﻿010 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  

  

  orders 
  of 
  tints, 
  by 
  compensating 
  them 
  with 
  the 
  corresponding 
  tints 
  of 
  plates 
  

   of 
  selenite. 
  

  

  In 
  other 
  discs 
  the 
  rings 
  2 
  and 
  3 
  have 
  each 
  the 
  same 
  colour 
  throughout, 
  — 
  the 
  

   one 
  generally 
  red 
  and 
  the 
  other 
  green, 
  and 
  having 
  no 
  relation, 
  either 
  to 
  the 
  central 
  

   tint 
  at 
  1, 
  or 
  to 
  one 
  another. 
  In 
  some 
  cases 
  the 
  order 
  of 
  colours 
  is 
  completely 
  

   inverted, 
  as 
  in 
  Fig. 
  4, 
  where 
  the 
  central 
  tints 
  are 
  a 
  blue 
  of 
  the 
  second 
  order, 
  gra- 
  

   dually 
  passing 
  through 
  red 
  and 
  yellow 
  to 
  a 
  brilliant 
  white 
  of 
  the 
  first 
  order. 
  In 
  

   other 
  crystals 
  I 
  have 
  found 
  the 
  central 
  tints 
  red, 
  green, 
  and 
  yellow, 
  of 
  high 
  orders 
  ; 
  

   but 
  in 
  these 
  cases 
  the 
  discs 
  are 
  not 
  regularly 
  formed, 
  and 
  the 
  elementary 
  crystals 
  

   not 
  wholly 
  in 
  optical 
  contact. 
  

  

  The 
  most 
  perfect 
  circular 
  crystals 
  are 
  those 
  in 
  which 
  the 
  central 
  tints 
  are 
  the 
  

   blue 
  and 
  white 
  of 
  the 
  first 
  order. 
  This 
  arises 
  from 
  the 
  extreme 
  minuteness 
  of 
  the 
  

   crystals, 
  which 
  thus 
  form 
  a 
  more 
  uniform 
  disc, 
  and 
  cause 
  the 
  black 
  cross 
  to 
  have 
  

   a 
  degree 
  of 
  sharpness, 
  which 
  it 
  requires 
  a 
  considerable 
  magnifying 
  power 
  to 
  ex- 
  

   hibit. 
  In 
  such 
  crystals, 
  the 
  central 
  portion 
  is 
  surrounded 
  with 
  a 
  black 
  and 
  narrow 
  

   ring, 
  beyond 
  which 
  there 
  is 
  another 
  annulus 
  of 
  sectors, 
  sometimes 
  white 
  like 
  the 
  

   inner 
  ones. 
  This 
  is 
  again 
  terminated 
  by 
  a 
  black 
  circle, 
  beyond 
  which 
  is 
  a 
  third 
  

   series 
  of 
  sectors, 
  sometimes 
  white 
  and 
  sometimes 
  a 
  blue 
  of 
  the 
  first 
  order. 
  This 
  

   structure 
  is 
  shewn 
  in 
  Fig. 
  5, 
  where 
  the 
  black 
  cross 
  starts 
  into 
  different 
  breadths, 
  

   in 
  passing 
  from 
  one 
  set 
  of 
  sectors 
  to 
  the 
  others, 
  — 
  an 
  effect 
  which 
  is 
  produced 
  by 
  an 
  

   inferior 
  degree 
  of 
  optical 
  contact 
  in 
  the 
  elementary 
  crystals 
  of 
  the 
  outer 
  sectors. 
  

  

  An 
  interesting 
  structure 
  is 
  shewn 
  in 
  Fig. 
  6, 
  where 
  all 
  the 
  tints 
  are 
  white, 
  the 
  

   central 
  ones 
  terminating 
  in 
  a 
  dark 
  circle, 
  beyond 
  which 
  are 
  four 
  large 
  sectors, 
  

   whose 
  tint 
  is 
  the 
  bluish 
  white 
  of 
  the 
  first 
  order, 
  lower 
  than 
  the 
  central 
  tint. 
  Each 
  

   of 
  these 
  sectors, 
  however, 
  is 
  divided 
  into 
  four 
  portions 
  by 
  very 
  faint 
  circular 
  lines, 
  

   which 
  scarcely 
  depolarise 
  the 
  incident 
  light, 
  the 
  tint 
  being 
  there 
  a 
  minimum, 
  and 
  

   increasing 
  to 
  the 
  middle 
  point 
  between 
  them. 
  

  

  In 
  Figs. 
  7 
  and 
  8, 
  we 
  have 
  represented 
  structures 
  consisting 
  of 
  crystals, 
  shoot- 
  

   ing 
  out, 
  as 
  it 
  were, 
  from 
  the 
  centre, 
  and 
  all 
  of 
  a 
  golden-yellow 
  colour. 
  In 
  Fig. 
  7, 
  

   the 
  black 
  cross 
  is 
  seen, 
  but 
  in 
  Fig. 
  8 
  there 
  is 
  such 
  imperfection 
  of 
  contact 
  be- 
  

   tween 
  all 
  the 
  radial 
  crystals, 
  that 
  the 
  darkness 
  of 
  those 
  under 
  the 
  black 
  cross 
  

   is 
  scarcely 
  visible. 
  

  

  In 
  discs 
  like 
  Fig. 
  7, 
  a 
  very 
  singular 
  effect 
  is 
  sometimes 
  produced, 
  as 
  shewn 
  in 
  

   Fig. 
  9, 
  where 
  the 
  black 
  cross 
  is 
  so 
  divergent 
  and 
  wide, 
  that 
  the 
  golden-coloured 
  

   crystals 
  half-way 
  between 
  its 
  branches, 
  have 
  the 
  appearance 
  of 
  a 
  yellow 
  rectan- 
  

   gular 
  cross. 
  

  

  Under 
  favourable 
  circumstances, 
  the 
  discs 
  assume 
  a 
  very 
  interesting 
  and 
  com- 
  

   plex 
  appearance, 
  as 
  shewn 
  in 
  Fig. 
  10. 
  Beyond 
  the 
  central 
  golden-yellow 
  radia- 
  

   tions 
  is 
  a 
  broad 
  annulus 
  of 
  pale 
  blue 
  of 
  the 
  first 
  order, 
  divided 
  by 
  a 
  faint, 
  dark, 
  and 
  

   narrow 
  band, 
  scarcely 
  luminous. 
  This 
  annulus 
  is 
  surrounded 
  by 
  a 
  sharp 
  and 
  

   broad 
  line, 
  perfectly 
  black, 
  which 
  is 
  succeeded 
  by 
  a 
  similar 
  line 
  separated 
  from 
  

  

  