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  XLIIL— 
  On 
  Circular 
  Crystals. 
  By 
  Sir 
  David 
  Brewster, 
  K.H., 
  D.C.L., 
  F.R.S., 
  

   V.P.R.S. 
  Edin., 
  and 
  Associate 
  of 
  the 
  Institute 
  of 
  France. 
  (With 
  Two 
  Plates.) 
  

  

  (Read 
  21st 
  March 
  1853.) 
  

  

  In 
  1836, 
  Mr 
  Fox 
  Talbot 
  communicated 
  to 
  the 
  Royal 
  Society 
  apaper 
  "On 
  the 
  

   Optical 
  Phenomena 
  of 
  certain 
  Crystals" 
  which 
  he 
  obtained 
  by 
  dissolving 
  a 
  crystal 
  

   of 
  Borax 
  in 
  a 
  drop 
  of 
  somewhat 
  diluted 
  Phosphoric 
  acid. 
  When 
  the 
  acid 
  and 
  the 
  

   salt 
  are 
  in 
  proper 
  proportions, 
  " 
  the 
  field 
  of 
  view 
  of 
  the 
  microscope 
  is 
  seen 
  co- 
  

   vered 
  with 
  minute 
  circular 
  spots, 
  each 
  of 
  which 
  is 
  like 
  a 
  tuft 
  of 
  silk 
  radiating 
  

   from 
  a 
  centre, 
  and 
  is 
  composed 
  of 
  a 
  close 
  assemblage 
  of 
  delicate 
  acicular 
  crystals 
  

   forming 
  a 
  star." 
  Among 
  these 
  crystals 
  are 
  seen 
  interspersed 
  " 
  a 
  number 
  of 
  cir- 
  

   cular 
  transparent 
  bodies, 
  which 
  are 
  tufts 
  or 
  stars 
  of 
  acicular 
  crystals, 
  in 
  such 
  close 
  

   assemblage 
  as 
  to 
  be 
  in 
  optical 
  contact 
  with 
  each 
  other, 
  and 
  to 
  produce 
  the 
  ap- 
  

   pearance 
  of 
  a 
  single 
  individual." 
  

  

  When 
  the 
  field 
  of 
  the 
  polarising 
  microscope 
  is 
  dark, 
  " 
  the 
  little 
  circles 
  become 
  

   luminous, 
  and 
  we 
  see 
  upon 
  each 
  of 
  them 
  a 
  well-defined 
  and 
  dark 
  cross, 
  dividing 
  

   the 
  crystal 
  into 
  four 
  equal 
  parts." 
  With 
  a 
  high 
  power, 
  Mr 
  Talbot 
  observed 
  upon 
  

   each 
  circle 
  one 
  or 
  more 
  coloured 
  rings 
  arranged 
  concentrically 
  ; 
  the 
  innermost, 
  

   which 
  was 
  deeply 
  coloured 
  or 
  black, 
  enclosing 
  a 
  central 
  space 
  of 
  white 
  light, 
  tra- 
  

   versed 
  by 
  the 
  black 
  cross 
  already 
  mentioned. 
  " 
  The 
  general 
  appearance," 
  he 
  adds, 
  

   " 
  resembles 
  the 
  Fig. 
  98* 
  in 
  Brewster's 
  Treatise 
  on 
  Optics, 
  which 
  is 
  a 
  represen- 
  

   tation 
  of 
  the 
  rings 
  in 
  uniaxal 
  crystals." 
  

  

  About 
  twenty 
  years 
  f 
  before 
  the 
  publication 
  of 
  Mr 
  Talbot's 
  paper, 
  I 
  had 
  pro- 
  

   duced 
  minute 
  circular 
  crystals 
  in 
  Oil 
  of 
  Mace, 
  — 
  in 
  a 
  mixture 
  of 
  Oil 
  of 
  Mace 
  and 
  

   Rosin, 
  and 
  also 
  in 
  Tallow, 
  when 
  these 
  substances 
  were 
  melted 
  between 
  two 
  plates 
  

   of 
  glass, 
  and 
  cooled 
  under 
  pressure. 
  The 
  crystals 
  thus 
  produced 
  were 
  not 
  them- 
  

   selves 
  visible 
  in 
  the 
  microscope, 
  but, 
  in 
  polarised 
  light, 
  they 
  exhibited 
  their 
  exist- 
  

   ence 
  and 
  their 
  structure, 
  in 
  the 
  formation 
  of 
  four 
  luminous 
  sectors 
  traversed 
  by 
  a 
  

   black 
  cross. 
  

  

  When 
  we 
  look 
  through 
  a 
  plate 
  of 
  Oil 
  of 
  Mace 
  properly 
  prepared 
  at 
  a 
  bright 
  

   and 
  small 
  disc 
  of 
  light, 
  the 
  light 
  is 
  generally 
  surrounded 
  with 
  a 
  single 
  halo, 
  

   though 
  sometimes 
  with 
  two. 
  When 
  the 
  disc 
  of 
  light 
  is 
  polarised 
  and 
  subse- 
  

   quently 
  analysed, 
  the 
  halo 
  is 
  divided 
  into 
  four 
  luminous 
  sectors, 
  separated 
  by 
  a 
  

   rectangular 
  black 
  cross, 
  which, 
  from 
  the 
  divergency 
  of 
  its 
  branches, 
  had 
  the 
  ap- 
  

   pearance 
  of 
  four 
  dark 
  sectors. 
  The 
  arms 
  of 
  the 
  black 
  cross 
  were 
  always 
  parallel 
  

  

  * 
  Fig. 
  120 
  in 
  the 
  New 
  Edition, 
  just 
  published. 
  

   f 
  Phil. 
  Trans., 
  1815, 
  p. 
  49, 
  38 
  ; 
  and 
  1816, 
  p. 
  97. 
  

  

  VOL. 
  XX. 
  PART 
  IV. 
  8 
  B 
  

  

  