﻿256 
  Brown: 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  

  

  i 
  

  

  m 
  

  

  Dammar; 
  Canada 
  balsam.— 
  It 
  is 
  unfortunate 
  that 
  the 
  refractive 
  

   index'of 
  both 
  dammar 
  and 
  Canada 
  balsam 
  approach 
  too 
  closely 
  

   those 
  of 
  the 
  stem 
  tissues 
  to 
  allow 
  their 
  use 
  for 
  mounting 
  unstained 
  

   sections 
  of 
  any 
  of 
  the 
  hygroscopic 
  tissues 
  of 
  the 
  stem. 
  Details 
  

   in 
  some 
  of 
  the 
  soft 
  tissues 
  of 
  the 
  bark, 
  as 
  will 
  be 
  shown 
  presently, 
  

   are 
  also 
  obscured 
  through 
  shrinkage 
  during 
  the 
  process 
  of 
  dehy- 
  

   dration. 
  These 
  media 
  have 
  a 
  refractive 
  index 
  about 
  .01 
  below 
  

   the 
  average 
  for 
  dehydrated 
  plant 
  tissues 
  and, 
  for 
  general 
  purposes, 
  

  

  for 
  mounting 
  stained 
  sections. 
  

  

  are 
  

  

  McLean 
  

  

  in 
  

  

  5 
  solution. 
  — 
  This 
  solution, 
  suggested 
  by 
  McLean 
  

   ('14), 
  IS 
  prepared 
  by 
  dissolving 
  in 
  a 
  saturated 
  aqueous 
  solution 
  of 
  

   potassium 
  iodide 
  as 
  much 
  mercury 
  biniodide 
  as 
  it 
  will 
  take 
  up. 
  

   This 
  solution 
  has 
  a 
  high 
  index 
  of 
  refraction 
  (1.6210 
  for 
  solution 
  of 
  

   full 
  strength), 
  equal 
  to 
  that 
  of 
  flint 
  glass 
  and 
  far 
  exceeding 
  that 
  

   of 
  any 
  of 
  the 
  tissues 
  examined; 
  it 
  is 
  readily 
  diluted 
  with 
  water, 
  

   the 
  index 
  of 
  refraction 
  of 
  which 
  is 
  lower 
  than 
  that 
  of 
  any 
  con- 
  

   stituent 
  of 
  plant 
  tissue. 
  This 
  liquid 
  may 
  therefore 
  be 
  regarded 
  as 
  

   one 
  of 
  the 
  best 
  for 
  preparing 
  a 
  series 
  of 
  aqueous 
  dilutions 
  for 
  

   measuring 
  refraction 
  in 
  tissues, 
  and 
  the 
  principle 
  involved 
  .„ 
  

   diluting 
  with 
  water 
  is 
  the 
  same 
  as 
  that 
  earlier 
  given 
  and 
  adopted 
  

   in 
  mixing 
  the 
  oils 
  of 
  a 
  refraction 
  series. 
  By 
  such 
  a 
  series 
  of 
  

   liquids 
  a 
  wide 
  range 
  in 
  refraction 
  (i. 
  33-1 
  .62) 
  may 
  be 
  covered, 
  

   and 
  if 
  successive 
  dilutions 
  were 
  made 
  to 
  differ 
  from 
  each 
  other 
  

   by 
  n 
  = 
  .005, 
  there 
  would 
  be 
  59 
  liquids 
  in 
  this 
  series. 
  As 
  before 
  

   mentioned, 
  tissues 
  imbibe 
  aqueous 
  solutions 
  more 
  readily 
  than 
  the 
  

   oils, 
  causing 
  wider 
  deviations 
  in 
  refraction, 
  but 
  the 
  mean 
  refrac- 
  

   tions 
  of 
  tissues 
  determined 
  by 
  their 
  use 
  were 
  the 
  same 
  as 
  those 
  

   determined 
  by 
  oils. 
  McLean's 
  solution 
  also 
  has 
  high 
  dispersion 
  

   or, 
  in 
  other 
  words, 
  a 
  wide 
  difference 
  between 
  the 
  index 
  of 
  refraction 
  

  

  for 
  light 
  of 
  long 
  wave-length 
  (red) 
  and 
  light 
  of 
  short 
  wave-length 
  

   (blue) 
  . 
  

  

  As 
  a 
  medium 
  for 
  mounting 
  soft 
  tissues, 
  this 
  solution 
  possesses 
  

   the 
  advantage 
  of 
  not 
  requiring 
  dehydration, 
  so 
  that 
  such 
  tissues 
  

   may 
  be 
  mounted 
  directly 
  without 
  danger 
  of 
  shrinkage. 
  But 
  

   It 
  has 
  the 
  serious 
  disadvantage 
  of 
  causing 
  ^ide 
  deviation 
  of 
  

   refraction 
  in 
  certain 
  tissues, 
  such 
  as 
  collenchyma, 
  as 
  a 
  result 
  of 
  

   which 
  the 
  outlines 
  may 
  become 
  quite 
  invisible. 
  

  

  