﻿134: 
  . 
  - 
  D. 
  A. 
  Kreider 
  — 
  Detection 
  and 
  

  

  tion 
  of 
  the 
  optical 
  nature 
  of 
  the 
  crystals 
  during 
  their 
  growth 
  

   or 
  at 
  tlie 
  instant 
  of 
  their 
  appearance, 
  without 
  disturbing 
  them 
  

   in 
  the 
  least 
  ; 
  thus 
  enabling 
  me 
  to 
  decide 
  a 
  point 
  which 
  had 
  

   previously 
  been 
  open 
  to 
  question. 
  It 
  is 
  also 
  more 
  delicate 
  

   than 
  any 
  polariscope 
  I 
  have 
  ever 
  used. 
  

  

  The 
  instruments 
  employed 
  for 
  the 
  determinations 
  recorded 
  

   in 
  the 
  article 
  referred 
  to 
  were 
  of 
  the 
  type 
  in 
  which 
  a 
  bi 
  quartz 
  

   or 
  half-shadow 
  field 
  is 
  used, 
  as 
  in 
  the 
  well 
  known 
  Scheibler 
  

   and 
  Laurent 
  polarimeters 
  ; 
  various 
  efforts 
  to 
  fit 
  up 
  something 
  

   simpler 
  having 
  proved 
  futile. 
  

  

  It 
  is 
  rather 
  surprising 
  to 
  find 
  others 
  still 
  emplo.ying 
  the 
  

   polarization-microscope 
  for 
  this 
  purpose 
  when 
  the 
  polarimeter 
  

   is 
  so 
  much 
  more 
  satisfactory. 
  After 
  being 
  adjusted 
  to 
  the 
  tint 
  

   of 
  passage, 
  a 
  very 
  small 
  crystal 
  simply 
  held 
  in 
  the 
  field 
  of 
  view 
  

   by 
  a 
  pair 
  of 
  forceps 
  or, 
  if 
  desired, 
  moved 
  from 
  one 
  to 
  the 
  other 
  

   half 
  of 
  the 
  illuminated 
  disc, 
  instantly 
  and 
  without 
  further 
  ad- 
  

   justment, 
  reveals 
  the 
  sense 
  of 
  its 
  rotation. 
  But 
  the 
  objection 
  

   to 
  its 
  use, 
  as 
  well 
  as 
  to 
  that 
  of 
  the 
  polarization-microscope, 
  is 
  

   that 
  it 
  necessitates 
  the 
  handling 
  of 
  each 
  individual 
  crystal 
  of 
  

   every 
  crop, 
  which 
  when 
  many 
  are 
  involved 
  demands 
  a 
  great 
  

   deal 
  of 
  time 
  and 
  labor. 
  

  

  The 
  characteristic 
  feature 
  of 
  the 
  device 
  which 
  I 
  proceed 
  to 
  

   describe, 
  is 
  a 
  larger 
  field 
  of 
  view 
  than 
  that 
  of 
  the 
  other 
  instru- 
  

   ments, 
  and 
  one 
  of 
  uniform 
  color 
  of 
  such 
  a 
  tint 
  that 
  when 
  a 
  

   mixture 
  of 
  the 
  crystals 
  is 
  placed 
  in 
  it 
  they 
  assume 
  different 
  

   colors 
  sufficiently 
  striking 
  to 
  permit 
  of 
  their 
  being 
  easily 
  

   distinguished 
  and 
  separated. 
  It 
  consists 
  of 
  two 
  Nicol's 
  

   prisms 
  and 
  some 
  optically 
  active 
  substance, 
  which 
  latter 
  

   may 
  as 
  well 
  be 
  a 
  crystal 
  of 
  the 
  kind 
  to 
  be 
  experimented 
  

   upon. 
  I 
  have 
  also 
  used 
  quartz 
  sections, 
  but 
  greater 
  sen- 
  

   sitiveness 
  seems 
  to 
  result 
  from 
  the 
  employment 
  of 
  the 
  sodium 
  

   chlorate 
  crystal 
  if 
  it 
  is 
  sufficiently 
  transparent. 
  This 
  will 
  

   be 
  appreciated 
  as 
  a 
  perfectly 
  satisfactory 
  and 
  economical 
  sub- 
  

   stitute 
  for 
  the 
  more 
  difficultly 
  attainable 
  bi-quartz 
  or 
  simple 
  

   quartz 
  section, 
  where 
  the 
  latter 
  are 
  not 
  at 
  hand. 
  The 
  crystal 
  

   is 
  mounted 
  over 
  the 
  polarizer 
  and 
  observed 
  directly 
  through 
  

   the 
  analyzer. 
  If 
  the 
  Nicols 
  are 
  crossed 
  and 
  the 
  crystal 
  then 
  

   inserted 
  between 
  them, 
  light 
  will 
  of 
  course 
  be 
  transmitted, 
  and 
  

   if 
  white 
  light 
  be 
  employed, 
  by 
  rotating 
  the 
  analyzer, 
  as 
  is 
  well 
  

   known, 
  the 
  various 
  colors 
  complementary 
  to 
  that 
  extinguished 
  

   will 
  appear 
  in 
  the 
  order 
  of 
  the 
  short 
  to 
  the 
  long 
  wave-lengths, 
  

   if 
  turned 
  to 
  the 
  right, 
  when 
  a 
  dextro-rotating 
  crystal 
  is 
  

   employed 
  and 
  in 
  the 
  reverse 
  order, 
  if 
  turned 
  to 
  the 
  left; 
  so 
  

   that 
  by 
  suitable 
  adjustment 
  the 
  desired 
  color 
  can 
  easily 
  be 
  pro- 
  

   duced, 
  as 
  is 
  explained 
  below. 
  If 
  the 
  crystal 
  is 
  thick 
  enough, 
  

   say 
  one 
  to 
  two 
  millimeters, 
  this 
  will 
  suffice 
  to 
  reveal 
  its 
  nature; 
  

   for 
  if 
  dextro, 
  the 
  blue 
  color 
  will 
  appear 
  shortly 
  on 
  turning 
  to 
  

   the 
  right 
  and 
  if 
  Isevo, 
  blue 
  will 
  appear 
  by 
  turning 
  to 
  the 
  left. 
  

  

  