﻿194 
  O. 
  JSf. 
  Rood— 
  Flicker 
  Photometer. 
  

  

  Art. 
  XIX. 
  — 
  On 
  the 
  Flicker 
  Photometer 
  ; 
  by 
  Ogden 
  N. 
  

   Rood, 
  Professor 
  of 
  Physics 
  in 
  Columbia 
  University. 
  

  

  In 
  the 
  Septenriber 
  number 
  of 
  this 
  Journal, 
  1893, 
  I 
  published 
  

   an 
  account 
  of 
  " 
  A. 
  photometric 
  method 
  which 
  is 
  independent 
  

   of 
  color," 
  the 
  principle 
  being, 
  that 
  when 
  two 
  colored 
  surfaces 
  

   are 
  presented 
  in 
  rapid 
  succession 
  to 
  the 
  eye, 
  the 
  resulting 
  flicker 
  

   vanishes 
  when 
  they 
  have 
  the 
  same 
  luminosity. 
  My 
  experi- 
  

   ments 
  were 
  afterward 
  repeated 
  and 
  amplified 
  by 
  Dr. 
  F. 
  L. 
  

   Tufts, 
  who 
  obtained 
  confirmatory 
  results.* 
  In 
  my 
  paper 
  I 
  also 
  

   remarked, 
  that 
  " 
  there 
  does 
  not 
  seem 
  to 
  be 
  any 
  reason 
  why 
  this 
  

   process, 
  with 
  the 
  aid 
  of 
  well 
  known 
  optical 
  devices, 
  should 
  not 
  

   be 
  applied 
  to 
  ordinary 
  photometric 
  work, 
  or 
  to 
  such 
  as 
  is 
  car- 
  

   ried 
  on 
  in 
  the 
  interior 
  of 
  the 
  observing 
  telescope 
  of 
  a 
  spectro- 
  

   scope." 
  

  

  In 
  1896 
  Prof. 
  F. 
  P. 
  Whitman 
  devised 
  a 
  photometer 
  with 
  a 
  

   revolving 
  card-board 
  disc, 
  in 
  which 
  the 
  proposed 
  flicker 
  

   method 
  was 
  utilized, 
  and 
  which 
  gave 
  satisfactory 
  results.f 
  On 
  

   IN'ov. 
  17th, 
  1896, 
  I 
  read 
  a 
  paper 
  before 
  the 
  National 
  Academy 
  

   of 
  Sciences, 
  in 
  which 
  were 
  described 
  four 
  methods 
  in 
  which 
  

   the 
  flicker 
  process 
  could 
  be 
  employed 
  photometrically: 
  {a) 
  by 
  

   the 
  use 
  of 
  convex 
  or 
  concave 
  lenses 
  oscillating 
  at 
  proper 
  rates, 
  

   {b) 
  by 
  a 
  vibrating 
  plane 
  mirror, 
  {&) 
  by 
  a 
  revolving 
  glass 
  prism 
  

   of 
  small 
  angle, 
  [d) 
  by 
  a 
  revolving 
  plane 
  mirror 
  set 
  excentrically 
  

   on 
  its 
  axis. 
  Apparatus 
  embodying 
  these 
  ideas 
  had 
  at 
  that 
  time 
  

   been 
  constructed 
  and 
  used 
  by 
  me. 
  In 
  1897 
  I 
  constructed 
  a 
  

   complete 
  photometer, 
  using 
  an 
  oscillating 
  convex 
  cylindrical 
  

   lens, 
  and 
  made 
  many 
  measurements 
  with 
  it. 
  The 
  instrument 
  

   was 
  briefly 
  described 
  in 
  Science, 
  June 
  3d, 
  1898, 
  and 
  a 
  week 
  

   later, 
  in 
  the 
  same 
  periodical, 
  appeared 
  a 
  short 
  account 
  of 
  some 
  

   quantitative 
  observations 
  made 
  with 
  it 
  on 
  color-blindness. 
  

   Since 
  that 
  time, 
  the 
  convex 
  cylindrical 
  lens 
  has 
  been 
  replaced 
  

   advantageously 
  by 
  one 
  that 
  is 
  concave^ 
  and 
  the 
  paper 
  prism 
  

   previously 
  used 
  by 
  one 
  of 
  plaster 
  of 
  Paris. 
  The 
  fleld 
  has 
  

   also 
  been 
  very 
  much 
  contracted, 
  and 
  other 
  improvements 
  

   introduced, 
  particularly 
  in 
  the 
  matter 
  of 
  uniformit}^ 
  of 
  rate 
  of 
  

   oscillation 
  and 
  choice 
  of 
  such 
  rate. 
  

  

  The 
  general 
  idea 
  of 
  the 
  photometer 
  is 
  that 
  the 
  differently 
  

   colored 
  beams 
  of 
  light 
  traversing 
  its 
  axis 
  should 
  illuminate 
  

   the 
  two 
  surfaces 
  of 
  a 
  rectangular 
  prism, 
  P, 
  facing 
  the 
  eye, 
  and 
  

   that 
  by 
  the 
  oscillations 
  of 
  a 
  cylindrical 
  concave 
  lens, 
  C, 
  in 
  front 
  

   of 
  the 
  prism, 
  its 
  illuminated 
  surfaces 
  should 
  alternately 
  and 
  in 
  

  

  * 
  Transactions 
  of 
  the 
  New 
  York 
  Academy 
  of 
  Sciences, 
  xvi, 
  190-212, 
  April, 
  1897. 
  

   t 
  Physical 
  Review, 
  vol. 
  iii, 
  No. 
  16, 
  Jan.-Feb., 
  1896. 
  

  

  