﻿Brown: 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  249 
  

  

  \ 
  

  

  they 
  are 
  immersed 
  and 
  to 
  become 
  invisible 
  in 
  two 
  or 
  more 
  liquids 
  

   of 
  the 
  series. 
  This 
  latter 
  phenomenon 
  is 
  attributable 
  to 
  the 
  fact 
  

   that 
  the 
  refraction 
  index 
  of 
  the 
  mass, 
  i.e., 
  the 
  dry 
  tissue 
  substance 
  

   together 
  with 
  whatever 
  of 
  the 
  refractive 
  liquid 
  may 
  have 
  been 
  

   imbibed, 
  here 
  approaches 
  that 
  of 
  the 
  liquid 
  in 
  which 
  it 
  is 
  mounted; 
  

   and 
  under 
  these 
  conditions 
  it 
  would 
  be 
  recorded 
  as 
  O 
  in 
  the 
  tabula- 
  

  

  i 
  

  

  tion 
  (see 
  Table 
  I). 
  In 
  such 
  cases 
  the 
  refractive 
  power 
  of 
  the 
  

   tissue 
  substance 
  is 
  taken 
  as 
  the 
  mean 
  between 
  the 
  liquid 
  of 
  highest 
  

   refraction 
  and 
  the 
  one 
  of 
  lowest 
  refraction 
  in 
  which 
  the 
  outlines 
  

   are 
  barely 
  distinguishable. 
  Further 
  observations 
  regarding 
  the 
  

   refraction 
  of 
  hygroscopic 
  bodies 
  are 
  made 
  below. 
  

  

  ft 
  

  

  D. 
  Relative 
  accuracy 
  of 
  the 
  methods 
  

  

  + 
  

  

  The 
  accuracy 
  with 
  which 
  the 
  index 
  of 
  refraction 
  of 
  an 
  object 
  

   may 
  be 
  determined 
  by 
  the 
  foregoing 
  methods 
  obviously 
  depends, 
  

   in 
  the 
  first 
  place, 
  upon 
  the 
  accuracy 
  of 
  the 
  calculated 
  indices 
  of 
  

   the 
  liquid 
  mixtures. 
  In 
  a 
  test 
  made 
  of 
  one 
  of 
  the 
  liquid 
  mixtures 
  

   (castor 
  oil, 
  5 
  volumes: 
  clove 
  oil, 
  5 
  volumes), 
  for 
  example, 
  all 
  the 
  

   figures 
  in 
  the 
  calculated 
  index 
  (1.512 
  +) 
  were 
  found 
  to 
  be 
  correct; 
  

  

  ■ 
  

  

  in 
  fact, 
  no 
  errors 
  were 
  found 
  in 
  the 
  calculated 
  indices 
  of 
  any 
  of 
  the 
  

   mixtures; 
  but 
  for 
  extreme 
  accuracy, 
  the 
  index 
  of 
  refraction 
  of 
  the 
  

   liquid 
  should 
  be 
  standardized 
  as 
  a 
  matter 
  of 
  precaution. 
  \ 
  

  

  With 
  

  

  Wrigh 
  

  

  with 
  an 
  exactness 
  of 
  ±.001 
  when 
  monochromatic 
  illumination 
  

  

  L 
  ^ 
  

  

  (strong 
  sodium 
  light) 
  is 
  used, 
  and 
  Wright 
  considers 
  the 
  method 
  

   by 
  oblique 
  illumination 
  about 
  as 
  accurate 
  as 
  the 
  Becke 
  line 
  

   method. 
  By 
  the 
  Becke 
  line 
  method 
  the 
  refraction 
  of 
  calcium 
  

   oxalate 
  trihydrate 
  was 
  found 
  by 
  the 
  writer 
  to 
  lie 
  between 
  1.568 
  

   and 
  1.552; 
  and 
  with 
  a 
  series 
  of 
  liquids 
  intermediate 
  in 
  refraction 
  

   between 
  the 
  ones 
  used, 
  it 
  would 
  doubtless 
  have 
  been 
  possible 
  

   to 
  have 
  made 
  a 
  more 
  exact 
  determination. 
  The 
  method 
  employed 
  

   to 
  determine 
  the 
  refraction 
  of 
  hygroscopic 
  bodies 
  is 
  less 
  exact 
  

   than 
  methods 
  applicable 
  to 
  crystalline 
  bodies, 
  but 
  determina- 
  

   tions 
  may 
  be 
  carried 
  to 
  the 
  hundredths 
  with 
  reasonable 
  accuracy. 
  

  

  / 
  

  

  / 
  

  

  III. 
  REFRACTION 
  OF 
  TISSUES 
  

  

  The 
  results 
  recorded 
  in 
  Table 
  1 
  bring 
  out 
  wide 
  relative 
  dif- 
  

   ferences 
  in 
  refraction 
  in 
  different 
  tissues 
  or 
  in 
  different 
  parts 
  of 
  

  

  