﻿248 
  Brown: 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  

  

  1 
  

  

  ^f 
  the 
  dispersion 
  (i.e., 
  the 
  difference 
  between 
  the 
  indices 
  of 
  

   refraction 
  of 
  light 
  of 
  long 
  and 
  of 
  short 
  wave-length) 
  of 
  the 
  liquid 
  

   is 
  not 
  the 
  same 
  as 
  that 
  of 
  the 
  object, 
  it 
  is 
  possible 
  to 
  find 
  a 
  common 
  

   index 
  of 
  refraction 
  for 
  light 
  of 
  one 
  wave-length 
  only. 
  In 
  this 
  

   case, 
  the 
  refracted 
  rays 
  give 
  fringes 
  of 
  color 
  such 
  as 
  blue 
  on 
  one 
  

   side 
  and 
  red 
  on 
  the 
  other. 
  McLean's 
  solution, 
  which 
  has 
  a 
  high 
  

   dispersion, 
  causes 
  wide 
  color 
  fringes 
  with 
  the 
  reserve 
  cellulose 
  of 
  

   date 
  seed, 
  in 
  dilutions 
  having 
  a 
  refraction 
  between 
  i 
  .531 
  and 
  i 
  .535. 
  

   This 
  method 
  is 
  particularly 
  useful 
  in 
  determining 
  whether 
  the 
  

   refraction 
  of 
  an 
  object 
  is 
  above 
  or 
  below 
  that 
  of 
  the 
  liquid. 
  

  

  h 
  

  

  1 
  

  

  B. 
  By 
  direct 
  illiimination 
  

   ' 
  Relative 
  bnghtness.— 
  In 
  studying 
  tissues, 
  the 
  relative 
  refrac- 
  

   tion 
  of 
  a 
  transparent 
  object 
  is 
  often 
  indicated 
  by 
  the 
  illumination 
  

   of 
  its 
  transverse 
  surface. 
  On 
  raising 
  the 
  tube 
  of 
  the 
  microscope 
  

   slightly 
  from 
  sharp 
  focus, 
  an 
  object 
  whose 
  refractive 
  index 
  is 
  

   higher 
  than 
  that 
  of 
  the 
  tissues 
  or 
  liquid 
  in 
  contact 
  with 
  it 
  appears 
  

   relatively 
  bright 
  except 
  at 
  the 
  border 
  (see 
  Figs, 
  i 
  and 
  2) 
  . 
  An 
  

   object 
  of 
  lower 
  refraction, 
  on 
  the 
  other 
  hand, 
  appears 
  relatively 
  

   dark. 
  On 
  lowering 
  the 
  tube 
  the 
  relative 
  brightness 
  is 
  reversed 
  

  

  in 
  each 
  case. 
  

  

  The 
  Becke 
  line.— 
  A 
  few 
  bodies, 
  such 
  as 
  the 
  crystals 
  of 
  calcium 
  

   oxalate 
  in 
  the 
  crystal-parenchyma 
  of 
  woody 
  tissues, 
  are 
  visible 
  in 
  

   all 
  liquids 
  of 
  the 
  series. 
  The 
  refraction 
  of 
  such 
  bodies 
  is 
  best 
  

   determined 
  as 
  follows: 
  An 
  uncovered 
  edge 
  of 
  the 
  crystal 
  is 
  found, 
  

   which 
  projects 
  into 
  the 
  liquid. 
  When 
  brought 
  into 
  sharp 
  focus, 
  a 
  

   narrow 
  band 
  of 
  bright 
  light-the 
  Becke 
  line-defines 
  the 
  outline. 
  

   On 
  focusing 
  upward, 
  this 
  band 
  of 
  light 
  appears 
  to 
  move 
  in 
  the 
  

   direction 
  of 
  the 
  body 
  of 
  highest 
  refraction, 
  i.e., 
  outward 
  from 
  the 
  

   border 
  into 
  the 
  liquid, 
  if 
  the 
  refraction 
  of 
  the 
  liquid 
  is 
  higher 
  than 
  

   that 
  of 
  the 
  cr>'stal, 
  or 
  inward 
  from 
  the 
  border 
  toward 
  the 
  centre 
  of 
  

   the 
  crystal 
  if 
  the 
  refraction 
  of 
  the 
  crystal 
  is 
  higher 
  than 
  that 
  of 
  the 
  

   liquid. 
  

  

  4 
  

  

  C. 
  Modifications 
  with 
  hygroscopic 
  tiss 
  

  

  lies 
  

  

  In 
  applying 
  the 
  methods 
  above 
  described 
  to 
  plant 
  tissues 
  

   certam 
  modifications 
  are 
  necessary, 
  due 
  to 
  the 
  fact 
  that 
  

   most 
  membranes 
  tend 
  to 
  imbibe 
  the 
  refractive 
  liquids 
  in 
  which 
  

  

  