﻿EQUILIBRIUM 
  OF 
  ELASTIC 
  SOLIDS. 
  97 
  

  

  Therefore, 
  if 
  the 
  solid 
  be 
  viewed 
  by 
  polarized 
  light 
  (transmitted 
  parallel 
  to 
  

   the 
  axis), 
  the 
  difference 
  of 
  retardation 
  of 
  the 
  oppositely 
  polarized 
  rays 
  at 
  any 
  

   point 
  in 
  the 
  solid 
  will 
  be 
  inversely 
  proportional 
  to 
  the 
  square 
  of 
  the 
  distance 
  from 
  

   the 
  axis 
  of 
  the 
  cylinder, 
  and 
  the 
  planes 
  of 
  polarization 
  of 
  these 
  rays 
  will 
  be 
  

   inclined 
  45° 
  to 
  the 
  radius 
  at 
  that 
  point. 
  

  

  The 
  general 
  appearance 
  is 
  therefore 
  a 
  system 
  of 
  coloured 
  rings 
  arranged 
  op- 
  

   positely 
  to 
  the 
  rings 
  in 
  uniaxal 
  crystals, 
  the 
  tints 
  ascending 
  in 
  the 
  scale 
  as 
  they 
  

   approach 
  the 
  centre, 
  and 
  the 
  distance 
  between 
  the 
  rings 
  decreasing 
  towards 
  the 
  

   centre. 
  The 
  whole 
  system 
  is 
  crossed 
  by 
  two 
  dark 
  bands 
  inclined 
  45° 
  to 
  the 
  plane 
  

   of 
  primitive 
  polarization, 
  when 
  the 
  plane 
  of 
  the 
  analysing 
  plate 
  is 
  perpendicular 
  

   to 
  that 
  of 
  the 
  first 
  polarizing 
  plate. 
  

  

  A 
  jelly 
  of 
  isinglass 
  poured 
  when 
  hot 
  between 
  two 
  concentric 
  cylinders 
  forms, 
  

   when 
  cold, 
  a 
  convenient 
  solid 
  for 
  this 
  experiment 
  ; 
  and 
  the 
  diameters 
  of 
  the 
  rings 
  

   may 
  be 
  varied 
  at 
  pleasure 
  by 
  changing 
  the 
  force 
  of 
  torsion 
  applied 
  to 
  the 
  interior 
  

   cylinder. 
  

  

  By 
  continuing 
  the 
  force 
  of 
  torsion 
  while 
  the 
  jelly 
  is 
  allowed 
  to 
  dry, 
  a 
  hard 
  

   plate 
  of 
  isinglass 
  is 
  obtained, 
  which 
  still 
  acts 
  in 
  the 
  same 
  way 
  on 
  polarized 
  light, 
  

   even 
  when 
  the 
  force 
  of 
  torsion 
  is 
  removed. 
  

  

  It 
  seems 
  that 
  this 
  action 
  cannot 
  be 
  accounted 
  for 
  by 
  supposing 
  the 
  interior 
  

   parts 
  kept 
  in 
  a 
  state 
  of 
  constraint 
  by 
  the 
  exterior 
  parts, 
  as 
  in 
  unannealed 
  and 
  

   heated 
  glass 
  ; 
  for 
  the 
  optical 
  properties 
  of 
  the 
  plate 
  of 
  isinglass 
  are 
  such 
  as 
  would 
  

   indicate 
  a 
  strain 
  preserving 
  in 
  every 
  part 
  of 
  the 
  plate 
  the 
  direction 
  of 
  the 
  original 
  

   strain, 
  so 
  that 
  the 
  strain 
  on 
  one 
  part 
  of 
  the 
  plate 
  cannot 
  be 
  maintained 
  by 
  an 
  op- 
  

   posite 
  strain 
  on 
  another 
  part. 
  

  

  Two 
  other 
  uncrystallised 
  substances 
  have 
  the 
  power 
  of 
  retaining 
  the 
  polarizing 
  

   structure 
  developed 
  by 
  compression. 
  The 
  first 
  is 
  a 
  mixture 
  of 
  wax 
  and 
  resin 
  

   pressed 
  into 
  a 
  thin 
  plate 
  between 
  two 
  plates 
  of 
  glass, 
  as 
  described 
  by 
  Sir 
  David 
  

   Brewster, 
  in 
  the 
  Philosophical 
  Transactions 
  for 
  1815 
  and 
  1830. 
  

  

  When 
  a 
  compressed 
  plate 
  of 
  this 
  substance 
  is 
  examined 
  with 
  polarized 
  light, 
  

   it 
  is 
  observed 
  to 
  have 
  no 
  action 
  on 
  light 
  at 
  a 
  perpendicular 
  incidence 
  ; 
  but 
  when 
  

   inclined, 
  it 
  shews 
  the 
  segments 
  of 
  coloured 
  rings. 
  This 
  property 
  does 
  not 
  belong 
  

   to 
  the 
  plate 
  as 
  a 
  whole, 
  but 
  is 
  possessed 
  by 
  every 
  part 
  of 
  it. 
  It 
  is 
  therefore 
  similar 
  

   to 
  a 
  plate 
  cut 
  from 
  a 
  uniaxal 
  crystal 
  perpendicular 
  to 
  the 
  axis. 
  

  

  I 
  find 
  that 
  its 
  action 
  on 
  light 
  is 
  like 
  that 
  of 
  & 
  positive 
  crystal, 
  while 
  that 
  of 
  a 
  

   plate 
  of 
  isinglass 
  similarly 
  treated 
  would 
  be 
  negative. 
  

  

  The 
  other 
  substance 
  which 
  possesses 
  similar 
  properties 
  is 
  gutta 
  percha. 
  This 
  

   substance 
  in 
  its 
  ordinary 
  state, 
  when 
  cold, 
  is 
  not 
  transparent 
  even 
  in 
  thin 
  films 
  ; 
  

   but 
  if 
  a 
  thin 
  film 
  be 
  drawn 
  out 
  gradually, 
  it 
  may 
  be 
  extended 
  to 
  more 
  than 
  double 
  

   its 
  length. 
  It 
  then 
  possesses 
  a 
  powerful 
  double 
  refraction, 
  which 
  it 
  retains 
  so 
  

   strongly 
  that 
  it 
  has 
  been 
  used 
  for 
  polarizing 
  light.* 
  As 
  one 
  of 
  its 
  refractive 
  in- 
  

  

  * 
  By 
  Dr 
  Wright, 
  I 
  believe. 
  

  

  