﻿196 
  0. 
  iV^. 
  Rood— 
  Flicker 
  Photometer. 
  

  

  parallel 
  to 
  the 
  line 
  where 
  the 
  prism 
  faces 
  meet 
  and 
  also 
  parallel 
  

   to 
  the 
  axis 
  of 
  the 
  instrument. 
  Its 
  distance 
  from 
  the 
  prism 
  

   was 
  16 
  centimeters. 
  

  

  The 
  observing 
  tube, 
  T, 
  had 
  a 
  length 
  of 
  21 
  and 
  a 
  diameter 
  

   of 
  4 
  centimeters. 
  It 
  was 
  placed 
  at 
  right 
  angles, 
  both 
  to 
  the 
  

   vertical 
  axis 
  of 
  the 
  prism 
  and 
  to 
  the 
  axis 
  of 
  the 
  photometer. 
  

   Both 
  ends 
  of 
  it 
  were 
  furnished 
  with 
  diaphragms 
  pierced 
  bj 
  

   apertures 
  5™™ 
  in 
  diameter; 
  the 
  end 
  next 
  to 
  the 
  cylindrical 
  lens 
  

   should 
  be 
  brought 
  as 
  near 
  to 
  the 
  lens 
  as 
  practicable. 
  The 
  

   visible 
  field 
  thus 
  obtained 
  is 
  rather 
  small, 
  but 
  quite 
  sufficient, 
  

   larger 
  fields 
  having 
  been 
  found 
  to 
  give 
  inferior 
  results, 
  par- 
  

   ticularly 
  in 
  the 
  hands 
  of 
  untrained 
  observers. 
  

  

  In 
  the 
  apparatus 
  as 
  at 
  present 
  arranged 
  a 
  small 
  electromotor, 
  

   E, 
  is 
  used 
  to 
  give 
  the 
  cylindrical 
  lens 
  the 
  proper 
  oscillatory 
  

   movement 
  through 
  the 
  intervention 
  of 
  a 
  train 
  of 
  toothed 
  

   wheels, 
  W. 
  The 
  speed 
  of 
  the 
  motor 
  is 
  regulated 
  in 
  the 
  follow- 
  

   ing 
  way 
  : 
  a 
  current 
  is 
  employed 
  more 
  than 
  sufficient 
  to 
  drive 
  

   the 
  motor 
  at 
  proper 
  speed, 
  and 
  the 
  strength 
  of 
  this 
  current 
  is 
  

   reduced 
  by 
  the 
  introduction 
  of 
  a 
  variable 
  resistance, 
  E, 
  com- 
  

   posed 
  of 
  five 
  glass 
  tubes, 
  each 
  of 
  which 
  is 
  13 
  centimeters 
  in 
  

   length 
  with 
  an 
  internal 
  diameter 
  of 
  2 
  centimeters. 
  The 
  tubes 
  

   are 
  filled 
  with 
  a 
  mixture 
  of 
  coarse 
  graphite 
  and 
  asbestos, 
  which 
  

   can 
  be 
  more 
  or 
  less 
  compressed 
  by 
  the 
  aid 
  of 
  five 
  screws 
  pro- 
  

   vided 
  with 
  plungers. 
  As 
  each 
  tube 
  carries 
  only 
  the 
  fifth 
  of 
  

   the 
  . 
  current, 
  the 
  rise 
  in 
  temperature 
  is 
  inconsiderable. 
  The 
  

   idea 
  of 
  employing 
  the 
  mixture 
  for 
  a 
  variable 
  resistance 
  was 
  

   borrowed 
  from 
  Dr. 
  Hallock. 
  In 
  this 
  way 
  the 
  speed 
  of 
  the 
  

   oscillating 
  lens 
  can 
  be 
  regulated 
  and 
  kept 
  constant 
  for 
  a 
  long 
  

   time. 
  For 
  the 
  sake 
  of 
  convenience, 
  a 
  rotating 
  card-board 
  disc, 
  

   D, 
  is 
  attached 
  to 
  W, 
  and 
  provided 
  with 
  a 
  proper 
  number 
  of 
  

   black 
  and 
  white 
  sectors. 
  Its 
  appearance 
  enables 
  the 
  observer 
  

   to 
  judge 
  whether 
  the 
  proper 
  rate 
  of 
  speed 
  is 
  being 
  employed. 
  

  

  The 
  photometer 
  is 
  most 
  sensitive, 
  that 
  is 
  the 
  flicker 
  is 
  

   strongest, 
  when 
  the 
  rate 
  of 
  oscillation 
  is 
  such 
  that 
  the 
  two 
  col- 
  

   ored 
  surfaces 
  in 
  the 
  field 
  are 
  just 
  fairly 
  blended 
  by 
  the 
  move- 
  

   ment 
  of 
  the 
  lens, 
  and 
  cannot 
  be 
  distinguished 
  separately. 
  The 
  

   rate 
  of 
  lens 
  oscillation 
  usually 
  employed 
  has 
  been 
  about 
  sixteen 
  

   per 
  second, 
  more 
  or 
  less. 
  When 
  the 
  illumination 
  is 
  feeble, 
  

   low 
  rates 
  are 
  preferable 
  ; 
  when 
  it 
  is 
  strong, 
  higher 
  rates 
  are 
  

   better, 
  and 
  some 
  persons 
  seem 
  to 
  have 
  a 
  general 
  preference 
  for 
  

   rather 
  high 
  rates, 
  which 
  while 
  diminishing 
  the 
  sensitiveness 
  

   somewhat, 
  also 
  abolish 
  fiicker 
  due 
  to 
  imperfection 
  in 
  the 
  prism. 
  

  

  Finally, 
  movable 
  screens 
  of 
  black 
  pasteboard 
  are 
  arranged 
  

   around 
  the 
  prism, 
  observing 
  tube, 
  and 
  partially 
  around 
  the 
  

   head 
  of 
  the 
  observer, 
  in 
  such 
  a 
  way 
  that 
  all 
  extraneous 
  light 
  is 
  

   cut 
  off. 
  

  

  