﻿Separcdion 
  of 
  Dextro- 
  and 
  Lmvo-rotating 
  Crystals. 
  135 
  

  

  However, 
  this 
  arrangement 
  would 
  not 
  be 
  siifficientlj 
  sensitive 
  

   for 
  smaller 
  crystals 
  and 
  moreover 
  necessitates 
  adjustment 
  of 
  

   the 
  analyzer. 
  

  

  But 
  if 
  now 
  the 
  crystal 
  is 
  mounted 
  over 
  the 
  polarizer, 
  and 
  the 
  

   analyzer 
  be 
  so 
  adjusted 
  as 
  to 
  bring 
  into 
  the 
  field 
  the 
  blue 
  color 
  

   and 
  then 
  turned 
  just 
  enough 
  to 
  cause 
  the 
  blue 
  to 
  merge 
  into 
  

   purple, 
  a 
  very 
  sensitive 
  field 
  is 
  obtained. 
  All 
  that 
  is 
  required 
  

   then 
  is 
  to 
  insert 
  a 
  crystal, 
  ever 
  so 
  small, 
  between 
  the 
  analyzer 
  

   and 
  " 
  field 
  crystal," 
  whereupon 
  either 
  the 
  blue 
  will 
  be 
  restored 
  

   or 
  a 
  deeper 
  red 
  obtained. 
  In 
  the 
  first 
  case 
  the 
  rotation 
  of 
  the 
  

   crystal 
  is 
  evidently 
  the 
  same 
  as 
  that 
  of 
  the 
  field 
  crystal, 
  and 
  in 
  

   the 
  other 
  case, 
  the 
  opposite. 
  For 
  larger 
  crystals 
  the 
  change 
  of 
  

   color 
  is 
  only 
  greater 
  and 
  not 
  less 
  certain 
  in 
  its 
  evidence 
  as 
  to 
  

   the 
  optical 
  properties 
  of 
  the 
  crystal 
  under 
  investigation. 
  

  

  It 
  is 
  thus 
  only 
  necessary 
  to 
  select 
  some 
  clear 
  and 
  good-sized 
  

   crystal 
  whose 
  rotation 
  may 
  easily 
  be 
  determined 
  as 
  above 
  stated 
  

   and 
  then 
  to 
  use 
  this 
  crystal 
  as 
  the 
  " 
  field 
  crystal." 
  Without 
  

   any 
  change 
  after 
  the 
  first 
  adjustment 
  above 
  described, 
  various 
  

   crystals 
  may 
  now 
  be 
  brought 
  into 
  the 
  field 
  of 
  view 
  and 
  the 
  

   color 
  they 
  assume 
  will 
  instantly 
  disclose 
  their 
  nature. 
  This 
  

   may 
  be 
  facilitated 
  by 
  spreading 
  them 
  on 
  a 
  long 
  piece 
  of 
  glass, 
  

   which 
  may 
  then 
  be 
  drawn 
  through 
  the 
  field, 
  and 
  if 
  it 
  is 
  desired 
  

   to 
  separate 
  them, 
  all 
  those 
  which, 
  for 
  example, 
  turn 
  blue 
  may 
  

   be 
  picked 
  out. 
  This 
  is 
  extremely 
  sensitive 
  ; 
  the 
  merest 
  specks, 
  

   if 
  of 
  the 
  same 
  rotation 
  as 
  the 
  field 
  crystal, 
  being 
  revealed. 
  

   Those 
  of 
  opposite 
  sense 
  are 
  less 
  easily 
  detected 
  ; 
  but 
  in 
  using 
  

   the 
  device 
  I 
  have 
  usually 
  employed 
  two 
  field 
  crystals 
  of 
  oppo- 
  

   site 
  rotation, 
  separately 
  mounted, 
  as 
  shown 
  at 
  m, 
  n^ 
  fig. 
  1, 
  so 
  

   that 
  either 
  could 
  be 
  swung 
  into 
  the 
  field 
  and 
  the 
  analyzer 
  ad- 
  

   justed. 
  This 
  however 
  is 
  only 
  rarely 
  desired 
  and 
  then 
  merely 
  

   for 
  confirmation, 
  where 
  a 
  considerable 
  number 
  of 
  very 
  small 
  

   crystals 
  occur. 
  

  

  The 
  sensitiveness 
  may 
  be 
  further 
  increased 
  by 
  placing 
  the 
  

   very 
  small 
  crystal 
  in 
  the 
  field 
  and 
  then 
  turning 
  the 
  analyzer 
  to 
  

   and 
  fro 
  over 
  the 
  turning 
  point 
  of 
  color, 
  wdien 
  the 
  speck 
  of 
  a 
  

   crystal, 
  and 
  its 
  optical 
  nature 
  as 
  well, 
  will 
  be 
  revealed 
  more 
  

   easily 
  by 
  the 
  color 
  there 
  preceding 
  or 
  lagging 
  behind 
  that 
  of 
  

   the 
  field. 
  In 
  this 
  way 
  I 
  have 
  often 
  been 
  able 
  to 
  locate 
  and 
  

   study 
  tiny 
  crystals 
  in 
  the 
  solution 
  which 
  were 
  so 
  small 
  as 
  to 
  

   escape 
  detection 
  by 
  reflected 
  light. 
  

  

  Fig. 
  1 
  illustrates 
  the 
  arrangements 
  of 
  the 
  parts 
  of 
  the 
  

   apparatus 
  just 
  described 
  \ 
  p) 
  is 
  a 
  wooden 
  handle 
  through 
  which 
  

   passes 
  the 
  brass 
  rod 
  r 
  s, 
  to 
  which 
  are 
  fastened 
  the 
  universal 
  

   clamps 
  and 
  s, 
  which 
  hold 
  small, 
  hollow, 
  wooden 
  cylinders 
  

   turned 
  to 
  fit 
  the 
  mounted 
  Nicols; 
  m 
  and 
  ?i 
  are 
  respectively 
  

   the 
  dextro 
  and 
  Isevo 
  crystals 
  of 
  sodium 
  chlorate, 
  chosen 
  for 
  

   transparency 
  and 
  suflficient 
  thickness, 
  say 
  2 
  to 
  4™™. 
  They 
  are 
  

  

  