﻿J. 
  M. 
  Blake 
  — 
  Crystal 
  Drawing 
  and 
  Modeling. 
  397 
  

  

  Art. 
  XXXYII. 
  — 
  Crystal 
  Drawing 
  and 
  Modeling; 
  by 
  John 
  

  

  M. 
  Blake. 
  (Article 
  5.) 
  

  

  It 
  now 
  remains 
  to 
  describe 
  two 
  other 
  methods 
  which 
  become 
  

   practicable 
  when 
  we 
  use 
  the 
  plotting 
  sphere. 
  

  

  Drawing. 
  

  

  In 
  a 
  former 
  article 
  a 
  practical 
  method 
  of 
  obtaining 
  the 
  gno- 
  

   monic 
  projection 
  was 
  given 
  which 
  was 
  based 
  upon 
  the 
  plotting 
  

   of 
  the 
  crystal 
  planes 
  on 
  the 
  sphere. 
  This 
  gnomonic 
  projec- 
  

   tion 
  can 
  be 
  utilized 
  as 
  an 
  aid 
  in 
  making 
  crystal 
  drawings. 
  The 
  

   general 
  principle 
  on 
  which 
  this 
  adaptation 
  is 
  based 
  is 
  that 
  the 
  

   direction 
  of 
  a 
  crystal-edge 
  formed 
  by 
  the 
  intersection 
  of 
  two 
  

   planes 
  will 
  be 
  at 
  right 
  angles 
  to 
  a 
  line 
  connecting 
  the 
  normal 
  

   points 
  of 
  the 
  planes 
  on 
  the 
  gnomonic 
  projection. 
  By 
  taking 
  

   advantage 
  of 
  this 
  fact, 
  we 
  can 
  greatly 
  simplify 
  the 
  making 
  of 
  

   drawings. 
  

  

  This 
  principle 
  was 
  used 
  by 
  others 
  in 
  a 
  limited 
  way 
  as 
  far 
  

   back 
  as 
  the 
  fifties 
  for 
  making 
  plan 
  drawings, 
  but 
  it 
  was 
  clone 
  

   under 
  exceptional 
  conditions. 
  We 
  would 
  here 
  draw 
  attention 
  

   to 
  the 
  more 
  general 
  adaptability 
  of 
  this 
  present 
  method 
  for 
  

   use 
  in 
  both 
  plan 
  and 
  perspective 
  drawing. 
  Either 
  kind 
  of 
  

   drawing 
  can 
  be 
  made 
  at 
  will, 
  provided 
  that 
  in 
  each 
  instance 
  

   we 
  start 
  from 
  a 
  tangent 
  plane 
  projection 
  made 
  from 
  the 
  proper 
  

   view-point. 
  The 
  method 
  in 
  which 
  a 
  templet 
  is 
  depended 
  upon 
  

   to 
  mark 
  out 
  the 
  direction 
  and 
  length 
  of 
  the 
  axes 
  can 
  be 
  entirely 
  

   dispensed 
  with, 
  since 
  the 
  new 
  way 
  is 
  so 
  entirely 
  different. 
  

   Furthermore, 
  it 
  may 
  be 
  said, 
  there 
  will 
  now 
  exist 
  no 
  valid 
  

   reason 
  for 
  being 
  restricted 
  to 
  certain 
  fixed 
  positions 
  and 
  axial 
  

   lengths, 
  such 
  as 
  would 
  be 
  the 
  case 
  when 
  using 
  the 
  templet. 
  

  

  It 
  may 
  be 
  stated 
  that 
  as 
  a 
  general 
  principle 
  the 
  planes 
  which 
  

   are 
  more 
  nearly 
  flat 
  on 
  when 
  seen 
  from 
  the 
  selected 
  view- 
  

   point, 
  will 
  be 
  most 
  clearly 
  presented 
  and 
  brought 
  out 
  by 
  their 
  

   intersections 
  ; 
  while 
  those 
  planes 
  which 
  are 
  viewed 
  nearly 
  

   edgewise, 
  and 
  are 
  marginal, 
  are 
  most 
  likely 
  to 
  be 
  confused 
  in 
  

   their 
  intersections 
  on 
  the 
  drawing. 
  It 
  is 
  well, 
  therefore, 
  to 
  

   make 
  a 
  point 
  of 
  avoiding 
  this 
  nearly 
  edgewise 
  position 
  for 
  any 
  

   of 
  the 
  planes. 
  

  

  When 
  we 
  have 
  the 
  position 
  of 
  the 
  planes 
  plotted 
  on 
  the 
  

   sphere, 
  it 
  is 
  easy 
  to 
  test 
  for 
  the 
  above 
  conditions 
  by 
  moving 
  

   the 
  sphere 
  in 
  the 
  equatorial 
  ring 
  until 
  a 
  position 
  is 
  found 
  where 
  

   a 
  projection 
  made 
  from 
  this 
  view-point 
  will 
  not 
  give 
  a 
  tangent 
  

   length 
  in 
  the 
  projection 
  exceeding, 
  at 
  the 
  utmost, 
  that 
  of 
  

   seventy-five 
  or 
  eighty 
  degrees. 
  In 
  case 
  of 
  a 
  plan 
  drawing, 
  we 
  

   would 
  select 
  the 
  view-point 
  that 
  would 
  give 
  the 
  most 
  compact 
  

  

  