﻿Colours 
  by 
  Differential 
  Visual 
  Difusivity. 
  421 
  

  

  can 
  see 
  yellow 
  being 
  similarly 
  transmitted, 
  as 
  yellow. 
  Red 
  

   and 
  green 
  light 
  acting 
  together 
  do 
  not 
  cause 
  a 
  yellow 
  

   reaction 
  or 
  the 
  breaking 
  down 
  of 
  a 
  " 
  yellow 
  substance 
  " 
  at 
  

   the 
  surface 
  of 
  incidence. 
  Neither 
  does 
  yellow 
  light 
  cause 
  at 
  

   this 
  surface 
  the 
  breaking 
  down 
  of 
  a 
  red 
  substance 
  and 
  a 
  

   green 
  substance. 
  If 
  a 
  trichromatic 
  mechanism 
  is 
  necessary, 
  as 
  

   it 
  appears 
  to 
  be 
  from 
  the 
  phenomena 
  of 
  colour 
  mixture 
  and 
  

   colour 
  blindness, 
  then 
  the 
  present 
  experiments 
  call 
  for 
  the 
  

   additional 
  complication 
  of 
  an 
  antecedent 
  transmitting 
  process 
  

   where 
  colours 
  retain 
  their 
  physical 
  {spectral) 
  individuality. 
  

  

  Along 
  the 
  lines 
  of 
  the 
  Schenck 
  modification 
  of 
  the 
  Young- 
  

   Helmholtz 
  theory 
  the 
  diffusivity 
  results 
  might 
  be 
  interpreted 
  

   as 
  showing 
  that 
  the 
  primitive 
  yellow 
  substance 
  is 
  still 
  present 
  

   in 
  the 
  retina, 
  as 
  well 
  as 
  the 
  red 
  and 
  green 
  substances 
  which 
  

   have 
  developed 
  from 
  it. 
  Only 
  upon 
  the 
  red 
  and 
  green 
  sub- 
  

   stances 
  reaching 
  a 
  certain 
  depth 
  is 
  their 
  equivalence 
  with 
  the 
  

   yellow 
  substance 
  established. 
  It 
  may 
  be 
  imagined 
  that 
  this 
  

   isolation 
  is 
  due 
  to 
  their 
  going 
  along 
  separate 
  channels. 
  This 
  

   interpretation 
  practically 
  amounts 
  to 
  assuming 
  four 
  reacting 
  

   substances 
  in 
  the 
  retina. 
  The 
  fourth 
  is 
  superfluous 
  from 
  the 
  

   standpoint 
  of 
  colour 
  mixture, 
  and 
  so 
  appears 
  in 
  contradiction 
  

   to 
  the 
  bases 
  of 
  the 
  three-colour 
  theory, 
  which 
  the 
  Schenck 
  mo- 
  

   dification 
  is 
  not. 
  This 
  throws 
  us 
  back, 
  if 
  we 
  hold 
  to 
  the 
  three- 
  

   colour 
  theory, 
  upon 
  the 
  above 
  suggestion 
  that 
  the 
  diffusivity 
  

   phenomena 
  take 
  place 
  before 
  the 
  trichromatic 
  mechanism 
  is 
  

   reached. 
  Unless, 
  therefore, 
  a 
  theory 
  appears 
  which 
  is 
  in 
  

   accord 
  with 
  all 
  the 
  facts 
  of 
  colour 
  mixture, 
  but 
  calls 
  for 
  a 
  

   wave-length 
  basis 
  of 
  colour 
  vision, 
  these 
  new 
  facts 
  appear 
  

   merely 
  to 
  add 
  complications 
  to 
  an 
  already 
  too 
  complex 
  

   problem. 
  Unfortunately, 
  this 
  has 
  been 
  the 
  almost 
  uniform 
  

   history 
  of 
  research 
  in 
  vision. 
  

   Physical 
  Laboratory, 
  

  

  The 
  United 
  Gas 
  Improvement 
  Company, 
  

   Philadelphia, 
  Pa., 
  Sept. 
  1917. 
  

  

  Mote 
  added 
  on 
  correction 
  of 
  proof 
  . 
  

  

  The 
  transmission 
  of 
  colour 
  impressions 
  in 
  order 
  of 
  wave- 
  

   length, 
  which 
  is 
  indicated 
  as 
  taking 
  place 
  antecedent 
  to 
  the 
  

   trichromatic 
  mechanism, 
  has 
  every 
  appearance 
  of 
  being 
  a 
  

   purely 
  optical 
  effect. 
  It 
  could 
  be 
  accounted 
  for 
  by 
  the 
  

   assumption 
  of 
  strong 
  chromatic 
  aberration 
  in 
  annular 
  focussing 
  

   elements 
  in 
  the 
  retina, 
  concentrating 
  the 
  incident 
  light 
  on 
  

   photo-sensitive 
  fibres 
  lying 
  in 
  the 
  axes 
  of 
  the 
  elements. 
  The 
  

   different 
  periods 
  of 
  transmission 
  of 
  the 
  various 
  colours 
  would 
  

   then 
  be 
  due 
  to 
  the 
  different 
  distances 
  the 
  resultant 
  decompo- 
  

   sition 
  products 
  had 
  to 
  travel 
  along 
  these 
  fibres. 
  Further 
  dis- 
  

   cussion 
  of 
  this 
  possible 
  explanation 
  is 
  deferred 
  for 
  the 
  present. 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  35. 
  No. 
  209. 
  May 
  1918. 
  2 
  G 
  

  

  