﻿614 
  SIR 
  DAVID 
  BREWSTER 
  ON 
  CIRCULAR 
  CRYSTALS. 
  

  

  18, 
  have 
  placed 
  themselves 
  in 
  these 
  positions 
  after 
  the 
  interior 
  crystal 
  has 
  been 
  

   formed 
  ; 
  that 
  is, 
  they 
  are 
  not 
  increments 
  deposited 
  by 
  the 
  solution, 
  but 
  have 
  been 
  

   formed 
  at 
  a 
  distance 
  from 
  the 
  crystal, 
  and 
  carried 
  to 
  their 
  new 
  position. 
  This 
  is 
  

   proved 
  by 
  the 
  fact 
  that 
  sometimes 
  a 
  mass 
  of 
  them 
  surround 
  several 
  of 
  the 
  square 
  

   crystals, 
  while 
  individual 
  ones 
  take 
  their 
  place 
  at 
  random 
  upon 
  the 
  face 
  of 
  the 
  

   square. 
  When 
  the 
  crystals 
  are 
  deposited 
  from 
  a 
  strong 
  solution, 
  the 
  square 
  ones 
  

   become 
  almost 
  opaque, 
  and 
  the 
  irregular 
  ones 
  highly 
  coloured, 
  and 
  of 
  exceedingly 
  

   various 
  shapes. 
  I 
  have 
  not 
  been 
  able 
  to 
  obtain 
  any 
  square 
  crystals 
  of 
  the 
  disul- 
  

   phate 
  of 
  mercury 
  from 
  its 
  solution 
  in 
  muriatic 
  acid.* 
  

  

  6. 
  Parmeline. 
  — 
  This 
  substance, 
  dissolved 
  in 
  water, 
  has 
  a 
  tendency 
  to 
  give 
  

   circular 
  crystals. 
  In 
  alcohol 
  it 
  gives 
  very 
  fine 
  ones, 
  producing, 
  when 
  small, 
  

   beautiful 
  halos 
  like 
  oil 
  of 
  mace, 
  with 
  blue 
  light 
  in 
  their 
  centre. 
  

  

  7. 
  Asparagine 
  and 
  Salicine 
  mixed. 
  — 
  After 
  standing 
  several 
  months, 
  this 
  mixed 
  

   salt 
  produced 
  small 
  circular 
  crystals, 
  apparently 
  of 
  asparagine. 
  These 
  crystals 
  

   gave 
  brilliant 
  halos 
  of 
  red 
  and 
  green 
  light, 
  of 
  such 
  a 
  diameter 
  that 
  the 
  individuals 
  

   were 
  only 
  y^ooth 
  of 
  an 
  inch 
  in 
  diameter. 
  Among 
  these 
  small 
  crystals 
  were 
  placed 
  

   large 
  circular 
  discs, 
  with 
  curved 
  sectors 
  and 
  black 
  crosses, 
  which 
  gave 
  them 
  the 
  

   appearance 
  of 
  the 
  corolla 
  of 
  a 
  flower 
  with 
  party-coloured 
  petals. 
  

  

  8. 
  Palmic 
  Acid. 
  — 
  This 
  substance, 
  when 
  melted 
  by 
  heat, 
  gives 
  very 
  fine 
  negative 
  

   circular 
  crystals 
  like 
  those 
  in 
  the 
  hexagonal 
  mosaic 
  of 
  manna. 
  Theinsulated 
  

   discs 
  have 
  a 
  rim 
  sometimes 
  divided 
  by 
  broad 
  black 
  bands, 
  where 
  the 
  substance 
  

   was 
  too 
  thin 
  to 
  polarise 
  the 
  light. 
  When 
  the 
  rim 
  is 
  broad 
  and 
  single, 
  it 
  is 
  com- 
  

   posed 
  of 
  narrow 
  luminous 
  sectors, 
  radiating 
  from 
  points 
  in 
  the 
  circumference 
  of 
  

   the 
  disc. 
  The 
  rims 
  are 
  sometimes 
  of 
  a 
  different 
  colour 
  from 
  the 
  principal 
  sectors, 
  

   and 
  the 
  latter 
  are 
  often 
  subdivided 
  by 
  numbers 
  of 
  black 
  and 
  equidistant 
  concentric 
  

   circles. 
  

  

  9. 
  Nitrate 
  of 
  Uranium. 
  — 
  This 
  salt 
  gives 
  fine 
  negative 
  circular 
  crystals 
  in 
  

   water, 
  alcohol, 
  and 
  ether. 
  The 
  crystals 
  formed 
  in 
  the 
  alcoholic 
  solution 
  deli- 
  

   quesced 
  in 
  an 
  instant, 
  forming 
  hemispherical 
  bells, 
  which 
  polarised 
  the 
  light 
  by 
  

   oblique 
  refraction, 
  giving 
  four 
  luminous 
  sectors, 
  and 
  a 
  black 
  cross 
  very 
  wide 
  at 
  

   the 
  centre, 
  like 
  the 
  sectors 
  and 
  cross 
  produced 
  by 
  the 
  hemispherical 
  cups 
  of 
  de- 
  

  

  * 
  In 
  making 
  these 
  observations, 
  and 
  on 
  many 
  other 
  occasions, 
  I 
  have 
  felt 
  the 
  great 
  inconvenience 
  

   of 
  the 
  present, 
  and 
  in 
  general, 
  perhaps 
  the 
  best, 
  arrangement 
  of 
  the 
  compound 
  microscope. 
  High 
  

   powers 
  being 
  always 
  obtained 
  by 
  object-glasses 
  of 
  short 
  focal 
  length, 
  it 
  is 
  almost 
  impossible, 
  in 
  

   transparent 
  structures, 
  to 
  develope 
  them, 
  when 
  they 
  consist 
  of 
  lines 
  or 
  parts 
  of 
  different 
  thickness. 
  

   Vision 
  is 
  destroyed 
  by 
  the 
  refractions 
  and 
  diffractions 
  of 
  the 
  intromitted 
  light. 
  The 
  only 
  remedy 
  for 
  

   this 
  is 
  to 
  use 
  ^-inch, 
  or 
  even 
  1 
  or 
  2-inch 
  object-glasses, 
  and 
  obtain 
  the 
  power 
  that 
  is 
  required 
  at 
  the 
  

   eye-piece, 
  by 
  means 
  of 
  grooved 
  and 
  other 
  lenses 
  of 
  diamond, 
  garnet, 
  &c. 
  

  

  