﻿0. 
  N. 
  Rood— 
  Flicker 
  Photometer, 
  197 
  

  

  The 
  mode 
  of 
  making 
  the 
  observations 
  is 
  much 
  the 
  same 
  as 
  

   with 
  ordinary 
  photometers 
  ; 
  by 
  a 
  preliminary 
  experiment 
  the 
  

   position 
  of 
  flicker 
  disappearance 
  is 
  approximately 
  ascertained, 
  

   and 
  then 
  the 
  moving 
  lamp 
  is 
  displaced 
  till 
  a 
  distinct 
  flicker 
  is 
  

   noticed 
  ; 
  it 
  is 
  then 
  brought 
  up 
  till 
  this 
  disappears, 
  and 
  the 
  

   operation 
  repeated 
  in 
  the 
  reversed 
  sense, 
  both 
  observations 
  

   being 
  recorded. 
  From 
  time 
  to 
  time 
  wider 
  excursions 
  are 
  

   made, 
  to 
  avoid 
  the 
  danger 
  of 
  " 
  falling 
  into 
  a 
  rut." 
  Two 
  fldu- 
  

   cial 
  marks 
  are 
  finally 
  impressed 
  on 
  the 
  fillet 
  of 
  paper, 
  which 
  is 
  

   removed 
  and 
  read 
  with 
  a 
  lens 
  and 
  millimeter 
  scale. 
  Persons 
  

   trained 
  in 
  laboratory 
  work 
  usually 
  obtain 
  good 
  results 
  in 
  their 
  

   first 
  trials 
  ; 
  this 
  was 
  the 
  case 
  with 
  Miss 
  Furness, 
  Dr. 
  Dennett, 
  

   Professor 
  Hallock 
  and 
  some 
  of 
  the 
  laboratory 
  assistants. 
  In 
  

   other 
  cases 
  a 
  certain 
  amount 
  of 
  training 
  was 
  found 
  necessary, 
  

   which 
  seemed 
  to 
  be 
  due 
  to 
  the 
  fact 
  that 
  the 
  attention 
  of 
  the 
  

   observer 
  was 
  allowed 
  to 
  wander 
  from 
  flicker-perception 
  to 
  

   color-perception, 
  and 
  to 
  the 
  changes 
  in 
  it 
  due 
  to 
  the 
  position 
  of 
  

   the 
  movable 
  lamp. 
  With 
  the 
  small 
  field 
  now 
  employed, 
  there 
  

   is 
  no 
  eye-fatigue, 
  except 
  that 
  incident 
  on 
  looking 
  steadily 
  into 
  

   any 
  kind 
  of 
  tube 
  for 
  some 
  minutes. 
  

  

  In 
  order 
  to 
  furnish 
  an 
  idea 
  of 
  the 
  degree 
  of 
  accuracy 
  attain- 
  

   able, 
  I 
  give 
  below 
  sets 
  of 
  results 
  obtained 
  by 
  myself 
  when 
  

   using 
  three 
  pairs 
  of 
  colors, 
  red 
  and 
  blue, 
  red 
  and 
  green, 
  green 
  

   and 
  blue. 
  The 
  colored 
  light 
  was 
  furnished 
  by 
  colored 
  glass, 
  

   and 
  was 
  to 
  all 
  appearance 
  spectral 
  in 
  its 
  purity. 
  One 
  face 
  of 
  

   the 
  prism 
  being 
  illuminated 
  with 
  red 
  light 
  from 
  the 
  stationary 
  

   lamp, 
  the 
  other 
  received 
  violet-blue 
  light 
  from 
  the 
  movable 
  

   lamp, 
  which 
  was 
  adjusted 
  till 
  the 
  flicker 
  vanished, 
  and 
  its 
  

   mean 
  distance 
  from 
  the 
  edge 
  of 
  the 
  prism 
  obtained, 
  ten 
  obser- 
  

   vations 
  being 
  made. 
  For 
  certain 
  purposes 
  this 
  operation 
  was 
  

   repeated 
  on 
  seven 
  difiierent 
  days. 
  Taking 
  the 
  mean 
  distance 
  

   of 
  all 
  seven 
  results 
  as 
  correct, 
  and 
  as 
  corresponding 
  when 
  

   squared 
  to 
  100 
  per 
  cent, 
  I 
  calculated 
  the 
  deviations 
  from 
  this 
  

   in 
  each 
  of 
  the 
  seven 
  cases 
  in 
  percentages. 
  

  

  Red 
  and 
  Blue. 
  

  

  Red 
  

  

  and 
  Green. 
  

  

  Blue 
  and 
  Green. 
  

  

  Diff. 
  

  

  

  Diff. 
  

  

  Diff. 
  

  

  Dec. 
  13 
  -0-26 
  

  

  Feb. 
  

  

  2 
  -0-S9 
  

  

  Jan. 
  5 
  -1-3 
  -5 
  

  

  " 
  16 
  +1-74 
  

  

  C( 
  

  

  3 
  +2-60 
  

  

  " 
  16 
  -fO-01 
  

  

  Jan. 
  5 
  +0-4 
  

  

  u 
  

  

  11 
  -M-10 
  

  

  " 
  17 
  —1-91 
  

  

  " 
  13 
  -0-26 
  

  

  a 
  

  

  15 
  -1-69 
  

  

  " 
  18 
  +0-84 
  

  

  " 
  16 
  -1-57 
  

  

  « 
  

  

  17 
  -0-35 
  

  

  '« 
  24 
  -1-26 
  

  

  '' 
  17 
  — 
  0-26 
  " 
  20 
  —1-95 
  " 
  25 
  —1-26 
  

  

  " 
  18 
  -fO-3 
  March 
  1 
  +1-29 
  

  

  Similar 
  results 
  are 
  also 
  given 
  above 
  for 
  the 
  pair 
  red-green 
  and 
  

   green-blue. 
  No 
  account 
  is 
  taken 
  of 
  possible 
  variations 
  in 
  the 
  

  

  Am. 
  Jour. 
  Sci. 
  — 
  Fourth 
  Series, 
  Vol. 
  YIII, 
  Xo. 
  45.— 
  September, 
  1899. 
  

   14 
  

  

  