﻿234 
  Scientific 
  Intelligence, 
  

  

  darkness 
  increases 
  it. 
  Or 
  that, 
  as 
  a 
  rule, 
  the 
  eye 
  is 
  less 
  sensitive 
  

   in 
  the 
  morning 
  than 
  at 
  night. 
  

  

  (3) 
  That 
  for 
  the 
  less 
  sensitive 
  condition, 
  the 
  minimum 
  tem- 
  

   perature 
  of 
  visibility 
  for 
  the 
  surface 
  of 
  a 
  solid 
  is 
  about 
  470° 
  C, 
  

   but 
  that 
  this 
  may 
  be 
  much 
  reduced 
  by 
  even 
  a 
  few 
  minutes 
  in 
  a 
  

   dark 
  room. 
  

  

  (4) 
  That 
  at 
  night, 
  a 
  surface 
  at 
  a 
  temperature 
  of 
  410° 
  is 
  visible, 
  

   and 
  that 
  by 
  resting 
  the 
  eyes 
  in 
  complete 
  darkness, 
  this 
  may 
  be 
  

   reduced 
  to 
  as 
  low 
  as 
  370° 
  nearly, 
  below 
  which 
  apparently 
  one 
  

   cannot 
  go, 
  since 
  ] 
  minutes' 
  rest 
  appears 
  to 
  be 
  almost 
  as 
  effica- 
  

   cious 
  as 
  3 
  hours'. 
  

  

  (5) 
  That 
  different 
  people's 
  eyes 
  (of 
  no 
  special 
  or 
  known 
  de- 
  

   parture 
  from 
  normality) 
  differ 
  somewhat 
  in 
  their 
  'minimum 
  tem- 
  

   perature 
  of 
  visibility,' 
  but 
  probably 
  not 
  to 
  any 
  great 
  extent, 
  if 
  

   tested 
  under 
  the 
  same 
  conditions 
  as 
  to 
  preparation, 
  etc. 
  . 
  . 
  . 
  

  

  The 
  loss 
  of 
  distinct 
  color 
  at 
  the 
  low 
  temperatures 
  is 
  very 
  strik- 
  

   ing; 
  the 
  appearance 
  to 
  the 
  author, 
  and 
  to 
  most 
  of 
  the 
  observers, 
  

   has 
  absolutely 
  nothing 
  of 
  red 
  in 
  it, 
  but 
  is 
  like 
  a 
  white 
  mist 
  — 
  the 
  

   nearest 
  comparison 
  that 
  can 
  be 
  made. 
  

  

  In 
  the 
  morning 
  observations, 
  however, 
  when 
  the 
  strip 
  disap- 
  

   peared 
  at 
  from 
  460° 
  to 
  470°, 
  the 
  last 
  appearance 
  was 
  distinctly 
  

   reddish 
  ; 
  and 
  this 
  agrees 
  with 
  one 
  observation 
  noted 
  at 
  night, 
  

   when 
  after 
  getting 
  the 
  visibility 
  critical-point 
  at 
  about 
  390° 
  C, 
  

   the 
  temperature 
  was 
  raised 
  until 
  one 
  could 
  declare 
  for 
  certain 
  

   that 
  the 
  light 
  looked 
  red 
  : 
  it 
  was 
  then 
  found 
  to 
  be 
  449°. 
  

  

  ' 
  Of 
  course, 
  in 
  all 
  the 
  observations, 
  the 
  luminous 
  area 
  was 
  most 
  

   distinctly 
  seen 
  by 
  somewhat 
  averting 
  the 
  gaze 
  from 
  it 
  ; 
  generally 
  

   it 
  was 
  found 
  best 
  to 
  look 
  in 
  the 
  direction 
  of 
  either 
  far 
  upper 
  

   corner 
  of 
  the 
  enclosure. 
  

  

  As 
  already 
  mentioned, 
  most 
  of 
  the 
  observers 
  pronounced 
  the 
  

   appearance 
  at 
  the 
  critical-point 
  to 
  be 
  that 
  of 
  a 
  ' 
  whitish 
  mist 
  ;' 
  

   one, 
  however, 
  thought 
  he 
  saw 
  a 
  slight 
  ' 
  lilac 
  tinge 
  ' 
  in 
  it 
  ; 
  and 
  

   ' 
  Case 
  G 
  ' 
  declared 
  it 
  to 
  be 
  decidedly 
  yellow, 
  which 
  is 
  interest- 
  

   ing, 
  because 
  to 
  him 
  a 
  red 
  mark 
  on 
  white 
  paper 
  (such 
  as 
  a 
  pip 
  on 
  

   a 
  card 
  belonging 
  to 
  one 
  of 
  the 
  red 
  suits 
  of 
  a 
  pack) 
  appears 
  yel- 
  

   low, 
  by 
  artificial 
  light 
  at 
  night. 
  

  

  In 
  one 
  experiment 
  a 
  plate 
  of 
  glass, 
  -J 
  inch 
  thick, 
  and 
  in 
  another 
  

   a 
  layer 
  of 
  water, 
  ■£• 
  inch 
  thick, 
  were 
  inserted 
  between 
  the 
  strip 
  

   and 
  the 
  eye, 
  without 
  making 
  the 
  slightest 
  difference 
  in 
  the 
  phe- 
  

   nomena; 
  showing 
  (1) 
  that 
  the 
  point 
  where 
  these 
  substances 
  

   begin 
  to 
  be 
  more 
  or 
  less 
  opaque 
  to 
  infra-red 
  radiation 
  had 
  not 
  

   been 
  reached 
  ; 
  (2) 
  that 
  the 
  small 
  difference 
  in 
  intensity 
  produced 
  

   by 
  their 
  insertion 
  had 
  no 
  appreciable 
  effect. 
  This 
  last 
  conclusion 
  

   is 
  far 
  more 
  strongly 
  borne 
  out 
  by 
  the 
  equality 
  of 
  temperature 
  in 
  

   the 
  case 
  of 
  the 
  bare 
  metallic 
  and 
  the 
  black 
  surfaces, 
  and 
  indicates 
  

   that 
  in 
  all 
  the 
  cases 
  it 
  was 
  wave-length, 
  and 
  not 
  intensity, 
  which 
  

   was 
  determinative 
  of 
  visibility, 
  so 
  disposing 
  of 
  the 
  possible 
  objec- 
  

   tion 
  that 
  the 
  difference 
  between 
  ' 
  morning 
  ' 
  and 
  ' 
  evening 
  ' 
  might 
  

   be 
  due 
  merely 
  to 
  the 
  state 
  of 
  enlargement 
  of 
  the 
  pupil 
  of 
  the 
  

   eye, 
  which 
  would 
  naturally 
  be 
  more 
  contracted 
  at 
  the 
  one 
  time 
  

  

  