﻿SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  611 
  

  

  the 
  other 
  by 
  a 
  faint 
  line 
  of 
  light, 
  which, 
  in 
  some 
  crystals, 
  reaches 
  the 
  yellow 
  of 
  the 
  

   first 
  order. 
  Beyond 
  this 
  is 
  another 
  annulus 
  of 
  pale 
  blue 
  light, 
  divided, 
  like 
  the 
  

   first, 
  by 
  a 
  faint 
  line. 
  In 
  some 
  discs 
  this 
  annulus 
  is 
  divided 
  into 
  three, 
  by 
  two 
  

   faint 
  bands. 
  Each 
  sector 
  of 
  this 
  annulus 
  is 
  subdivided 
  by 
  dark 
  radial 
  lines, 
  into 
  

   four 
  or 
  five 
  spaces, 
  and, 
  sometimes, 
  beyond 
  this 
  there 
  is 
  another 
  annulus 
  simi- 
  

   larly 
  divided, 
  the 
  tint 
  of 
  both 
  being 
  a 
  white 
  of 
  the 
  first 
  order. 
  

  

  The 
  interesting 
  fact 
  in 
  this 
  description, 
  and 
  which 
  will 
  afterwards 
  occupy 
  our 
  

   attention, 
  is, 
  that 
  the 
  two 
  sharp 
  black 
  circular 
  lines, 
  or 
  spaces, 
  are 
  wholly 
  devoid 
  of 
  

   matter, 
  and 
  that 
  the 
  interior 
  part 
  of 
  the 
  disc 
  is 
  separated 
  by 
  them 
  from 
  the 
  exterior 
  

   part. 
  

  

  Among 
  the 
  almost 
  infinite 
  variety 
  of 
  crystallisations 
  which 
  this 
  substance 
  pre- 
  

   sents 
  to 
  us, 
  I 
  shall 
  describe 
  only 
  another 
  which, 
  though 
  we 
  shall 
  afterwards 
  find 
  

   it 
  fully 
  developed 
  in 
  other 
  substances, 
  occurs 
  only 
  in 
  circular 
  sectors 
  of 
  30° 
  or 
  45°. 
  

   It 
  is 
  represented 
  in 
  Fig. 
  11, 
  in 
  its 
  complete 
  state, 
  and 
  consists 
  of 
  a 
  series 
  of 
  con- 
  

   centric 
  circles, 
  composed 
  of 
  crystalline 
  patches, 
  which 
  generally 
  polarise 
  tints 
  not 
  

   higher 
  than 
  the 
  yellow 
  of 
  the 
  first 
  order. 
  Each 
  concentric 
  circle 
  appears 
  at 
  first 
  

   to 
  be 
  separated 
  from 
  its 
  neighbour, 
  and 
  each 
  crystalline 
  patch 
  from 
  those 
  adjacent 
  

   to 
  it 
  ; 
  but 
  though 
  this 
  is 
  in 
  some 
  crystallisations 
  the 
  case, 
  yet 
  in 
  general, 
  we 
  can 
  

   observe 
  between 
  the 
  patches, 
  in 
  all 
  directions, 
  crystalline 
  matter 
  so 
  exceedingly 
  

   attenuated, 
  that 
  its 
  existence 
  is 
  not 
  made 
  visible 
  by 
  its 
  action 
  on 
  polarised 
  light. 
  

  

  2. 
  Salicine. 
  — 
  In 
  this 
  substance, 
  whether 
  dissolved 
  in 
  water 
  or 
  in 
  alcohol, 
  I 
  

   have 
  found 
  the 
  most 
  splendid 
  circular 
  crystallisations. 
  They 
  are 
  generally 
  very 
  

   large, 
  and 
  their 
  character 
  is 
  negative, 
  like 
  the 
  rings 
  in 
  calcareous 
  spar. 
  When 
  

   the 
  crystals 
  are 
  small, 
  and 
  require 
  a 
  considerable 
  power 
  to 
  be 
  seen, 
  their 
  tint 
  is 
  

   the 
  palest 
  blue 
  of 
  the 
  first 
  order, 
  but 
  when 
  their 
  diameter 
  is 
  between 
  the 
  one-fifth 
  

   and 
  the 
  one-thirtieth 
  of 
  an 
  inch, 
  and 
  their 
  tints 
  those 
  of 
  the, 
  first 
  and 
  second 
  orders, 
  

   they 
  form, 
  in 
  the 
  estimation 
  of 
  all 
  who 
  have 
  seen 
  them, 
  one 
  of 
  the 
  finest 
  objects 
  

   for 
  the 
  polarising 
  microscope. 
  

  

  One 
  of 
  the 
  smaller 
  crystals 
  is 
  shewn 
  in 
  Fig. 
  12, 
  where 
  the 
  tint 
  of 
  the 
  four 
  

   sectors 
  is 
  bluish-white, 
  while 
  that 
  of 
  the 
  circular 
  rim 
  is 
  absolutely 
  black, 
  arising 
  

   from 
  the 
  great 
  thinness 
  of 
  the 
  crystals 
  which 
  compose 
  it. 
  That 
  they 
  are 
  trans- 
  

   parent 
  crystals 
  and 
  not 
  opaque 
  matter 
  is 
  proved 
  in 
  this, 
  and 
  in 
  all 
  similar 
  cases, 
  

   by 
  turning 
  round 
  the 
  analyser 
  when 
  the 
  light 
  freely 
  permeates 
  the 
  rim, 
  and 
  has 
  

   a 
  slightly 
  yellow 
  tinge, 
  being 
  complementary 
  to 
  what 
  Newton 
  calls, 
  in 
  his 
  Table 
  

   of 
  Periodical 
  Colours, 
  the 
  Beginning 
  of 
  Black. 
  

  

  A 
  larger 
  disc 
  of 
  Salicine 
  is 
  shewn 
  in 
  Fig. 
  13, 
  where 
  there 
  is 
  a 
  sharp 
  black 
  cross 
  

   in 
  the 
  centre, 
  surrounded 
  with 
  five 
  or 
  six 
  narrow 
  and 
  concentric 
  black 
  rings, 
  which 
  

   become 
  white 
  by 
  turning 
  the 
  analyser 
  ; 
  or 
  we 
  shall 
  in 
  future 
  express 
  it, 
  in 
  the 
  

   white 
  field. 
  Beyond 
  these 
  central 
  sectors, 
  the 
  black 
  cross 
  is 
  wide 
  and 
  divergent. 
  

  

  VOL. 
  XX. 
  PART 
  IV. 
  8 
  C 
  

  

  