﻿44 
  Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  

  

  and 
  three 
  purple 
  asters, 
  using 
  a 
  screen 
  which 
  gave 
  the 
  effect 
  

   of 
  relative 
  luminosity 
  only. 
  Photograph 
  A, 
  PL 
  V 
  gives 
  the 
  

   result 
  which 
  shows 
  that 
  the 
  purple 
  flowers 
  were 
  certainly 
  not 
  

   specially 
  conspicuous 
  on 
  account 
  of 
  their 
  luminosity. 
  By 
  

   using 
  a 
  set 
  of 
  spectroscopically 
  tested 
  screens 
  I 
  was 
  then 
  able 
  

   to 
  secure 
  photographs 
  representing 
  the 
  relative 
  luminosities 
  

   of 
  the 
  flowers 
  when 
  certain 
  rays 
  were 
  ehminated. 
  Thus 
  in 
  

   photograph 
  B 
  all 
  the 
  light 
  rays 
  except 
  the 
  yellow-green 
  were 
  

   eliminated, 
  and 
  the 
  plate 
  was 
  therefore 
  red 
  and 
  blue 
  blind. 
  

   In 
  C 
  only 
  the 
  red 
  and 
  part 
  of 
  the 
  green 
  rays 
  were 
  used, 
  the 
  

   plate 
  being 
  thus 
  bhnd 
  to 
  blue 
  and 
  blue-green. 
  In 
  D 
  all 
  

   the 
  rays 
  except 
  red 
  were 
  used, 
  the 
  plate 
  being 
  thus 
  bhnd 
  

   to 
  red. 
  I 
  leave 
  it 
  to 
  the 
  reader 
  to 
  judge 
  whether 
  an 
  eye, 
  

   blind 
  to 
  any 
  of 
  the 
  selected 
  parts 
  of 
  the 
  spectrum, 
  would 
  

   be 
  Hkely 
  to 
  see 
  the 
  purple 
  asters 
  better 
  than 
  one 
  which 
  had 
  

   a 
  normal 
  range. 
  So 
  far 
  as 
  luminosity 
  is 
  concerned, 
  the 
  

   purple 
  flowers 
  are 
  less 
  conspicuous 
  in 
  all 
  the 
  abnormally 
  

   produced 
  photographs 
  than 
  in 
  that 
  which 
  gives 
  the 
  value 
  

   with 
  all 
  the 
  rays 
  in 
  use. 
  

  

  I 
  consider 
  it 
  a 
  fair 
  assumption, 
  therefore, 
  that 
  if 
  the 
  

   preference 
  for 
  the 
  purple 
  flowers 
  is 
  due 
  to 
  the 
  greater 
  ease 
  

   with 
  which 
  they 
  can 
  be 
  seen, 
  the 
  cause 
  lies 
  in 
  the 
  same 
  

   property 
  which 
  makes 
  them 
  conspicuous 
  to 
  my 
  own 
  eyes, 
  

   viz. 
  the 
  colour. 
  

  

  Before 
  leaving 
  the 
  subject 
  of 
  flowers 
  I 
  may 
  mention 
  a 
  few 
  

   isolated 
  observations 
  which 
  seem 
  to 
  have 
  a 
  bearing 
  on 
  the 
  

   issue. 
  An 
  vrticae 
  picked 
  out 
  and 
  dived 
  at 
  with 
  great 
  

   accuracy, 
  from 
  a 
  distance 
  of 
  about 
  four 
  feet, 
  a 
  purple 
  aster 
  

   which 
  lay 
  between 
  two 
  scarlet 
  flowers. 
  Occasionally 
  an 
  

   example 
  would 
  settle 
  — 
  though 
  not 
  approaching 
  from 
  any 
  

   distance 
  — 
  on 
  the 
  disc 
  of 
  an 
  aster 
  from 
  which 
  I 
  had 
  removed 
  

   the 
  ray 
  florets. 
  On 
  the 
  other 
  hand, 
  the 
  yellow 
  disc 
  is 
  not 
  

   the 
  guiding 
  attraction, 
  since 
  they 
  frequently 
  flew 
  to 
  and 
  

   settled 
  on 
  an 
  aster 
  bud 
  which 
  showed 
  no 
  yellow 
  disc, 
  and 
  was 
  

   only 
  distinguished 
  from 
  the 
  rest 
  of 
  the 
  foliage 
  by 
  the 
  coloured 
  

   tips 
  of 
  the 
  ray 
  florets. 
  On 
  another 
  occasion 
  I 
  observed 
  an 
  

   urticae 
  pick 
  out 
  with 
  certainty 
  a 
  very 
  small 
  yeflow 
  flower 
  on 
  

   the 
  lawn, 
  the 
  latter 
  being 
  itself 
  greenish 
  yellow, 
  and 
  after 
  

   tasting 
  the 
  flower 
  it 
  flew 
  to 
  and 
  inspected 
  a 
  bit 
  of 
  yellow 
  leaf 
  

   lying 
  some 
  httle 
  distance 
  away. 
  Again 
  I 
  observed 
  P. 
  rapae 
  

   flying 
  over 
  the 
  lawn 
  and 
  picking 
  out 
  with 
  precision 
  some 
  

   scattered 
  and 
  very 
  minute 
  yellow 
  flowers 
  whose 
  colour 
  was 
  

   the 
  only 
  feature 
  which 
  appeared 
  to 
  distinguish 
  them 
  from 
  

   their 
  surroundings. 
  Every 
  one 
  knows 
  how 
  our 
  white 
  

  

  