﻿202 
  

  

  On 
  the 
  Lines 
  of 
  the 
  Solar 
  Spectrum. 
  

  

  [Jan. 
  28, 
  

  

  the 
  spectrum 
  at 
  sunset, 
  and 
  any 
  additional 
  lines 
  which 
  might 
  appear 
  in 
  

   the 
  latter 
  case 
  should 
  be 
  noted 
  with 
  reference 
  to 
  Kirchhoff's 
  map. 
  

  

  Accordingly 
  the 
  author 
  set 
  to 
  work 
  with 
  the 
  spectroscope 
  first 
  supplied 
  

   to 
  him, 
  and 
  in 
  the 
  autumns 
  of 
  1868 
  and 
  1869 
  mapped 
  the 
  differences 
  in 
  

   question 
  from 
  the 
  extreme 
  red 
  to 
  D. 
  These 
  results 
  appeared 
  in 
  the 
  

   ' 
  Proceedings 
  of 
  the 
  Royal 
  Society 
  ' 
  for 
  June 
  16, 
  1870, 
  and 
  the 
  map 
  of 
  the 
  

   spectra, 
  sun 
  high 
  and 
  sun 
  low, 
  of 
  the 
  region 
  in 
  question 
  forms 
  plate 
  1 
  

   of 
  the 
  19th 
  volume. 
  

  

  The 
  instrument 
  first 
  supplied 
  to 
  the 
  author 
  was 
  found 
  in 
  practice 
  to 
  

   be 
  of 
  insufficient 
  power 
  to 
  permit 
  of 
  ready 
  identification 
  of 
  the 
  lines 
  

   seen 
  in 
  the 
  spectrum 
  of 
  the 
  sun 
  when 
  high 
  with 
  those 
  represented 
  in 
  

   Kirchhoff's 
  map 
  ; 
  and 
  a 
  new 
  spectroscope 
  of 
  greater 
  power 
  was 
  supplied 
  

   to 
  him, 
  which 
  reached 
  him 
  at 
  the 
  end 
  of 
  the 
  year 
  1871. 
  Observa- 
  

   tions 
  for 
  a 
  continuation 
  of 
  his 
  map 
  had, 
  in 
  the 
  mean 
  time, 
  been 
  taken 
  

   with 
  the 
  old 
  instrument 
  in 
  the 
  autumns 
  of 
  1870 
  and 
  1871, 
  and 
  the 
  spec- 
  

   trum 
  mapped 
  from 
  D 
  to 
  F, 
  in 
  continuation 
  of 
  the 
  former 
  map. 
  But 
  the 
  

   new 
  instrument 
  proved 
  so 
  superior 
  to 
  the 
  old, 
  that 
  the 
  author 
  deter- 
  

   mined 
  to 
  map 
  the 
  whole 
  spectrum 
  afresh 
  from 
  observations 
  made 
  with 
  

   it, 
  using 
  the 
  former 
  maps 
  merely 
  as 
  skeleton 
  forms. 
  The 
  observations 
  

   with 
  the 
  new 
  instrument 
  were 
  carried 
  on 
  in 
  the 
  autumns 
  of 
  1872 
  and 
  

   1873, 
  and 
  the 
  map 
  now 
  presented 
  is 
  the 
  result. 
  

  

  Observations 
  were 
  also 
  made 
  to 
  ascertain 
  whether 
  any 
  of 
  the 
  lines 
  

   which 
  came 
  out 
  when 
  the 
  sun 
  is 
  low, 
  especially 
  those 
  which 
  are 
  also 
  

   seen, 
  but 
  narrower 
  and 
  less 
  conspicuous, 
  when 
  the 
  sun 
  is 
  high, 
  could 
  be 
  

   due, 
  not 
  to 
  specific 
  atmospheric 
  absorption, 
  but 
  to 
  the 
  general 
  weakening 
  

   of 
  the 
  light, 
  causing 
  parts 
  of 
  the 
  spectrum 
  already 
  weakened 
  by 
  sola?* 
  

   absorption 
  to 
  appear 
  dark 
  when 
  a 
  general 
  weakening 
  of 
  the 
  light 
  was 
  

   superinduced, 
  though 
  they 
  had 
  appeared 
  bright 
  when 
  the 
  light 
  was 
  

   strong. 
  For 
  this 
  purpose 
  the 
  spectrum 
  of 
  the 
  sun 
  when 
  high, 
  as 
  seen 
  in 
  

   the 
  usual 
  way, 
  was 
  compared 
  with 
  the 
  spectrum 
  when 
  the 
  intensity 
  was 
  

   artificially 
  reduced 
  in 
  various 
  ways. 
  The 
  best 
  comparison 
  was 
  obtained 
  

   by 
  taking 
  advantage 
  of 
  a 
  natural 
  phenomenon. 
  At 
  Mussoorie, 
  late 
  in 
  

   the 
  autumn, 
  a 
  haze, 
  visible 
  at 
  sunset, 
  extends 
  over 
  the 
  low 
  country, 
  

   and 
  grows 
  day 
  by 
  day 
  in 
  height, 
  till 
  it 
  causes 
  the 
  sun 
  virtually 
  to 
  set 
  

   in 
  haze 
  while 
  still 
  3° 
  or 
  more 
  above 
  the 
  horizon, 
  whereas 
  in 
  the 
  clear 
  

   season 
  it 
  is 
  visible 
  till 
  it 
  attains 
  a 
  depression 
  of 
  1|°. 
  The 
  result 
  of 
  the 
  

   comparison 
  was, 
  that 
  none 
  of 
  the 
  additional 
  fines 
  were 
  discovered 
  to 
  

   have 
  any 
  other 
  origin 
  than 
  selective 
  atmospheric 
  absorption. 
  

  

  