﻿Chemistry 
  and 
  Physics. 
  233 
  

  

  his 
  neck 
  and 
  under 
  his 
  chin 
  so 
  that 
  not 
  a 
  ray 
  of 
  light 
  could 
  pene- 
  

   trate 
  the 
  enclosure. 
  The 
  box 
  was 
  about 
  48 
  cm 
  long, 
  30 
  broad, 
  

   and 
  22 
  high, 
  and 
  ordinarily 
  the 
  eyes, 
  in 
  making 
  an 
  observation, 
  

   would 
  be 
  about 
  30 
  cm 
  from 
  the 
  strip. 
  The 
  other 
  end 
  of 
  the 
  

   box 
  was 
  provided 
  with 
  a 
  hinged 
  shutter, 
  which 
  was 
  lifted 
  imme- 
  

   diately 
  after 
  an 
  observation 
  had 
  been 
  made, 
  for 
  the 
  purpose 
  of 
  

   noting 
  the 
  temperature 
  of 
  the 
  strip. 
  

  

  The 
  strip 
  itself 
  was 
  further 
  protected 
  from 
  draughts, 
  etc., 
  by 
  

   means 
  of 
  a 
  piece 
  of 
  brass, 
  bent 
  twice 
  at 
  right 
  angles, 
  and 
  resting 
  

   on 
  the 
  slate 
  block 
  below 
  the 
  strip, 
  as 
  in 
  the 
  calibration 
  experi- 
  

   ments. 
  The 
  angular 
  dimensions 
  of 
  the 
  surface 
  of 
  platinum, 
  as 
  

   seen 
  in 
  any 
  experiment, 
  were 
  therefore 
  : 
  

  

  Apparent 
  length 
  = 
  3° 
  49' 
  approximately 
  ; 
  

   " 
  width 
  = 
  1 
  54 
  " 
  

  

  so 
  that 
  the 
  apparent 
  area 
  subtended 
  was 
  about 
  36 
  times 
  that 
  of 
  

   the 
  full 
  moon. 
  

  

  The 
  current 
  by 
  which 
  the 
  strip 
  was 
  heated 
  ran 
  through 
  a 
  vari- 
  

   able 
  carbon 
  resistance, 
  the 
  handle 
  of 
  which 
  was 
  within 
  convenient 
  

   reach 
  of 
  the 
  observer 
  as 
  he 
  sat 
  with 
  his 
  head 
  under 
  the 
  black 
  

   cloth. 
  He 
  could 
  thus 
  alter 
  the 
  temperature 
  of 
  the 
  platinum 
  until 
  

   it 
  was 
  on 
  the 
  very 
  verge 
  of 
  invisibility, 
  a 
  very 
  small 
  fraction 
  of 
  a 
  

   turn 
  being 
  then 
  sufficient 
  to 
  produce 
  utter 
  darkness 
  where 
  before 
  

   the 
  area 
  of 
  faint 
  light 
  had 
  been. 
  A 
  contact-breaker 
  was 
  also 
  

   within 
  convenient 
  reach, 
  so 
  that 
  the 
  current 
  could 
  be 
  broken 
  or 
  

   made 
  at 
  pleasure, 
  and 
  the 
  objective 
  reality 
  of 
  the 
  faint 
  luminosity 
  

   at 
  the 
  limiting-point 
  thus 
  demonstrated. 
  When 
  he 
  was 
  satisfied 
  

   that 
  the 
  limiting 
  point 
  had 
  been 
  reached 
  the 
  hinged 
  end 
  of 
  the 
  

   box 
  was 
  opened, 
  a 
  beam 
  of 
  light 
  sent 
  to 
  the 
  mirror 
  connected 
  

   with 
  the 
  strip, 
  and 
  the 
  deflection, 
  giving 
  the 
  temperature, 
  read 
  on 
  

   the 
  scale. 
  The 
  possible- 
  error 
  in 
  the 
  estimation 
  of 
  the 
  absolute 
  

   value 
  of 
  the 
  temperature 
  may 
  be 
  taken 
  as 
  certainly 
  not 
  more 
  

   than 
  2°. 
  . 
  . 
  . 
  

  

  The 
  general 
  conclusions 
  reached 
  are 
  as 
  follows 
  : 
  — 
  

  

  " 
  (1) 
  That 
  the 
  minimum 
  temperature 
  of 
  visibility 
  is 
  the 
  same 
  

   for 
  a 
  bright 
  polished 
  metallic 
  surface 
  as 
  for 
  one 
  covered 
  with 
  

   lampblack, 
  although 
  the 
  intensity 
  of 
  the 
  radiation 
  in 
  the 
  two 
  

   cases 
  may 
  be 
  different. 
  

  

  This 
  result 
  may 
  at 
  first 
  be, 
  to 
  some, 
  unexpected, 
  but 
  a 
  little 
  

   consideration 
  will 
  show 
  that 
  it 
  might 
  have 
  been, 
  d 
  priori, 
  antici- 
  

   pated. 
  For 
  probably 
  temperature 
  governs 
  the 
  highest 
  wave- 
  

   length 
  from 
  a 
  radiating 
  body, 
  and 
  wave-length 
  governs 
  visibility, 
  

   at 
  least 
  after 
  an 
  extremely 
  small 
  intensity 
  of 
  radiation 
  has 
  been 
  

   passed. 
  

  

  (2) 
  That 
  the 
  visible 
  limit 
  at 
  the 
  red 
  end 
  of 
  the 
  spectrum 
  varies 
  

   greatly 
  for 
  a 
  normal 
  eye, 
  according 
  to 
  its 
  state 
  of 
  preparation, 
  

   i. 
  e. 
  according 
  to 
  the 
  intensity 
  of 
  the 
  light 
  in 
  which 
  the 
  observer 
  

   has 
  been 
  before 
  making 
  the 
  observation. 
  . 
  . 
  . 
  

  

  Speaking 
  generally, 
  we 
  may 
  say 
  that 
  a 
  bright 
  light 
  diminishes 
  

   the 
  sensitiveness 
  of 
  the 
  eye 
  to 
  radiation 
  of 
  low 
  frequency 
  ; 
  that 
  

  

  Am. 
  Jour. 
  Sci— 
  Third 
  Series, 
  Vol. 
  XLIX, 
  No. 
  291.— 
  Maroh, 
  1894. 
  

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