﻿548 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  THE 
  OPTICAL 
  PHENOMENA 
  OF 
  

  

  The 
  other 
  crystals 
  of 
  tourmaline 
  which 
  I 
  have 
  discovered 
  in 
  mica 
  have 
  a 
  very 
  

   different 
  character 
  : 
  They 
  have 
  been 
  formed 
  subsequently 
  to 
  the 
  crystallisation 
  

   of 
  the 
  mica, 
  and 
  exist 
  only 
  between 
  its 
  laminae. 
  I 
  have 
  not 
  been 
  able 
  to 
  discover 
  

   any 
  cavities 
  in 
  mica 
  containing 
  fluids 
  or 
  gases, 
  but 
  I 
  have 
  found 
  thousands 
  from 
  

   which 
  the 
  fluids 
  and 
  gases 
  have 
  escaped, 
  — 
  the 
  one 
  crystallising 
  into 
  hexagonal 
  

   plates 
  of 
  tourmaline, 
  and 
  the 
  other 
  separating 
  the 
  laminae, 
  or 
  running 
  between 
  

   them, 
  and 
  carrying 
  along 
  with 
  it 
  minute 
  portions 
  of 
  crystallisable 
  matter. 
  

  

  The 
  hexagonal 
  crystals 
  thus 
  formed 
  have 
  their 
  faces 
  perpendicular 
  to 
  the 
  axis 
  

   of 
  double 
  refraction, 
  which 
  is 
  the 
  axis 
  of 
  the 
  prism 
  ; 
  and 
  what 
  is 
  peculiarly 
  in- 
  

   teresting, 
  the 
  fluid 
  from 
  which 
  they 
  were 
  formed 
  has 
  insinuated 
  itself 
  between 
  

   several 
  of 
  the 
  laminae, 
  and 
  the 
  different 
  plates 
  of 
  tourmaline 
  which 
  they 
  formed 
  

   have, 
  of 
  course, 
  the 
  sides 
  of 
  the 
  hexagon 
  incoincident. 
  Sometimes 
  these 
  crystals 
  

   extend 
  to 
  different 
  distances 
  from 
  the 
  centre 
  of 
  the 
  original 
  cavity, 
  and 
  are 
  occa- 
  

   sionally 
  formed 
  round 
  it 
  in 
  a 
  circular 
  group. 
  See 
  Plate 
  XV., 
  Fig. 
  1. 
  

  

  The 
  centre 
  of 
  the 
  cavity 
  from 
  which 
  these 
  crystals 
  have 
  been 
  projected 
  is 
  oc- 
  

   cupied 
  by 
  a 
  spherical 
  group 
  of 
  granular 
  or 
  capillary 
  crystals, 
  which 
  is 
  generally 
  

   very 
  opaque, 
  though 
  such 
  groups 
  sometimes 
  exhibit, 
  in 
  particular 
  spots, 
  double 
  

   refraction, 
  and 
  a 
  speck 
  of 
  light 
  is 
  occasionally 
  seen 
  through 
  the 
  centre 
  of 
  the 
  

   group. 
  In 
  some 
  cases 
  I 
  have 
  observed 
  these 
  very 
  thin 
  hexagonal 
  plates 
  without 
  

   this 
  opaque 
  centre 
  ; 
  and 
  they 
  have 
  probably 
  been 
  formed 
  by 
  a 
  portion 
  of 
  the 
  fluid 
  

   projected 
  to 
  a 
  distance 
  between 
  faces 
  of 
  easy 
  cleavage. 
  The 
  black 
  spherical 
  

   group 
  already 
  mentioned 
  has 
  its 
  outward 
  surface 
  bristled 
  with 
  points, 
  which 
  are 
  

   the 
  extremities 
  of 
  the 
  crystals 
  radiating 
  from 
  its 
  centre 
  ; 
  and 
  in 
  one 
  fine 
  specimen 
  

   to 
  be 
  farther 
  described, 
  it 
  is 
  surrounded 
  with 
  a 
  ring 
  of 
  less 
  opacity 
  than 
  the 
  nu- 
  

   cleus, 
  and 
  analogous 
  to 
  what 
  is 
  common 
  in 
  circular 
  crystals. 
  See 
  Fig. 
  1. 
  

  

  The 
  thin 
  plates 
  thus 
  formed 
  between 
  the 
  laminae, 
  whether 
  hexagonal 
  or 
  pris- 
  

   matic, 
  are 
  always 
  of 
  a 
  faint 
  brownish 
  yellow, 
  which 
  at 
  an 
  increased 
  thickness 
  be- 
  

   comes 
  green 
  ; 
  and 
  so 
  exceedingly 
  thin 
  are 
  these 
  plates, 
  especially 
  those 
  farthest 
  

   from 
  the 
  nucleus, 
  that 
  with 
  a 
  power 
  of 
  400, 
  it 
  is 
  often 
  very 
  difficult 
  to 
  see 
  their 
  

   terminal 
  lines. 
  

  

  In 
  order 
  to 
  convey 
  an 
  idea 
  of 
  these 
  phenomena, 
  I 
  have 
  given 
  a 
  drawing 
  in 
  

   Fig. 
  1 
  of 
  a 
  very 
  interesting 
  one, 
  where 
  the 
  prismatic 
  crystal 
  nearest 
  the 
  black 
  

   central 
  group 
  is 
  a 
  bright 
  green 
  in 
  all 
  azimuths 
  with 
  polarised 
  light, 
  surrounded 
  

   with 
  three 
  or 
  four 
  larger 
  prismatic 
  yellowish 
  plates, 
  growing 
  fainter 
  both 
  in 
  tint 
  

   and 
  outline 
  as 
  they 
  recede. 
  In 
  some 
  cases 
  the 
  crystals 
  are 
  brown, 
  and 
  in 
  others 
  

   beautifully 
  dichroitic, 
  being 
  bright 
  green 
  and 
  pink 
  in 
  the 
  different 
  azimuths 
  of 
  

   polarised 
  light. 
  

  

  As 
  considerable 
  forces 
  must 
  have 
  been 
  in 
  operation 
  during 
  the 
  production 
  of 
  

   these 
  phenomena, 
  we 
  may 
  expect 
  to 
  see 
  the 
  effects 
  of 
  them 
  upon 
  the 
  surrounding 
  

   mica. 
  We 
  accordingly 
  observe 
  the 
  polarisation 
  produced 
  by 
  pressure 
  round 
  

   almost 
  all 
  of 
  these 
  crystalline 
  groups. 
  Rents 
  and 
  other 
  marks 
  of 
  violence 
  are 
  dis- 
  

  

  