﻿32 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  spectroscope. 
  The 
  rays 
  are 
  bent 
  or 
  deflected 
  from 
  their 
  

   original 
  course, 
  but 
  all 
  are 
  not 
  equally 
  deflected. 
  Those 
  

   of 
  the 
  shortest 
  wave 
  length 
  are 
  deflected 
  most, 
  and 
  hence 
  

   it 
  is 
  possible 
  to 
  project 
  on 
  a 
  screen 
  a 
  band 
  of 
  hght, 
  one 
  

   end 
  of 
  which 
  consists 
  of 
  the 
  rays 
  of 
  shortest 
  wave 
  length, 
  

   the 
  other 
  of 
  those 
  of 
  the 
  longest, 
  those 
  of 
  intermediate 
  

   wave 
  lengths 
  occupying 
  the 
  intermediate 
  positions. 
  

  

  We 
  are 
  now, 
  however, 
  aware 
  of 
  a 
  new 
  fact, 
  the 
  band 
  of 
  

   light 
  is 
  no 
  longer 
  white, 
  but 
  the 
  different 
  portions 
  of 
  it 
  

   produce 
  in 
  us 
  different 
  sensations 
  which 
  we 
  call 
  colour. 
  

   Thus 
  the 
  band 
  to 
  a 
  normal 
  eye 
  appears 
  deep 
  red 
  at 
  one 
  

   end, 
  and 
  changes 
  towards 
  the 
  other 
  end 
  into 
  orange, 
  

   yellow, 
  green, 
  blue, 
  dark 
  blue, 
  and 
  finally 
  violet. 
  An 
  

   object 
  such 
  as 
  a 
  flower 
  appears 
  to 
  us 
  to 
  be 
  coloured 
  because 
  

   its 
  substance 
  absorbs 
  some 
  of 
  the 
  light 
  falHng 
  upon 
  it, 
  

   and 
  reflects 
  the 
  rest. 
  Such 
  colours 
  in 
  nature 
  are 
  rarely 
  

   pure, 
  i. 
  e. 
  they 
  consist 
  of 
  mixtures 
  of 
  reflected 
  rays, 
  thus 
  

   producing 
  what 
  we 
  term 
  various 
  " 
  shades 
  " 
  of 
  colour. 
  

   These 
  mixtures 
  of 
  colours 
  can 
  be 
  analysed 
  by 
  looking 
  at 
  

   the 
  objects 
  through 
  special 
  light 
  filters 
  which 
  entirely 
  

   absorb 
  certain 
  rays, 
  while 
  allowing 
  the 
  rest 
  to 
  pass 
  un- 
  

   altered. 
  Some 
  rather 
  unsuspected 
  effects 
  may 
  in 
  this 
  

   way 
  be 
  obtained. 
  Thus 
  a 
  deep 
  blue 
  lobelia 
  examined 
  . 
  

   through 
  a 
  screen 
  which 
  is 
  impervious 
  to 
  blue 
  rays, 
  looks 
  

   deep 
  red, 
  showing 
  that 
  its 
  colour 
  consists 
  partly 
  of 
  red 
  

   rays. 
  Similarly 
  certain 
  forms 
  of 
  pale 
  pink 
  pelargonium 
  

   when 
  examined 
  through 
  a 
  filter 
  impervious 
  to 
  red, 
  

   appear 
  bright 
  blue, 
  although 
  we 
  are 
  not 
  conscious 
  of 
  

   the 
  blue 
  component 
  of 
  the 
  colour 
  when 
  seen 
  under 
  normal 
  

   conditions. 
  

  

  It 
  may 
  thus 
  be 
  seen 
  that 
  we 
  may 
  be 
  said 
  to 
  possess 
  

   two 
  separate 
  visual 
  faculties 
  in 
  relation 
  to 
  fight. 
  One, 
  the 
  

   fundamental 
  perception 
  of 
  fight 
  as 
  such, 
  the 
  other 
  the 
  

   capacity 
  for 
  distinguishing, 
  apart 
  from 
  mere 
  luminosity, 
  

   fights 
  of 
  different 
  wave 
  lengths 
  ; 
  and 
  a 
  totally 
  colour-bfind 
  

   person 
  is 
  one 
  who 
  possesses 
  only 
  the 
  former 
  of 
  these 
  two 
  

   faculties. 
  Such 
  cases 
  are 
  extremely 
  rare, 
  though 
  partial, 
  

   and 
  especially 
  " 
  red-green 
  " 
  bfindness 
  is 
  of 
  frequent 
  occur- 
  

   rence. 
  In 
  one 
  case 
  of 
  total 
  colour 
  blindness 
  or 
  mono- 
  

   chromatic 
  vision, 
  instanced 
  by 
  Sir 
  Wilfiam 
  Abney 
  (" 
  Re- 
  

   searches 
  in 
  Colour 
  Vision," 
  London, 
  1913), 
  the 
  subject 
  had 
  

   no 
  sense 
  of 
  any 
  colour, 
  and 
  moreover 
  his 
  sensation 
  of 
  

   luminosity 
  was 
  about 
  one-thirtieth 
  of 
  that 
  of 
  a 
  normal 
  

   person, 
  and 
  much 
  the 
  same 
  condition 
  was 
  observed 
  in 
  a 
  

  

  