  in 
  obtaining 
  the 
  direction 
  

   of 
  a 
  crystal 
  edge. 
  

  

  One 
  or 
  both 
  drawings 
  can 
  be 
  in 
  plan, 
  or 
  both 
  be 
  made 
  in 
  

   perspective. 
  It 
  is 
  often 
  well 
  to 
  make 
  one 
  in 
  perspective 
  and 
  

  

  