﻿Colours 
  by 
  Differential 
  Visual 
  Diffusivity. 
  419 
  

  

  with 
  other 
  colours. 
  The 
  results 
  from 
  the 
  yellow 
  light 
  expe- 
  

   riments 
  are, 
  however, 
  of 
  great 
  interest 
  from 
  the 
  standpoint 
  

   of 
  theories 
  of 
  colour 
  vision. 
  

  

  If 
  we 
  consider 
  these 
  experiments 
  without 
  reference 
  to 
  any 
  

   other 
  work 
  on 
  Colour 
  vision, 
  it 
  is 
  evident 
  that 
  the 
  explanation 
  

   called 
  for 
  is 
  one 
  in 
  which 
  wave-length 
  composition 
  rather 
  than 
  

   appearance 
  or 
  colour-sensation 
  analysis 
  determines 
  behaviour. 
  

   Recently 
  Houstoun 
  has 
  suggested 
  a 
  theory 
  of 
  colour 
  vision 
  

   according 
  to 
  which 
  colours 
  do 
  maintain 
  their 
  physical 
  wave- 
  

   length 
  identity 
  in 
  transmission 
  to 
  the 
  brain. 
  He 
  supposes 
  

   the 
  spectral 
  colours 
  cause 
  retinal 
  vibrators 
  " 
  to 
  set 
  up 
  waves 
  

   in 
  the 
  nerves 
  and 
  that 
  the 
  nerves 
  carry 
  these 
  waves 
  to 
  the 
  

   brain," 
  the 
  dominant 
  wave-length 
  of 
  the 
  transmitted 
  dis- 
  

   turbance 
  being 
  that 
  of 
  the 
  incident 
  light. 
  The 
  results 
  of 
  the 
  

   present 
  paper 
  might 
  be 
  interpreted 
  as 
  supporting 
  this 
  theory. 
  

   A 
  place 
  at 
  which 
  the 
  new 
  phenomena 
  might 
  at 
  first 
  sight 
  

   appear 
  to 
  fit 
  into 
  Houstoun's 
  theory 
  maybe 
  noted 
  as 
  suggestive. 
  

   If 
  the 
  writer 
  understands 
  the 
  theory 
  correctly, 
  it 
  calls 
  for 
  

   the 
  production 
  of 
  fluorescence 
  at 
  the 
  retina, 
  which 
  illuminates 
  

   the 
  brain 
  through 
  the 
  transparent 
  nerves 
  (for 
  the 
  " 
  waves 
  in 
  

   the 
  nerves 
  ; 
  ' 
  must, 
  because 
  of 
  their 
  frequency, 
  be 
  light 
  waves) 
  . 
  

   Now 
  it 
  is 
  one 
  of 
  the 
  elementary 
  facts 
  of 
  physical 
  optics 
  that 
  

   a 
  reduced 
  rate 
  of 
  transmission 
  of 
  light 
  waves 
  occurs 
  in 
  a 
  

   medium 
  of 
  high 
  refractive 
  index, 
  and 
  that 
  a 
  different 
  rate 
  of 
  

   transmission 
  of 
  colours, 
  in 
  the 
  order 
  of 
  wave-length, 
  occurs 
  

   in 
  a 
  medium 
  possessing, 
  as 
  most 
  transparent 
  media 
  do, 
  an 
  

   absorption-band 
  in 
  the 
  ultra-violet. 
  Tims 
  the 
  velocity 
  of 
  light 
  

   in 
  carbon 
  bisulphide 
  is 
  less 
  than 
  in 
  air, 
  and 
  red 
  light 
  travels 
  

   faster 
  than 
  blue. 
  If, 
  then, 
  to 
  the 
  transparent 
  nerves, 
  which 
  

   appear 
  to 
  be 
  a 
  part 
  of 
  Houstoun's 
  theory, 
  we 
  ascribe 
  a 
  high 
  

   refractive 
  index 
  varying 
  in 
  the 
  usual 
  manner 
  through 
  the 
  

   spectrum, 
  we 
  have 
  a 
  system 
  which 
  would 
  give 
  the 
  differential 
  

   speed 
  effects 
  found 
  by 
  the 
  present 
  experiments. 
  Unfortu- 
  

   nately 
  the 
  necessary 
  refractive 
  index 
  to 
  reduce 
  the 
  speed 
  of 
  

   light 
  to 
  the 
  order 
  of 
  magnitude 
  such 
  that 
  even 
  a 
  hundredth 
  

   of 
  a 
  second 
  lag 
  of 
  blue 
  light 
  would 
  be 
  possible 
  in 
  any 
  distance 
  

   available 
  in 
  the 
  human 
  head, 
  is 
  enormous 
  and 
  fantastic. 
  

   Apart 
  from 
  this 
  quantitative 
  difficulty 
  , 
  the 
  parallelism 
  between 
  

   the 
  behaviour 
  of 
  light 
  in 
  the 
  eye 
  and 
  in 
  the 
  highly 
  dis- 
  

   persive 
  medium 
  is 
  close. 
  There 
  is, 
  however, 
  no 
  obvious 
  

   explanation 
  along 
  this 
  line 
  of 
  the 
  variation 
  of 
  speed 
  of 
  

   transmission 
  with 
  the 
  intensity 
  of 
  the 
  stimulus. 
  

  

  Whether 
  or 
  not 
  the 
  visual 
  diffusivity 
  phenomena 
  agree 
  with 
  

   the 
  Houstoun 
  theory, 
  the 
  latter 
  requires, 
  in 
  the 
  writer's 
  

   opinion, 
  much 
  more 
  support 
  before 
  it 
  is 
  profitable 
  to 
  attempt 
  

   to 
  harmonize 
  new 
  observations 
  with 
  it. 
  The 
  objections 
  

  

  