﻿612 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  

  

  The 
  whole 
  of 
  this 
  annulus, 
  which 
  forms 
  the 
  greater 
  part 
  of 
  the 
  disc, 
  is 
  composed 
  

   of 
  crystals 
  radiating 
  from 
  the 
  centre, 
  and 
  of 
  inequal 
  thickness 
  in 
  their 
  breadth, 
  so 
  

   that 
  we 
  have 
  the 
  luminous 
  sectors 
  not 
  of 
  one 
  colour, 
  as 
  in 
  the 
  Lithoxanthate 
  of 
  

   Ammonia, 
  but 
  of 
  various 
  tints 
  from 
  white 
  of 
  the 
  first 
  to 
  blue 
  of 
  the 
  second 
  order. 
  

   The 
  radiating 
  crystals 
  are 
  sometimes 
  sectors 
  of 
  10° 
  or 
  15°, 
  of 
  uniform 
  thickness, 
  

   and 
  giving 
  the 
  same 
  colour 
  ; 
  and 
  hence, 
  the 
  black 
  cross 
  is 
  composed 
  of 
  sectors 
  of 
  

   different 
  degrees 
  of 
  blackness 
  as 
  they 
  are 
  brought 
  into 
  the 
  plane 
  of 
  primitive 
  

   polarisation. 
  Beyond 
  this 
  annulus, 
  the 
  disc 
  terminates 
  in 
  a 
  rim, 
  like 
  that 
  of 
  a 
  

   carriage-wheel, 
  composed 
  of 
  two 
  or 
  more 
  concentric 
  circles, 
  between 
  which 
  the 
  

   crystals 
  are 
  disposed 
  in 
  radial 
  lines, 
  sometimes 
  not 
  in 
  optical 
  contact, 
  but 
  exhibit- 
  

   ing 
  the 
  same 
  colours 
  as 
  those 
  in 
  the 
  larger 
  annulus. 
  In 
  discs 
  of 
  a 
  considerable 
  

   size, 
  there 
  are 
  seen 
  exceedingly 
  minute 
  and 
  dark 
  circles, 
  about 
  ten 
  or 
  twelve 
  in 
  

   number, 
  which 
  I 
  have 
  found 
  to 
  be 
  cracks 
  or 
  lines 
  of 
  cleavage, 
  and 
  which 
  are 
  ac- 
  

   companied 
  with 
  short 
  lines 
  of 
  cleavage, 
  passing 
  radially 
  from 
  the 
  one 
  to 
  the 
  other. 
  

  

  In 
  these 
  discs, 
  there 
  is 
  another 
  peculiarity 
  which 
  deserves 
  to 
  be 
  noticed. 
  In 
  

   the 
  coloured 
  sectors, 
  there 
  are 
  often 
  circular 
  spots 
  and 
  rings, 
  in 
  which 
  the 
  tint 
  

   descends 
  to 
  zero, 
  as 
  if 
  a 
  drop 
  of 
  some 
  solvent 
  had 
  fallen 
  upon 
  the 
  crystal 
  : 
  and 
  

   there 
  are 
  spots 
  of 
  an 
  opposite 
  kind, 
  where 
  the 
  tint 
  rises 
  from 
  that 
  of 
  the 
  sector 
  

   to 
  higher 
  tints, 
  an 
  effect 
  probably 
  produced 
  by 
  a 
  particle 
  of 
  the 
  crystal 
  forming 
  

   around 
  itself, 
  while 
  dissolving, 
  a 
  thicker 
  film, 
  becoming 
  thinner 
  as 
  it 
  recedes 
  from 
  

   the 
  particle. 
  

  

  In 
  some 
  of 
  the 
  circular 
  discs 
  of 
  Salicine, 
  I 
  have 
  found 
  the 
  outer 
  rim 
  as 
  wide 
  as 
  

   the 
  interior 
  portion, 
  and 
  in 
  this 
  case 
  it 
  polarises 
  a 
  bluish-white 
  of 
  the 
  first 
  order 
  ; 
  

   but, 
  what 
  is 
  peculiarly 
  worthy 
  of 
  notice, 
  this 
  rim 
  is 
  subdivided 
  by 
  faint 
  concen- 
  

   tric 
  rings 
  of 
  different 
  degrees 
  of 
  darkness, 
  into, 
  sometimes, 
  twelve 
  or 
  fifteen 
  annuli 
  

   of 
  different 
  degrees 
  of 
  brightness. 
  This 
  seldom 
  takes 
  place 
  in 
  the 
  interior 
  portion 
  

   of 
  the 
  disc, 
  but 
  when 
  it 
  does 
  occur, 
  and 
  the 
  tints 
  are 
  brilliant, 
  the 
  subdivison 
  of 
  

   the 
  annulus 
  into 
  a 
  number 
  of 
  concentric 
  circles 
  of 
  different 
  colours 
  is 
  singular^ 
  

   beautiful. 
  

  

  3. 
  Asparagine. 
  — 
  The 
  circularly 
  polarising 
  discs 
  which 
  this 
  substance 
  displays, 
  

   resemble 
  very 
  much 
  those 
  of 
  Salicine. 
  They 
  are 
  more 
  varied 
  in 
  their 
  structure, 
  

   and 
  more 
  beautiful 
  in 
  their 
  tints. 
  The 
  rims 
  of 
  the 
  discs 
  are 
  more 
  highly 
  coloured, 
  

   and 
  more 
  uniform 
  in 
  their 
  texture 
  ; 
  and 
  the 
  concentric 
  tints, 
  whether 
  they 
  are 
  all 
  

   of 
  different 
  degrees 
  of 
  whiteness, 
  or 
  of 
  higher 
  orders 
  of 
  colours, 
  are 
  so 
  perfectly 
  

   regular, 
  and 
  so 
  sharply 
  defined, 
  that 
  the 
  observer 
  stands 
  before 
  them 
  in 
  mute 
  ad- 
  

   miration, 
  and 
  feels 
  himself 
  unable 
  either 
  to 
  describe 
  or 
  to 
  draw 
  them. 
  There 
  are 
  

   two 
  peculiarities, 
  however, 
  which 
  deserve 
  to 
  be 
  noticed 
  ; 
  the 
  one, 
  the 
  existence 
  of 
  

   discs 
  in 
  which 
  there 
  is 
  no 
  circularly 
  polarising 
  structure 
  ; 
  and 
  the 
  other, 
  of 
  discs 
  

   exactly 
  resembling, 
  in 
  the 
  succession 
  of 
  black 
  and 
  white 
  narrow 
  rings, 
  the 
  systems 
  

  

  