﻿608 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  

  

  and 
  perpendicular 
  to 
  the 
  plane 
  of 
  primitive 
  polarisation, 
  and 
  the 
  disc 
  of 
  bright 
  

   light 
  disappeared 
  at 
  their 
  point 
  of 
  intersection. 
  In 
  the 
  opposite 
  position 
  of 
  the 
  

   analyser, 
  the 
  luminous 
  disc 
  appeared 
  at 
  the 
  point 
  of 
  intersection, 
  and 
  the 
  two 
  

   luminous 
  sectors 
  that 
  were 
  horizontal 
  were 
  brighter 
  than 
  the 
  two 
  vertical 
  ones. 
  

   These 
  phenomena 
  are 
  shewn 
  in 
  Figs. 
  1 
  and 
  2, 
  Plate 
  XVI. 
  

  

  In 
  some 
  cases, 
  the 
  circular 
  space 
  enclosing 
  the 
  sectors 
  was 
  very 
  small, 
  and 
  in 
  

   others 
  large, 
  and 
  frequently 
  when 
  two 
  halos 
  were 
  produced, 
  there 
  were 
  two 
  sets 
  

   of 
  luminous 
  sectors, 
  separated 
  by 
  an 
  interval 
  equal 
  to 
  that 
  between 
  the 
  halos. 
  

  

  It 
  is 
  very 
  obvious 
  that 
  the 
  halos 
  were 
  produced 
  by 
  the 
  crystals 
  of 
  the 
  Oil 
  of 
  

   Mace, 
  the 
  smaller 
  halo 
  by 
  the 
  larger 
  crystals, 
  and 
  the 
  larger 
  halo 
  by 
  the 
  smaller 
  

   crystals 
  existing 
  among 
  the 
  larger 
  ones. 
  In 
  order 
  to 
  explain 
  the 
  luminous 
  sec- 
  

   tors, 
  I 
  inferred 
  that 
  each 
  halo 
  was 
  composed 
  of 
  two, 
  the 
  one 
  lying 
  above 
  the 
  other, 
  

   and 
  having 
  every 
  alternate 
  sector 
  polarised 
  in 
  opposite 
  planes 
  ; 
  or, 
  in 
  other 
  words, 
  

   that 
  these 
  two 
  halos 
  were 
  the 
  two 
  images 
  formed 
  by 
  the 
  double 
  refraction 
  of 
  the 
  

   elementary 
  crystals, 
  and 
  were 
  oppositely 
  polarised, 
  as 
  all 
  such 
  images 
  are. 
  But 
  

   though 
  this 
  inference 
  was 
  correct, 
  as 
  I 
  afterwards 
  proved, 
  yet 
  I 
  could 
  not 
  see 
  with 
  

   the 
  microscope 
  the 
  actual 
  form 
  of 
  the 
  circular 
  crystals 
  by 
  which 
  the 
  double 
  re- 
  

   fraction 
  and 
  polarisation 
  were 
  produced. 
  

  

  After 
  the 
  publication 
  of 
  Mr 
  Talbot's 
  paper, 
  I 
  repeated 
  the 
  experiment 
  with 
  

   oil 
  of 
  mace, 
  and 
  having 
  adopted 
  different 
  methods 
  of 
  cooling 
  it 
  under 
  pressure, 
  I 
  

   soon 
  discovered 
  with 
  the 
  microscope, 
  and 
  by 
  the 
  aid 
  of 
  polarising 
  films 
  when 
  the 
  

   microscope 
  could 
  not 
  alone 
  detect 
  the 
  structure, 
  that 
  the 
  phenomena 
  which 
  I 
  have 
  

   described 
  were 
  produced 
  by 
  circular 
  crystals 
  varying 
  from 
  invisibility 
  to 
  the 
  200th 
  

   or 
  300th 
  of 
  an 
  inch 
  in 
  diameter, 
  and 
  exhibiting, 
  when 
  of 
  this 
  size, 
  distinct 
  and 
  

   beautiful 
  sectors 
  in 
  polarised 
  light. 
  

  

  Having 
  thus 
  discovered 
  a 
  method* 
  of 
  distinguishing 
  true 
  quaquaversus 
  po- 
  

   larisation, 
  or 
  that 
  which 
  is 
  produced 
  by 
  invisible 
  crystalline 
  particles 
  with 
  their 
  

   axes 
  lying 
  in 
  all 
  directions, 
  from 
  that 
  apparent 
  quaquaversus 
  polarisation 
  which 
  

   is 
  produced 
  by 
  the 
  same 
  class 
  of 
  particles 
  combined 
  in 
  circular 
  crystals, 
  I 
  was 
  

   anxious 
  to 
  prosecute 
  the 
  subject 
  of 
  circular 
  crystallisation, 
  by 
  examining 
  a 
  great 
  

   number 
  of 
  doubly-refracting 
  substances. 
  

  

  With 
  this 
  view 
  I 
  received 
  from 
  Mr 
  Talbot 
  the 
  preparation 
  of 
  Borax 
  and 
  Phos- 
  

   phoric 
  acid 
  which 
  he 
  had 
  found 
  to 
  give 
  the 
  best 
  circular 
  crystals, 
  and 
  from 
  Dr 
  

   Dowler 
  of 
  Richmond 
  a 
  quantity 
  of 
  the 
  Lithoxanthate 
  of 
  Ammonia, 
  which 
  yields 
  

   circular 
  crystals 
  with 
  more 
  certainty 
  and 
  less 
  trouble 
  than 
  the 
  preparation 
  of 
  

  

  * 
  This 
  method 
  consists 
  in 
  placing 
  a 
  film 
  of 
  selenite 
  (sulphate 
  of 
  lime) 
  between 
  the 
  polariser 
  

   and 
  the 
  substance 
  to 
  be 
  examined. 
  If 
  the 
  polarising 
  structure 
  is 
  produced 
  by 
  circular 
  crystals, 
  it 
  

   will 
  appear 
  covered 
  with 
  spots, 
  or 
  minute 
  sectors, 
  of 
  two 
  different 
  colours, 
  the 
  one 
  being 
  a 
  tint 
  a 
  

   little 
  lower, 
  and 
  the 
  other 
  a 
  tint 
  a 
  little 
  higher, 
  than 
  that 
  of 
  the 
  selenite. 
  The 
  higher 
  tint 
  is 
  the 
  sum 
  

   of 
  the 
  tints 
  of 
  the 
  two 
  substances, 
  and 
  the 
  lower 
  their 
  difference, 
  the 
  tint 
  of 
  the 
  selenite 
  being 
  in- 
  

   creased 
  by 
  that 
  of 
  two 
  of 
  the 
  sectors, 
  and 
  diminished 
  by 
  that 
  of 
  the 
  other 
  two. 
  

  

  