﻿472 
  Mr. 
  F. 
  W. 
  Very 
  on 
  the 
  Greenhouse 
  

  

  Professor 
  Poynting 
  objects* 
  to 
  Lowell's 
  value 
  for 
  the 
  

   reflexion 
  of 
  light 
  by 
  the 
  air 
  as 
  being 
  too 
  large. 
  " 
  On 
  another 
  

   point/' 
  he 
  says, 
  " 
  common 
  observation 
  appears, 
  at 
  any 
  rate 
  at 
  

   first 
  sight, 
  to 
  contradict 
  Professor 
  Lowell. 
  He 
  assumes 
  that 
  

   the 
  loss 
  in 
  the 
  visible 
  spectrum 
  radiation 
  in 
  its 
  passage 
  through 
  

   the 
  atmosphere 
  is 
  practically 
  all 
  due 
  to 
  reflexion, 
  and 
  he 
  puts 
  

   it 
  down 
  as 
  about 
  0*7 
  of 
  the 
  whole 
  in 
  clear 
  sky. 
  If 
  this 
  were 
  

   true 
  the 
  reflexion 
  from 
  the 
  sky 
  opposite 
  to 
  the 
  sun 
  would 
  I 
  

   think 
  be 
  vastly 
  greater 
  than 
  it 
  is. 
  White 
  cardboard 
  reflects 
  

   diffusely 
  about 
  07 
  of 
  sunlight. 
  But 
  when 
  a 
  piece 
  of 
  white 
  

   cardboard 
  is 
  exposed 
  normally 
  to 
  the 
  sun's 
  rays, 
  it 
  is 
  several 
  

   times 
  brighter 
  than 
  the 
  cloudless 
  sky." 
  

  

  This 
  is 
  true 
  if 
  the 
  white 
  surface 
  is 
  compared 
  with 
  a 
  deep- 
  

   blue 
  sky 
  far 
  away 
  from 
  the 
  sun 
  ; 
  but 
  it 
  is 
  not 
  always 
  true 
  for 
  

   the 
  sky 
  near 
  the 
  sun, 
  and 
  the 
  upward 
  reflexion 
  towards 
  the 
  

   sun 
  is 
  probably 
  nearly 
  proportional 
  to 
  the 
  downward 
  reflexion 
  

   when 
  the 
  sky 
  is 
  only 
  slightly 
  hazy. 
  I 
  tried 
  the 
  following- 
  

   experiment 
  on 
  a 
  day 
  with 
  hazy 
  blue 
  sky, 
  whitish 
  near 
  the 
  

   horizon: 
  — 
  The 
  sun 
  being 
  at 
  an 
  altitude 
  of 
  20° 
  and 
  shining 
  on 
  

   fresh 
  snow, 
  I 
  measured 
  with 
  a 
  wedge-photometer 
  the 
  visibility 
  

   (a) 
  of 
  a 
  wide 
  area 
  of 
  bright 
  background, 
  and 
  (7>) 
  that 
  of 
  a 
  

   narrow 
  dark 
  bar 
  projected 
  on 
  the 
  same 
  background, 
  getting 
  for 
  

  

  Sky 
  near 
  the 
  Sun 
  (a) 
  13-5 
  mag. 
  (b) 
  13*0, 
  

  

  Sky 
  at 
  horizon^ 
  azimuth 
  45° 
  from 
  sun 
  . 
  . 
  120 
  11*5, 
  

  

  Snow 
  in 
  sunshine 
  12-0 
  11*5. 
  

  

  Taking 
  the 
  ratio 
  of 
  brightness 
  corresponding 
  to 
  the 
  differ- 
  

   ence 
  of 
  magnitudes, 
  the 
  sky 
  near 
  the 
  sun 
  was 
  four 
  times 
  as 
  

   bright 
  as 
  the 
  sky 
  at 
  the 
  horizon 
  ; 
  but 
  the 
  latter 
  was 
  as 
  bright 
  

   as 
  the 
  sunlit 
  snow. 
  I 
  confess 
  that 
  I 
  was 
  surprised 
  at 
  the 
  last 
  

   equality, 
  for 
  the 
  sunlit 
  snow 
  gave 
  me 
  the 
  impression 
  of 
  

   greater 
  brightness 
  ; 
  but 
  this 
  impression 
  is 
  no 
  doubt 
  due 
  to 
  

   the 
  absence 
  of 
  colour 
  in 
  the 
  snow, 
  while 
  the 
  blue 
  tint 
  of 
  the 
  

   sky 
  gives 
  one 
  a 
  conception 
  that 
  the 
  sky-light 
  is 
  dimmer 
  than 
  

   it 
  really 
  is. 
  The 
  sky 
  at 
  the 
  horizon 
  was 
  half 
  a 
  magnitude 
  

   brighter, 
  or 
  1'6 
  times 
  as 
  bright 
  as 
  the 
  sky 
  at 
  an 
  altitude 
  of 
  

   20° 
  and 
  azimuth 
  90° 
  from 
  the 
  sun. 
  

  

  A 
  repetition 
  of 
  the 
  experiment 
  on 
  another 
  occasion 
  gave 
  

   me 
  the 
  following 
  results 
  : 
  — 
  The 
  sky 
  was 
  a 
  pale 
  blue 
  with 
  a 
  

   few 
  fracto- 
  cumuli, 
  but 
  without 
  appreciable 
  cirrus-haze. 
  The 
  

   observations 
  were 
  made 
  through 
  a 
  slit 
  3 
  cm. 
  wide, 
  viewed 
  

   at 
  a 
  distance 
  of 
  1 
  metre 
  by 
  means 
  of 
  the 
  wedge-photometer. 
  

   The 
  point 
  in 
  the 
  sky 
  selected 
  for 
  measurement 
  was 
  distant 
  

   from 
  the 
  sun 
  about 
  four 
  diameters, 
  or 
  far 
  enough 
  away 
  for 
  

   * 
  Phil. 
  Mag. 
  [6] 
  vol. 
  xiv. 
  p. 
  750. 
  

  

  