﻿SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  621 
  

  

  It 
  is 
  equally 
  difficult 
  to 
  assign 
  any 
  reason 
  for 
  the 
  production 
  of 
  the 
  concentric 
  

   bands 
  of 
  a 
  uniform 
  tint, 
  which 
  suddenly 
  pass 
  to 
  another 
  tint 
  belonging 
  to 
  a 
  

   different 
  order 
  of 
  colours, 
  and 
  produced 
  by 
  a 
  different 
  thickness 
  of 
  material. 
  A 
  

   circular 
  ring 
  of 
  green, 
  for 
  example, 
  will 
  pass 
  per 
  saltum, 
  to 
  a 
  red 
  of 
  the 
  next 
  order, 
  

   from 
  a 
  thickness 
  of 
  9 
  to 
  a 
  thickness 
  of 
  18 
  ; 
  and 
  this, 
  according 
  to 
  a 
  law 
  which 
  

   operates 
  at 
  every 
  point 
  of 
  the 
  circumference 
  of 
  the 
  ring. 
  Nor 
  is 
  this 
  phenomenon 
  

   less 
  remarkable 
  when 
  the 
  transition 
  takes 
  place 
  in 
  the 
  very 
  lowest 
  order 
  of 
  tints, 
  

   and 
  at 
  the 
  smallest 
  thickness 
  of 
  the 
  film, 
  as 
  shewn 
  in 
  Figs. 
  6, 
  10, 
  and 
  13, 
  where 
  

   the 
  tint 
  passes 
  in 
  repeated 
  alternations 
  from 
  the 
  pale 
  blue 
  to 
  the 
  beginning 
  of 
  

   black, 
  rising 
  to 
  a 
  maximum 
  of 
  blue, 
  and 
  again 
  descending 
  to 
  the 
  minimum 
  of 
  

   black. 
  

  

  The 
  black 
  rings 
  or 
  circles 
  shewn 
  in 
  Figs. 
  5, 
  6, 
  10, 
  and 
  13, 
  require 
  to 
  be 
  care- 
  

   fully 
  studied, 
  and 
  with 
  the 
  finest 
  microscopes. 
  In 
  most 
  cases 
  they 
  seem 
  to 
  be 
  

   spaces 
  devoid 
  of 
  crystalline 
  matter 
  ; 
  but 
  they 
  have 
  in 
  general 
  another 
  origin. 
  A 
  

   line 
  often 
  appears 
  perfectly 
  black, 
  when 
  it 
  corresponds 
  with 
  the 
  violet 
  of 
  the 
  

   second 
  order, 
  which 
  separates 
  the 
  indigo 
  of 
  the 
  same 
  order 
  from 
  the 
  red 
  of 
  the 
  

   first 
  order. 
  Another 
  set 
  of 
  lines 
  appear 
  black, 
  from 
  their 
  being 
  the 
  junction 
  lines 
  

   of 
  crystals 
  not 
  in 
  perfect 
  optical 
  contact. 
  A 
  third 
  set 
  of 
  black 
  circles 
  are 
  pro- 
  

   duced 
  by 
  the 
  extreme 
  thinness 
  of 
  the 
  substance, 
  which 
  is 
  not 
  capable 
  of 
  polarising 
  

   the 
  very 
  black 
  of 
  Newton's 
  scale, 
  and 
  the 
  existence 
  of 
  which 
  upon 
  the 
  glass 
  plate 
  

   can 
  be 
  ascertained 
  only 
  by 
  the 
  highest 
  powers 
  of 
  a 
  fine 
  microscope. 
  But 
  though 
  in 
  

   all 
  these 
  examples 
  there 
  is 
  no 
  breach 
  of 
  continuity 
  in 
  the 
  circular 
  disc, 
  yet 
  there 
  

   are 
  cases, 
  as 
  in 
  the 
  double 
  black 
  ring 
  in 
  Fig. 
  10, 
  where 
  the 
  corresponding 
  space 
  is 
  

   devoid 
  of 
  all 
  crystalline 
  matter. 
  The 
  crystallisation 
  of 
  the 
  disc 
  had 
  been 
  completed 
  

   at 
  the 
  inner 
  margin 
  of 
  the 
  first 
  black 
  ring, 
  and 
  by 
  some 
  repulsive 
  power 
  the 
  

   molecules 
  in 
  the 
  solution 
  were 
  kept 
  at 
  a 
  distance 
  from 
  the 
  completed 
  disc, 
  and 
  de- 
  

   posited 
  themselves 
  in 
  a 
  scarcely 
  visible 
  ring 
  around 
  the 
  outer 
  margin 
  of 
  the 
  first 
  

   black 
  ring. 
  The 
  repulsive 
  power 
  again 
  came 
  into 
  play 
  ; 
  and 
  another 
  black 
  ring 
  

   intervened, 
  the 
  molecules 
  being 
  deposited 
  at 
  the 
  same 
  distance 
  as 
  formerly 
  from 
  

   the 
  last-formed 
  ring. 
  What 
  repulsive 
  power 
  this 
  is, 
  if 
  it 
  is 
  not 
  electrical, 
  and 
  

   how 
  it 
  operates, 
  if 
  it 
  is 
  electrical, 
  we 
  cannot 
  even 
  conjecture. 
  

  

  Another 
  remarkable 
  peculiarity 
  in 
  circular 
  crystals 
  is 
  shewn 
  in 
  Figs. 
  11 
  and 
  

   12, 
  where, 
  as 
  in 
  chromic 
  acid, 
  the 
  disc 
  consists 
  of 
  alternations 
  of 
  dark 
  and 
  lumi- 
  

   nous 
  circles, 
  equidistant 
  from 
  each 
  other. 
  The 
  dark 
  circles 
  are 
  composed 
  of 
  the 
  

   acid 
  in 
  particles 
  too 
  small 
  to 
  polarise 
  light, 
  and 
  the 
  luminous 
  ones 
  of 
  separate 
  

   patches 
  of 
  crystalline 
  matter 
  thick 
  enough 
  to 
  give 
  the 
  blue 
  and 
  sometimes 
  the 
  

   white 
  of 
  the 
  first 
  order, 
  and 
  separated 
  from 
  one 
  another 
  by 
  matter 
  too 
  thin 
  to 
  

   polarise 
  light. 
  In 
  some 
  rare 
  cases, 
  the 
  spaces 
  between 
  the 
  circles 
  and 
  between 
  

   the 
  patches 
  are, 
  like 
  the 
  black 
  rings 
  formerly 
  described, 
  devoid 
  of 
  crystalline 
  

   matter. 
  The 
  separation 
  of 
  the 
  patches 
  in 
  this 
  case, 
  is 
  no 
  less 
  remarkable 
  than 
  

   the 
  separation 
  of 
  the 
  luminous 
  circles. 
  In 
  the 
  Adipocire 
  from 
  Paris, 
  the 
  tint 
  of 
  

  

  