﻿176 
  Mr. 
  Prentice 
  Reeves 
  on 
  the 
  

  

  the 
  quality 
  of 
  the 
  light 
  through 
  this 
  part 
  of 
  the 
  system. 
  Sj. 
  

   is 
  a 
  gas-filled 
  tungsten 
  lamp 
  the 
  light 
  from 
  which 
  passes 
  

   through 
  a 
  daylight 
  filter, 
  F, 
  and 
  by 
  means 
  of 
  the 
  collimating 
  

   lens 
  Ci 
  strikes 
  the 
  matte 
  surface 
  on 
  the 
  Whitman 
  disk 
  "W. 
  

   This 
  disk, 
  shown 
  in 
  fig. 
  1 
  a, 
  is 
  rotated 
  hy 
  a 
  motor 
  belted 
  to 
  

   M 
  and 
  turns 
  so 
  that 
  at 
  one 
  instant 
  a 
  reflecting 
  quadrant 
  

   sends 
  a 
  beam 
  of 
  standard 
  white 
  light 
  from 
  Si 
  to 
  the 
  eye, 
  

   and 
  the 
  next 
  instant 
  a 
  blank 
  quadrant 
  allows 
  the 
  coloured 
  

   light 
  through 
  P 
  to 
  reach 
  the 
  eye. 
  The 
  eye 
  sees 
  the 
  light 
  

   image 
  at 
  2 
  , 
  which 
  is 
  screened 
  down 
  so 
  as 
  to 
  restrict 
  vision 
  

   to 
  the 
  fovea, 
  and 
  an 
  artificial 
  pupil 
  A 
  was 
  used. 
  

  

  The 
  white 
  light 
  source 
  S 
  x 
  was 
  regulated 
  so 
  as 
  to 
  give 
  an 
  

   illumination 
  of 
  13 
  foot 
  candles 
  at 
  W, 
  and 
  was 
  kept 
  constant 
  

   at 
  this 
  intensity 
  by 
  means 
  of 
  connexions 
  through 
  a 
  Wolff: 
  

   potentiometer 
  and 
  a 
  sensitive 
  galvanometer. 
  With 
  both 
  

   light 
  sources 
  constant 
  and 
  three 
  independent 
  series 
  taken 
  on 
  

   different 
  days, 
  we 
  may 
  safely 
  assume 
  the 
  resultant 
  average 
  

   curves 
  to 
  be 
  representative 
  of 
  the 
  observers. 
  

  

  A 
  constant 
  width 
  of 
  slit 
  was 
  used 
  throughout. 
  In 
  order 
  

   to 
  determine 
  the 
  necessary 
  slit-width 
  correction, 
  the 
  wave- 
  

   length 
  interval 
  corresponding 
  to 
  the 
  width 
  of 
  the 
  slit, 
  as 
  one 
  

   edge 
  of 
  the 
  image 
  is 
  moved 
  across 
  the 
  field, 
  was 
  determined 
  

   throughout 
  the 
  spectrum 
  by 
  sighting 
  on 
  certain 
  lines 
  of 
  mer- 
  

   cury, 
  hydrogen, 
  and 
  helium. 
  The 
  relative 
  slit- 
  width 
  for 
  any 
  

   wave-length 
  thus 
  determined 
  multiplied 
  by 
  the 
  corresponding 
  

   acetylene 
  emission 
  gave 
  the 
  value 
  of 
  the 
  relative 
  energy. 
  

  

  When 
  taking 
  a 
  series 
  of 
  observations 
  the 
  necessary 
  pre- 
  

   liminary 
  adjustments 
  were 
  made, 
  the 
  movable 
  nicol 
  N 
  (fig. 
  1) 
  

   was 
  set 
  for 
  maximum 
  intensity 
  and 
  the 
  balance 
  was 
  made 
  by 
  

   shifting 
  the 
  wave-length 
  drum. 
  For 
  observations 
  between 
  

   wave-lengths 
  500 
  and 
  680 
  the 
  balance 
  was 
  made 
  by 
  setting 
  

   the 
  drum 
  and 
  moving 
  the 
  nicol. 
  An 
  electric 
  tachometer 
  

   was 
  belted 
  to 
  the 
  motor 
  and 
  the 
  speed 
  could 
  be 
  changed 
  by 
  

   variable 
  resistance. 
  As 
  all 
  observers 
  were 
  familiar 
  with 
  

   the 
  theory 
  of 
  the 
  so-called 
  " 
  critical 
  frequency 
  " 
  in 
  flicker 
  

   balances, 
  each 
  observer 
  regulated 
  the 
  speed, 
  but 
  no 
  record 
  

   was 
  kept 
  of 
  these 
  values. 
  The 
  relative 
  energy 
  for 
  a 
  given 
  

   wave-length 
  and 
  the 
  sine 
  square 
  of 
  the 
  angle 
  read 
  at 
  V 
  

   gives 
  us 
  the 
  relative 
  energy 
  for 
  equivalent 
  luminosity. 
  The 
  

   values 
  for 
  the 
  three 
  independent 
  series 
  are 
  averaged 
  and 
  

   plotted 
  for 
  each 
  individual's 
  curve. 
  From 
  this 
  average 
  curve 
  

   the 
  visibility 
  curve 
  for 
  that 
  observer 
  was 
  obtained, 
  and 
  the 
  

   visibility 
  curves 
  were 
  reduced 
  to 
  equal 
  areas 
  by 
  weighting 
  

   the 
  ordinates 
  according 
  to 
  height 
  in 
  order 
  to 
  compare 
  

   separate 
  curves 
  and 
  obtain 
  the 
  average 
  curve 
  of 
  the 
  group. 
  

   These 
  results 
  are 
  shown 
  in 
  Table 
  I., 
  and 
  fig. 
  2 
  shows 
  the 
  

  

  