﻿J. 
  M. 
  Blake 
  — 
  Crystal 
  Drawing 
  and 
  Modeling. 
  401 
  

  

  obtained 
  by 
  measuring 
  the 
  perpendicular 
  distance 
  from 
  each 
  

   face 
  to 
  its 
  opposite. 
  The 
  measuring 
  apparatus 
  can 
  be 
  made 
  to 
  

   enlarge 
  this 
  radial 
  distance 
  in 
  any 
  desired 
  proportion, 
  and 
  with 
  

   this 
  help, 
  a 
  model 
  can 
  be 
  made 
  to 
  any 
  desired 
  scale. 
  If 
  the 
  

   same 
  radius 
  is 
  kept 
  throughout, 
  the 
  planes 
  will 
  necessarily 
  be 
  

   all 
  tangent 
  to 
  a 
  sphere 
  of 
  that 
  radius. 
  

  

  A 
  slice 
  of 
  material 
  is 
  cut 
  off 
  with 
  the 
  saw 
  and 
  the 
  file 
  follows 
  

   at 
  the 
  proper 
  depth. 
  The 
  file 
  is 
  rubbed 
  across 
  a 
  stiff 
  brush 
  

   after 
  each 
  stroke 
  to 
  clear 
  the 
  teeth 
  of 
  plaster. 
  

  

  Plaster 
  has 
  certain 
  qualities 
  which 
  render 
  it 
  ideal 
  for 
  cutting 
  

   models. 
  It 
  has 
  no 
  grain, 
  and 
  has 
  a 
  certain 
  toughness 
  that 
  

   prevents 
  crumbling 
  when 
  cutting 
  an 
  acute 
  edge. 
  A 
  medium 
  

   degree 
  of 
  hardness 
  is 
  best 
  for 
  the 
  reason 
  that 
  it 
  is 
  more 
  easily 
  

   cut. 
  When 
  there 
  is 
  to 
  be 
  considerable 
  handling, 
  a 
  harder 
  vari- 
  

   ety 
  of 
  plaster 
  can 
  be 
  used. 
  If 
  desired, 
  models 
  can 
  be 
  mounted 
  

   on 
  a 
  stem 
  with 
  a 
  base 
  to 
  secure 
  them 
  from 
  injury. 
  

  

  The 
  components 
  of 
  a 
  compound 
  crystal 
  can 
  be 
  modeled 
  on 
  

   the 
  twin 
  plane 
  and 
  the 
  parts 
  assembled. 
  The 
  model 
  can 
  be 
  cut 
  

   for 
  this 
  purpose 
  with 
  the 
  twin 
  plane 
  square 
  to 
  the 
  supporting 
  

   stem. 
  This 
  can 
  be 
  done 
  by 
  making 
  a 
  projection 
  from 
  the 
  twin 
  

   plane 
  view-point 
  by 
  which 
  to 
  set 
  the 
  machine. 
  

  

  These 
  models 
  admit 
  of 
  having 
  the 
  indices 
  written 
  on 
  the 
  

   planes, 
  and 
  two 
  sets 
  of 
  indices 
  in 
  different 
  colors 
  are 
  sometimes 
  

   helpful 
  in 
  comparing 
  two 
  separate 
  descriptions 
  of 
  the 
  same 
  

   mineral 
  species. 
  Salts 
  that 
  are 
  not 
  permanent 
  may 
  in 
  this 
  way 
  

   be 
  preserved 
  in 
  model. 
  When 
  tested 
  by 
  the 
  hand 
  goniometer 
  

   these 
  models 
  show 
  a 
  very 
  good 
  approximation 
  to 
  the 
  correct 
  

   angles. 
  

  

  It 
  may 
  be 
  of 
  interest, 
  as 
  bearing 
  on 
  future 
  applications, 
  to 
  

   mention 
  that 
  for 
  the 
  purpose 
  of 
  testing 
  and 
  identifying 
  some 
  

   minute 
  artificial 
  Gray-Lussite 
  crystals 
  a 
  crystal 
  was 
  mounted 
  on 
  

   a 
  pointed 
  lead 
  wire 
  and 
  brought 
  under 
  a 
  one-fifth 
  objective. 
  

   The 
  selected 
  faces 
  were 
  leveled 
  and 
  compared 
  directly 
  with 
  a 
  

   plaster 
  model. 
  Calculation 
  of 
  the 
  angles 
  bordering 
  the 
  plane 
  

   faces 
  was 
  in 
  this 
  instance 
  rendered 
  unnecessary 
  by 
  the 
  posses- 
  

   sion 
  of 
  the 
  model. 
  

  

  There 
  are 
  some 
  further 
  suggestions 
  that 
  have 
  not 
  as 
  yet 
  been 
  

   presented 
  relating 
  to 
  the 
  possibility 
  of 
  more 
  exact 
  determina- 
  

   tion 
  of 
  certain 
  crystal 
  constants. 
  

  

  New 
  Haven, 
  Conn., 
  Feb., 
  1917. 
  

  

  