﻿4:66 
  G. 
  E. 
  Hale— 
  The 
  Ultra- 
  Violet 
  Spectrum 
  

  

  with, 
  a 
  belief 
  in 
  a 
  shallow 
  reversing 
  layer, 
  where 
  absorption 
  

   ordinarily 
  takes 
  place, 
  for 
  the 
  H 
  and 
  K 
  bands 
  are 
  unique 
  in 
  

   the 
  solar 
  spectrum. 
  It 
  rests, 
  however, 
  on 
  somewhat 
  insecure 
  

   foundations, 
  and 
  cannot 
  be 
  credited 
  with 
  much 
  weight. 
  

  

  On 
  account 
  of 
  the 
  dark 
  shades 
  at 
  H 
  and 
  K 
  it 
  has 
  proved 
  

   quite 
  easy 
  to 
  photograph 
  prominence 
  forms 
  with 
  an 
  open 
  slit. 
  

   With 
  other 
  prominence 
  lines 
  the 
  brilliancy 
  of 
  the 
  background 
  

   is 
  much 
  increased 
  when 
  the 
  slit 
  is 
  opened, 
  but 
  this 
  is 
  not 
  the 
  

   case 
  with 
  H 
  and 
  K, 
  and 
  it 
  is 
  often 
  possible 
  to 
  use 
  a 
  slit 
  nearly 
  

   a 
  quarter 
  of 
  an 
  inch 
  wide. 
  The 
  fourth 
  order 
  spectrum 
  has 
  

   been 
  employed 
  for 
  this 
  work, 
  and 
  the 
  best 
  results 
  are 
  obtained 
  

   with 
  an 
  exposure 
  of 
  about 
  one 
  second. 
  It 
  is 
  considered 
  that 
  

   great 
  advantage 
  will 
  result 
  from 
  a 
  material 
  reduction 
  of 
  this 
  

   exposure, 
  as 
  the 
  disturbances 
  in 
  our 
  atmosphere 
  have 
  as 
  yet 
  

   made 
  it 
  impossible 
  to 
  secure 
  the 
  finest 
  details 
  of 
  structure. 
  

  

  It 
  is 
  of 
  interest 
  to 
  note, 
  however, 
  that 
  the 
  first 
  photograph 
  

   ever 
  taken 
  of 
  the 
  rapid 
  development 
  of 
  a 
  prominence 
  was 
  

   made 
  in 
  this 
  way 
  by 
  my 
  assistants 
  on 
  July 
  8, 
  1891, 
  at 
  23h. 
  

   45m., 
  Chicago 
  M. 
  T. 
  As 
  at 
  first 
  observed 
  through 
  C, 
  the 
  

   prominence 
  was 
  low, 
  but 
  very 
  bright, 
  and 
  changing 
  rapidly. 
  

   A 
  great 
  tongue 
  moved 
  rapidly 
  out 
  to 
  an 
  elevation 
  of 
  about 
  

   80,000 
  miles, 
  and 
  at 
  this 
  time 
  the 
  extension 
  was 
  photographed 
  

   through 
  H 
  and 
  K. 
  In 
  fifteen 
  minutes 
  the 
  prominence 
  had 
  

   returned 
  to 
  its 
  original 
  form. 
  A 
  reproduction 
  of 
  the 
  photo- 
  

   graph 
  is 
  given 
  in 
  the 
  August 
  number 
  of 
  this 
  Journal, 
  and 
  

   though 
  much 
  has 
  been 
  lost 
  in 
  the 
  printing 
  process, 
  some 
  idea 
  

   of 
  the 
  actual 
  appearance 
  of 
  the 
  prominence 
  may 
  be 
  gained. 
  

   A 
  new 
  apparatus 
  for 
  photographing 
  the 
  prominences 
  is 
  now 
  

   being 
  constructed 
  as 
  the 
  outcome 
  of 
  my 
  investigations 
  on 
  this 
  

   subject, 
  and 
  this 
  is 
  expected 
  to 
  do 
  away 
  with 
  many 
  of 
  the 
  

   difficulties 
  previously 
  encountered. 
  It 
  will 
  consist 
  of 
  two 
  

   curved 
  slits, 
  moved 
  in 
  opposite 
  directions 
  across 
  the 
  ends 
  of 
  

   the 
  stationary 
  collimator 
  and 
  observing 
  telescope 
  by 
  means 
  of 
  

   a 
  peculiar 
  form 
  of 
  clepsydra. 
  The 
  sun's 
  image 
  and 
  photo- 
  

   graphic 
  plate 
  will 
  be 
  stationary, 
  and 
  the 
  apparatus 
  is 
  thus 
  to 
  

   be 
  constructed 
  on 
  the 
  principle 
  of 
  the 
  second 
  method 
  devised 
  

   by 
  myself 
  in 
  1889, 
  but 
  so 
  altered 
  as 
  to 
  avoid 
  the 
  defects 
  of 
  the 
  

   original 
  scheme.* 
  

  

  Decision 
  must 
  be 
  reserved 
  for 
  the 
  present 
  as 
  to 
  the 
  line 
  at 
  

   X 
  3970*11. 
  The 
  wave-length 
  has 
  been 
  determined 
  from 
  four 
  

   plates, 
  and 
  its 
  probable 
  error 
  is 
  0'030, 
  but 
  as 
  the 
  line 
  is 
  not 
  far 
  

   from 
  where 
  a 
  ghost 
  of 
  H 
  should 
  fall, 
  I 
  cannot 
  be 
  certain 
  that 
  

   it 
  belongs 
  to 
  the 
  prominence 
  spectrum. 
  At 
  the 
  same 
  time 
  it 
  is 
  

  

  * 
  For 
  previous 
  papers 
  on 
  promiuence 
  photography 
  see 
  — 
  Technology 
  Quarterly, 
  

   vol. 
  iii., 
  No. 
  4, 
  1890; 
  Astronomische 
  Nachrichten, 
  Nos. 
  3006, 
  3037 
  and 
  3053; 
  

   Sidereal 
  Messenger, 
  June, 
  1391; 
  This 
  Journal, 
  August, 
  1891. 
  

  

  