﻿Brown: 
  The 
  refraction 
  of 
  light 
  in 
  plant 
  tissues 
  245 
  

  

  S 
  

  

  I. 
  Refractive 
  liquids 
  

  

  To 
  measure 
  the 
  refl-actlon 
  of 
  an 
  object 
  microscopically 
  a 
  

   gradient 
  series 
  of 
  refractive 
  liquids 
  must 
  first 
  be 
  prepared. 
  For 
  

   plant 
  tissues 
  in 
  the 
  dry 
  condition, 
  a 
  series 
  of 
  twenty 
  liquids 
  dif- 
  

   fering 
  from 
  each 
  other 
  in 
  refraction 
  by 
  about 
  .005 
  and 
  ranging 
  

   from 
  rr499 
  to 
  1.598 
  was 
  found 
  generally 
  satisfactory. 
  Either 
  

   aqueous 
  solutions 
  or 
  oils 
  may 
  be 
  used 
  in 
  making 
  up 
  such 
  a 
  series, 
  

   both 
  water 
  and 
  oil 
  possessing 
  certain 
  advantages 
  over 
  the 
  alter- 
  

   native 
  solvent. 
  Aqueous 
  solutions, 
  for 
  instance, 
  would 
  probably 
  

   be 
  best 
  for 
  measuring 
  the 
  refractive 
  powers 
  of 
  the 
  living 
  contents 
  

   of 
  a 
  cell. 
  But 
  such 
  solutions 
  are 
  absorbed 
  by 
  many 
  tissues 
  more, 
  

   than 
  are 
  oils. 
  One 
  in 
  particular 
  of 
  the 
  solutions, 
  to 
  be 
  described 
  

   presently, 
  the 
  McLean 
  solution, 
  causes 
  great 
  swelling 
  of 
  muci- 
  

   laginous 
  membranes, 
  which, 
  for 
  certain 
  purposes, 
  is 
  desirable- 
  

  

  grams 
  

  

  Water 
  solutions 
  are 
  open 
  to 
  the 
  objection 
  that 
  they 
  tend 
  to 
  

   evaporate, 
  with 
  the 
  result 
  that 
  the 
  concentration 
  and 
  refraction 
  

   of 
  the 
  liquids 
  are 
  changed. 
  For 
  general 
  purposes, 
  therefore, 
  

   non-volatile 
  oil 
  mixtures 
  proved 
  to 
  be 
  superior 
  to 
  aqueous 
  solutions,' 
  

   and 
  a 
  series 
  made 
  up 
  from 
  mixtures 
  qf 
  castor 
  oil 
  (n 
  = 
  1.49) 
  and 
  

   clove 
  oil 
  (w 
  = 
  1.535) 
  for 
  refractions 
  below 
  1.535, 
  and 
  of 
  clove 
  oil 
  

   and 
  naphthalene 
  a 
  monobromated 
  (n 
  - 
  1.65) 
  for 
  the 
  higher 
  

   refractions 
  gave 
  excellent 
  results 
  {n 
  indicating 
  index 
  of 
  refraction). 
  

   The 
  mixtures 
  and 
  their 
  respective 
  indices 
  of 
  refraction 
  are 
  indi- 
  

   cated 
  in 
  Table 
  I. 
  

  

  A 
  gradient 
  series 
  of 
  refractive 
  liquids 
  may 
  be 
  prepared 
  by 
  

   mixing 
  a 
  liquid 
  of 
  high 
  refraction 
  with 
  one 
  of 
  low 
  refraction. 
  

   When 
  castor 
  oil 
  is 
  mixed 
  with 
  clove 
  oil, 
  or 
  clove 
  oil 
  with 
  naph- 
  

   thalene 
  a 
  monobromated, 
  the 
  index 
  of 
  refraction 
  of 
  the 
  mixture 
  is 
  

   proportional 
  to 
  the 
  volumes 
  of 
  the 
  liquids 
  mixed. 
  - 
  Or, 
  if 
  V„ 
  and 
  F5 
  

   are 
  the 
  volumes 
  of 
  the 
  liquids 
  mixed 
  and 
  fta 
  and 
  ttb 
  the 
  respective 
  

   indices 
  of 
  refraction, 
  then 
  

  

  Va{n„ 
  -1) 
  + 
  V, 
  (n, 
  - 
  i) 
  

  

  If 
  a 
  series 
  of 
  mixtures 
  is 
  now 
  prepared, 
  in 
  each 
  of 
  which 
  mixtures 
  

   the 
  relative 
  volume 
  of 
  one 
  of 
  the 
  oils 
  of 
  known 
  refraction 
  is 
  de- 
  

   creased 
  by 
  a 
  constant 
  quantity 
  (e.g., 
  10, 
  9, 
  8, 
  ... 
  o) 
  while 
  the 
  

  

  