﻿0. 
  N. 
  Rood 
  — 
  Flicker 
  Photometer. 
  

  

  195 
  

  

  rapid 
  succession 
  be 
  presented 
  to 
  the 
  eye. 
  One 
  incandescent 
  

   lamp, 
  L^, 
  remains 
  stationary 
  ; 
  the 
  other 
  is 
  moved 
  by 
  the 
  observer 
  

   till 
  all 
  flicker 
  disappears, 
  when 
  a 
  reading 
  is 
  registered 
  on 
  a 
  

   fillet 
  of 
  paper, 
  which 
  is 
  not 
  inspected 
  by 
  the 
  observer 
  till 
  the 
  

   conclusion 
  of 
  the 
  operation. 
  In 
  many 
  of 
  my 
  experiments 
  the 
  

   light 
  was 
  colored 
  by 
  transmission 
  through 
  plates 
  of 
  colored 
  

   glass, 
  G, 
  G^, 
  and 
  was 
  almost 
  spectral 
  in 
  hue. 
  On 
  a 
  strip 
  of 
  

   board, 
  B, 
  8 
  c. 
  broad 
  and 
  3*3 
  m. 
  in 
  length, 
  graduated 
  in 
  centi- 
  

   meters, 
  the 
  incandescent 
  lamps 
  are 
  placed, 
  suitable 
  guides 
  

   keeping 
  them 
  in 
  the 
  axis 
  of 
  the 
  instrument. 
  One 
  of 
  the 
  lamps 
  

   can 
  be 
  moved 
  by 
  a 
  long 
  slender 
  strip 
  of 
  wood, 
  S, 
  to 
  which 
  is 
  

   attached 
  a 
  fillet 
  of 
  paper 
  to 
  receive 
  the 
  observations, 
  which 
  are 
  

   impressed 
  on 
  it 
  by 
  a 
  pin-point 
  suitably 
  arranged. 
  The 
  other 
  

   lamp 
  is 
  stationed 
  where 
  in 
  each 
  case 
  it 
  is 
  found 
  desirable. 
  

  

  The 
  prism, 
  P, 
  is 
  fastened 
  on 
  the 
  center 
  of 
  the 
  long 
  board 
  at 
  

   the 
  common 
  zero 
  point 
  of 
  the 
  two 
  sets 
  of 
  graduations, 
  and 
  the 
  

   most 
  important 
  thing 
  about 
  it 
  is, 
  that 
  its 
  two 
  faces 
  should 
  

   meet 
  in 
  an 
  invisible 
  line 
  ; 
  otherwise 
  the 
  juncture, 
  not 
  being 
  

   illuminated, 
  appears 
  in 
  the 
  field 
  as 
  a 
  vertical 
  black 
  line, 
  and 
  

   this 
  will 
  of 
  itself 
  furnish 
  a 
  faint 
  flicker, 
  that 
  becomes 
  apparent 
  

   after 
  the 
  flicker 
  from 
  the 
  faces 
  of 
  the 
  prism 
  has 
  disappeared. 
  

   Good 
  prisms, 
  free 
  from 
  this 
  defect, 
  can 
  be 
  made 
  by 
  casting 
  

   plaster 
  of 
  Paris 
  in 
  a 
  mould 
  composed 
  of 
  glass 
  plates, 
  suitably 
  

   ground 
  on 
  their 
  edges. 
  These 
  plates 
  can 
  be 
  removed 
  from 
  the 
  

   plaster 
  by 
  the 
  gentle 
  heat 
  of 
  a 
  spirit 
  lamp, 
  without 
  damage 
  to 
  

   the 
  dividing 
  line 
  between 
  the 
  faces 
  of 
  the 
  still 
  moist 
  prism. 
  

   When 
  heat 
  is 
  applied, 
  the 
  plates 
  fall 
  ofl 
  of 
  themselves. 
  With 
  

   paper 
  prisms, 
  however 
  well 
  prepared, 
  it 
  is 
  impossible 
  entirely 
  

   to 
  get 
  rid 
  of 
  the 
  black 
  dividing 
  line, 
  and 
  the 
  observer 
  is 
  

   obliged 
  to 
  rely 
  on 
  obtaining 
  a 
  minimum 
  flicker. 
  

  

  With 
  the 
  above 
  object 
  still 
  in 
  view, 
  it 
  is 
  best 
  to 
  use 
  a 
  con- 
  

   cave 
  cylindrical 
  oscillating 
  lens, 
  which 
  diminishes 
  still 
  further 
  

   the 
  bad 
  effects 
  of 
  any 
  want 
  of 
  perfection 
  in 
  the 
  prism 
  ; 
  that 
  

   employed 
  by 
  me 
  had 
  a 
  focus 
  of 
  13 
  centimeters 
  ; 
  its 
  outline 
  was 
  

   square, 
  the 
  sides 
  being 
  4 
  centimeters 
  in 
  length. 
  It 
  was 
  placed 
  

  

  