﻿Solar 
  Prominences 
  and 
  their 
  Spectra. 
  163 
  

  

  to 
  agree 
  in 
  position 
  with 
  narrow 
  dark 
  lines 
  at 
  the 
  center 
  of 
  the 
  

   dark 
  shades. 
  Above 
  in 
  the 
  ultra-violet 
  the 
  photographs 
  bring 
  

   out 
  three 
  new 
  lines, 
  which 
  there 
  are 
  good 
  reasons 
  to 
  regard 
  as 
  

   the 
  first 
  three 
  lines 
  of 
  the 
  hydrogen 
  stellar 
  series, 
  though 
  their 
  

   wave-lengths 
  have 
  not 
  been 
  determined 
  as 
  yet. 
  The 
  lowest 
  of 
  

   the 
  three, 
  which 
  probably 
  corresponds 
  with 
  the 
  line 
  called 
  

   hydrogen 
  a 
  in 
  Dr. 
  Huggins's 
  map, 
  has 
  occasionally 
  been 
  

   glimpsed 
  in 
  the 
  prominence 
  spectrum 
  by 
  Professor 
  Young, 
  

   and 
  its 
  identity 
  can 
  now 
  be 
  certainly 
  determined 
  for 
  the 
  first 
  

   time.* 
  But 
  the 
  photographs 
  have 
  also 
  revealed 
  a 
  new 
  and 
  

   interesting 
  fact. 
  On 
  all 
  the 
  plates 
  made 
  with 
  the 
  focus 
  of 
  the 
  

   observing 
  telescope 
  accurately 
  adjusted 
  for 
  this 
  region, 
  the 
  

   first 
  line 
  above 
  K 
  is 
  shown 
  to 
  be 
  a 
  fine, 
  sharp 
  double, 
  the 
  sepa- 
  

   ration 
  of 
  the 
  components 
  amounting 
  to 
  a 
  few 
  tenths 
  of 
  a 
  tenth- 
  

   metre. 
  A 
  special 
  study 
  of 
  this 
  double 
  will 
  be 
  made 
  when 
  a 
  

   new 
  photographic 
  object-glass 
  of 
  six 
  feet 
  focus 
  has 
  been 
  com- 
  

   pleted 
  for 
  the 
  spectroscope. 
  The 
  fourth 
  order 
  spectrum 
  of 
  

   our 
  concave 
  grating 
  of 
  ten 
  feet 
  radius 
  will 
  also 
  probably 
  be 
  

   brought 
  into 
  service 
  for 
  work 
  on 
  the 
  solar 
  spectrum 
  in 
  this 
  

   region. 
  

  

  As 
  already 
  suggested, 
  the 
  two 
  upper 
  prominence 
  lines 
  are 
  

   probably 
  coincident 
  with 
  two 
  lines 
  in 
  the 
  hydrogen 
  series. 
  

   Only 
  one 
  of 
  these 
  appears 
  in 
  fig. 
  2, 
  where 
  it 
  is 
  very 
  faint. 
  

   A 
  photographic 
  search 
  for 
  the 
  remaining 
  lines 
  of 
  the 
  series 
  is 
  

   now 
  in 
  progress 
  at 
  the 
  Kenwood 
  Physical 
  Observatory. 
  

  

  The 
  important 
  variations 
  in 
  the 
  relative 
  intensities 
  of 
  promi- 
  

   nence 
  lines 
  revealed 
  in 
  eclipse 
  photographs 
  have 
  been 
  partially 
  

   confirmed 
  by 
  my 
  photographs. 
  So 
  far 
  only 
  one 
  prominence 
  

   has 
  appeared 
  in 
  which 
  the 
  ultra-violet 
  hydrogen 
  lines 
  could 
  be 
  

   photographed, 
  and 
  this 
  showed 
  a 
  corresponding 
  increase 
  of 
  

   brilliancy 
  in 
  the 
  visual 
  spectrum. 
  But 
  the 
  H 
  and 
  K 
  reversals 
  

   are 
  invariably 
  strong, 
  and 
  easily 
  photographed. 
  Preliminary 
  

   measures 
  show 
  that 
  both 
  lines 
  probably 
  belong 
  to 
  calcium, 
  but 
  

   this 
  is 
  yet 
  to 
  be 
  definitely 
  determined, 
  and 
  the 
  origin 
  of 
  the 
  

   broad 
  dark 
  shades 
  in 
  the 
  solar 
  spectrum 
  is 
  decidedly 
  uncertain. 
  

   In 
  spite 
  of 
  the 
  constant 
  presence 
  of 
  the 
  H 
  and 
  K 
  bright 
  lines 
  

   in 
  prominences, 
  it 
  can 
  hardly 
  be 
  supposed 
  that 
  the 
  substance 
  

   producing 
  them 
  can 
  be 
  ordinary 
  hydrogen, 
  for 
  several 
  reasons. 
  

   In 
  the 
  first 
  place 
  there 
  is 
  no 
  provision 
  for 
  K 
  in 
  Banner's 
  series, 
  

   and 
  H 
  certainly 
  does 
  not 
  fall 
  in 
  the 
  position 
  of 
  the 
  hydrogen 
  

   line, 
  as 
  it 
  is 
  about 
  1*5 
  tenth-metre 
  more 
  refrangible. 
  Again, 
  

   H 
  and 
  K 
  do 
  not 
  follow 
  the 
  hydrogen 
  lines 
  in 
  their 
  intensity 
  

  

  * 
  Great 
  confusion 
  is 
  likely 
  to 
  result 
  from 
  the 
  indiscriminate 
  use 
  of 
  the 
  letter 
  H 
  

   for 
  "hydrogen 
  " 
  o' 
  for 
  Fraunhofer's 
  H 
  line, 
  and 
  also 
  in 
  applying 
  the 
  Greek 
  letters 
  

   to 
  the 
  hydrogen 
  lines, 
  for 
  some 
  call 
  the 
  c 
  line 
  a, 
  and 
  others 
  apply 
  the 
  same 
  letter 
  

   to 
  the 
  first 
  hydrogen 
  line 
  in 
  the 
  ultra-violet. 
  It 
  is 
  desirable 
  to 
  adopt 
  some 
  com- 
  

   mon 
  nomenclature, 
  and 
  probably 
  the 
  most 
  natural 
  is 
  to 
  begin 
  with 
  c, 
  and 
  call 
  this 
  

   line 
  "hydrogen 
  a," 
  or 
  else 
  refer 
  to 
  each 
  line 
  by 
  its 
  wave-length. 
  

  

  