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  VIII. 
  The 
  Flicker 
  Photometer 
  and 
  the 
  Iris. 
  

   By 
  H. 
  C. 
  Stevens 
  *. 
  

  

  IN 
  a 
  recent 
  number 
  of 
  this 
  Journal, 
  the 
  suggestion 
  was. 
  

   made 
  by 
  an 
  eminent 
  authority 
  f 
  that 
  the 
  " 
  physiological 
  

   process 
  " 
  which 
  affords 
  the 
  common 
  basis 
  by 
  which 
  

   coloured 
  and 
  uncoloured 
  lights 
  are 
  measured 
  by 
  means 
  of 
  

   the 
  flicker 
  phenomenon, 
  is 
  the 
  iris. 
  The 
  writer 
  states 
  that 
  

   " 
  The 
  iris 
  contracts 
  when 
  the 
  eye 
  is 
  exposed 
  to 
  a 
  bright 
  red 
  

   or 
  to 
  a 
  bright 
  green 
  light. 
  There 
  must, 
  therefore, 
  be 
  some 
  

   relative 
  brightness 
  of 
  the 
  two 
  lights 
  which 
  tends 
  equally 
  to 
  

   close 
  the 
  iris, 
  and 
  this 
  may 
  afford 
  the 
  measure 
  required. 
  The 
  

   flicker 
  adjustment 
  is 
  complete 
  when 
  the 
  iris 
  has 
  no 
  tendency 
  

   to 
  alter 
  under 
  the 
  alternating 
  illumination." 
  And 
  further,. 
  

   " 
  It 
  is 
  clear, 
  I 
  think, 
  that 
  we 
  have 
  here 
  a 
  common 
  element 
  

   in 
  variously 
  coloured 
  lights, 
  such 
  as 
  might 
  serve 
  as 
  the 
  basis 
  

   of 
  coloured 
  photometry." 
  It 
  is 
  undoubtedly 
  true 
  that 
  the 
  

   iris 
  responds 
  differently 
  to 
  lights 
  of 
  different 
  intensities.. 
  

   Indeed, 
  Haycraft 
  % 
  has 
  shown 
  that 
  the 
  diameter 
  of 
  the 
  

   pupil, 
  as 
  measured 
  by 
  instantaneous 
  photograms, 
  is 
  a 
  function 
  

   of 
  the 
  intensity 
  of 
  the 
  light 
  falling 
  upon 
  the 
  eye. 
  The 
  

   brightnesses 
  of 
  coloured 
  lights 
  determined 
  by 
  this 
  method 
  

   agree 
  tolerably 
  well 
  with 
  those 
  obtained 
  by 
  the 
  flicker 
  photo- 
  

   meter. 
  It 
  might 
  well 
  seem, 
  therefore, 
  that 
  the 
  iris 
  were 
  

   indeed 
  the 
  "physiological 
  process" 
  which 
  serves 
  as 
  the 
  

   common 
  element 
  in 
  the 
  measurement 
  of 
  intensities 
  by 
  the 
  

   flicker 
  photometer. 
  

  

  The 
  hypothesis 
  is 
  susceptible 
  of 
  an 
  experimental 
  test 
  such 
  

   as 
  to 
  leave 
  no 
  doubt 
  as 
  to 
  the 
  role 
  played 
  by 
  the 
  iris 
  in 
  the 
  

   flicker 
  phenomenon. 
  By 
  the 
  simple 
  expedient 
  of 
  paralysing 
  

   with 
  atropin 
  the 
  musculus 
  sphincter 
  papilla?, 
  an 
  iris 
  is 
  secured 
  

   which 
  is 
  entirely 
  immobile 
  in 
  the 
  presence 
  of 
  lights 
  of 
  

   different 
  intensities. 
  Thus 
  one 
  is 
  able 
  by 
  means 
  of 
  an 
  

   experimentum 
  crucis 
  to 
  eliminate 
  from 
  consideration 
  the 
  

   muscular 
  action 
  of 
  the 
  iris, 
  according 
  to 
  the 
  logical 
  principle 
  

   of 
  the 
  method 
  of 
  difference. 
  If 
  the 
  flicker 
  phenomenon 
  

   persists 
  in 
  an 
  eye 
  with 
  a 
  paralysed 
  iris, 
  obviously 
  some 
  other 
  

   physiological 
  process 
  must 
  be 
  sought 
  as 
  the 
  cause 
  of 
  the 
  

   phenomenon. 
  Inasmuch 
  as 
  both 
  irides 
  react 
  when 
  light 
  is 
  

   thrown 
  exclusively 
  upon 
  either 
  eye, 
  by 
  virtue 
  of 
  the 
  so-called 
  

   consensual 
  reaction 
  of 
  the 
  irides, 
  it 
  is 
  obvious 
  that 
  both 
  eyes 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  t 
  Lord 
  Eayleigh, 
  ' 
  Coloured 
  Photometry/ 
  vol. 
  xxiv. 
  no. 
  140, 
  p. 
  301,, 
  

   Aug. 
  1912. 
  

  

  I 
  Schaefer, 
  ' 
  Text-book 
  of 
  Physiology,' 
  vol. 
  ii. 
  p. 
  1078. 
  

  

  