﻿Theory 
  and 
  Planetary 
  Temperatures. 
  473 
  

  

  the 
  brightness 
  to 
  be 
  nearly 
  uniform. 
  Mean 
  altitude 
  of 
  

   sun 
  = 
  28°. 
  

  

  Sky. 
  13-0 
  mag. 
  

  

  Snow 
  117 
  mag. 
  

  

  Sky 
  

  

  12-8 
  mag. 
  

  

  Snow 
  11*8 
  mag. 
  

  

  13-4 
  

  

  in 
  114 
  

  

  

  13-0 
  

  

  in 
  116 
  

  

  132 
  

  

  Sun, 
  11-5 
  

  

  

  131 
  

  

  Sun, 
  11-7 
  

  

  13-0 
  

  

  11-3 
  

  

  

  13-1 
  

  

  118 
  

  

  128 
  

  

  11-8 
  

  

  

  130 
  

  

  IIS 
  

  

  Mean 
  13"08 
  

  

  11-54 
  

  

  

  13-00 
  

  

  11-74 
  

  

  Difference 
  of 
  magnitude, 
  1st 
  set, 
  Sky=+l 
  - 
  o4 
  mag-. 
  

   2nd 
  „ 
  „ 
  4-1-26 
  „ 
  

  

  The 
  mean 
  difference 
  is 
  sky 
  = 
  +1*4 
  magnitude, 
  or 
  the 
  sky 
  

   had 
  3'63 
  times 
  the 
  brightness 
  of 
  sunlit 
  snow. 
  

   With 
  a 
  clear 
  blue 
  sky, 
  I 
  found 
  for 
  — 
  

  

  Sun's 
  altitude 
  =30°, 
  snow 
  1*6 
  tiroes 
  brightness 
  of 
  sky, 
  

   „ 
  ,, 
  =10°, 
  sky 
  1*35 
  ,, 
  „ 
  snow. 
  

  

  It 
  appears 
  from 
  these 
  comparisons 
  that 
  the 
  discrepancy 
  

   between 
  the 
  brightness 
  of 
  the 
  sky 
  and 
  of 
  a 
  white 
  sun-illumined 
  

   surface 
  is 
  not 
  as 
  great 
  as 
  one 
  is 
  apt 
  to 
  guess. 
  

  

  If 
  one 
  stands 
  with 
  his 
  back 
  to 
  the 
  sun 
  and 
  compares 
  the 
  

   light 
  reflected 
  from 
  the 
  sky 
  with 
  that 
  from 
  a 
  piece 
  of 
  white 
  

   paper, 
  the 
  former 
  comes 
  principally 
  from 
  remote 
  depths 
  of 
  

   air, 
  and 
  has 
  suffered 
  further 
  depletion 
  in 
  returning 
  thence 
  

   to 
  the 
  eye. 
  It 
  is 
  therefore 
  riot 
  to 
  be 
  expected 
  that 
  the 
  air 
  

   should 
  reflect 
  as 
  well 
  as 
  a 
  piece 
  of 
  white 
  paper 
  held 
  close 
  to 
  

   the 
  eye. 
  

  

  It 
  will 
  no 
  doubt 
  be 
  agreed 
  that 
  the 
  depletion 
  suffered 
  by 
  

   the 
  sun's 
  rays 
  in 
  the 
  middle 
  of 
  the 
  visible 
  spectrum 
  is 
  not 
  

   due 
  to 
  telluric 
  absorption-bands, 
  but 
  results 
  solely 
  from 
  

   diffraction 
  and 
  scattering 
  reflexion. 
  I 
  measured 
  the 
  light 
  

   from 
  the 
  sun, 
  when 
  it 
  was 
  about 
  to 
  set, 
  and 
  compared 
  its 
  

   intensity 
  in 
  the 
  green 
  of 
  the 
  spectrum 
  with 
  that 
  which 
  the 
  

   same 
  region 
  had 
  when 
  the 
  sun 
  was 
  30° 
  high. 
  The 
  intensity 
  

   of 
  green 
  light 
  from 
  the 
  nearly 
  setting 
  sun 
  was 
  0*0000065 
  

   times 
  that 
  at 
  the 
  higher 
  altitude. 
  The 
  relative 
  air-masses 
  

   were 
  2*0 
  and 
  24*4, 
  that 
  at 
  the 
  zenith 
  being 
  unity. 
  The 
  

   zenithal 
  transmission 
  corresponding 
  to 
  that 
  for 
  air-mass 
  24*4 
  

   was 
  therefore 
  

  

  (0-000 
  0065) 
  sfi^o 
  =0-587, 
  

  

  and 
  the 
  transmission 
  at 
  a 
  middle 
  altitude 
  of 
  30° 
  was 
  

   (0'587) 
  2 
  = 
  0*345, 
  or 
  the 
  depletion 
  equal 
  to 
  0"655, 
  which 
  must 
  

  

  