﻿Dr. 
  H. 
  Eltringham 
  on 
  Butterfly 
  Vision. 
  23 
  

  

  previous 
  results. 
  I 
  have 
  already 
  stated 
  that 
  I 
  was 
  able 
  

   to 
  see 
  and 
  photograph 
  the 
  image 
  in 
  the 
  glow-worm's 
  eye, 
  

   and 
  my 
  observations 
  on 
  that 
  insect 
  convince 
  m,e 
  that 
  

   there 
  is 
  no 
  doubt 
  whatever 
  of 
  the 
  correctness 
  of 
  Exner's 
  

   conclusions, 
  both 
  practical 
  and 
  theoretical, 
  in 
  regard 
  to 
  

   the 
  nature 
  of 
  the 
  superposition 
  image. 
  

  

  The 
  problem 
  of 
  the 
  apposition 
  image 
  is 
  much 
  less 
  easy 
  

   to 
  solve. 
  Exner 
  had 
  already 
  utiUsed 
  the 
  idea 
  of 
  freezing 
  

   the 
  eye 
  in 
  order 
  to 
  maintain 
  the 
  cones 
  and 
  other 
  struc- 
  

   tures 
  in 
  their 
  relative 
  positions. 
  The 
  same 
  idea 
  occurred 
  

   independently 
  to 
  my 
  friend 
  Prof. 
  Poulton, 
  though 
  with 
  

   the 
  additional 
  complication 
  of 
  maintaining 
  the 
  eye 
  in 
  a 
  

   frozen 
  condition 
  during 
  examination. 
  A 
  large 
  dragon- 
  

   fly's 
  eye 
  seemed 
  to 
  promise 
  the 
  greatest 
  facihty 
  in 
  handling, 
  

   and 
  I 
  therefore 
  designed 
  and 
  constructed 
  a 
  somewhat 
  

   elaborate 
  apparatus 
  for 
  carrying 
  out 
  the 
  experiment. 
  A 
  

   special 
  hollow 
  stage 
  was 
  made, 
  having 
  a 
  central 
  aperture 
  

   into 
  which 
  were 
  fitted 
  type-metal 
  blocks 
  pierced 
  through 
  

   the 
  centre 
  for 
  light, 
  and 
  accurately 
  cast 
  to 
  fit 
  the 
  eyes 
  of 
  

   various 
  large 
  species 
  of 
  dragon-flies. 
  Small 
  copper 
  pipes 
  

   attached 
  to 
  the 
  stage 
  conveyed 
  and 
  withdrew 
  a 
  freezing 
  

   mixture 
  of 
  ice 
  and 
  ammonium 
  chloride, 
  which 
  was 
  kept 
  

   circulating 
  by 
  means 
  of 
  a 
  small 
  gunmetal 
  force-pump 
  

   actuated 
  by 
  an 
  electro 
  -motor. 
  The 
  lower 
  side 
  of 
  the 
  

   freezing 
  stage 
  was 
  fitted 
  with 
  a 
  glass 
  box 
  having 
  an 
  annular 
  

   space 
  containing 
  calcium 
  chloride, 
  so 
  that 
  the 
  corneal 
  layer 
  

   of 
  the 
  eye 
  should 
  be 
  in 
  perfectly 
  dry 
  air 
  and 
  free 
  from 
  the 
  

   obscuration 
  caused 
  by 
  condensation. 
  It 
  was 
  hoped 
  by 
  

   these 
  means 
  to 
  maintain 
  the 
  eye 
  in 
  a 
  frozen 
  state 
  whilst 
  

   the 
  nervous 
  and 
  other 
  tissue 
  was 
  gradually 
  brushed 
  away 
  

   from 
  the 
  back, 
  until 
  the 
  level 
  of 
  the 
  cone 
  apices 
  was 
  

   reached. 
  

  

  In 
  its 
  primary 
  object 
  the 
  apparatus 
  was 
  entirely 
  success- 
  

   ful, 
  the 
  eye 
  being 
  completely 
  frozen 
  in 
  a 
  few 
  minutes. 
  

   I 
  may 
  also 
  mention 
  that 
  both 
  carbon 
  dioxide 
  and 
  sulphur 
  

   dioxide 
  were 
  tried 
  as 
  alternative 
  freezing 
  media, 
  but 
  the 
  

   method 
  described 
  proved 
  much 
  superior 
  to 
  either. 
  

  

  Unfortunately 
  the 
  results 
  obtained 
  with 
  this 
  apparatus 
  

   were 
  not 
  commensurate 
  with 
  the 
  time 
  and 
  labour 
  expended 
  

   in 
  its 
  construction 
  and 
  use. 
  With 
  the 
  eye 
  of 
  LibellvJa 
  

   depressa 
  the 
  apices 
  of 
  the 
  cones 
  could 
  be 
  seen 
  as 
  bright 
  

   points 
  of 
  hght, 
  but 
  no 
  image, 
  inverted 
  or 
  erect, 
  could 
  

   be 
  observed. 
  A 
  pencil 
  or 
  other 
  object 
  moved 
  in 
  front 
  

   of 
  the 
  eye 
  caused 
  a 
  shadow, 
  not 
  well 
  enough 
  defined 
  

  

  