﻿460 
  G. 
  K 
  Bale— 
  The 
  Ultra- 
  Violet 
  Spectrum 
  

  

  spectra 
  are 
  brought 
  into 
  the 
  field 
  of 
  view 
  by 
  rotating 
  the 
  

   grating. 
  A 
  diagonal 
  eye-piece 
  at 
  the 
  end 
  of 
  the 
  observing 
  

   telescope 
  allows 
  the 
  spectrum 
  to 
  be 
  observed 
  after 
  the 
  photo- 
  

   graphic 
  plate 
  is 
  in 
  position. 
  

  

  In 
  photographing 
  the 
  spectrum 
  of 
  a 
  prominence 
  the 
  follow- 
  

   ing 
  is 
  the 
  ordinary 
  process. 
  Let 
  us 
  suppose 
  that 
  it 
  is 
  desired 
  

   to 
  use 
  a 
  radial 
  slit, 
  in 
  the 
  H 
  and 
  K 
  region 
  of 
  the 
  spectrum. 
  

   The 
  C 
  line 
  in 
  the 
  second 
  order 
  is 
  brought 
  into 
  the 
  field, 
  and 
  

   while 
  observing 
  this 
  line 
  the 
  spectroscope 
  is 
  rotated 
  until 
  the 
  

   slit 
  is 
  radial 
  at 
  some 
  point 
  on 
  the 
  limb 
  where 
  a 
  prominence 
  is 
  

   seen. 
  The 
  driving-clock 
  is 
  then 
  started, 
  and 
  the 
  telescope 
  

   clamped, 
  so 
  that 
  the 
  sun's 
  image 
  is 
  kept 
  as 
  nearly 
  as 
  possible 
  

   stationary 
  on 
  the 
  slit 
  plate. 
  A 
  small 
  strip 
  of 
  metal, 
  pushed 
  in 
  

   just 
  behind 
  the 
  slit, 
  excludes 
  the 
  direct 
  solar 
  light, 
  except 
  from 
  

   a 
  small 
  region 
  near 
  the 
  limb. 
  The 
  whole 
  collimator 
  is 
  next 
  

   moved 
  by 
  a 
  screw 
  until 
  the 
  slit 
  is 
  brought 
  to 
  the 
  proper 
  focus 
  

   of 
  the 
  equatorial 
  for 
  K, 
  and 
  the 
  collimator 
  and 
  observing 
  tel- 
  

   escope 
  are 
  set 
  at 
  the 
  focus 
  for 
  the 
  same 
  line, 
  the 
  positions 
  being 
  

   taken 
  from 
  a 
  table 
  of 
  foci, 
  determined 
  by 
  experiment, 
  for 
  the 
  

   principal 
  lines 
  in 
  the 
  spectrum. 
  After 
  placing 
  the 
  sensitive 
  

   plate 
  in 
  position, 
  the 
  grating 
  is 
  rotated 
  until 
  the 
  K 
  line 
  in 
  the 
  

   fourth 
  order 
  is 
  in 
  the 
  middle 
  of 
  the 
  field, 
  the 
  slit 
  is 
  covered, 
  the 
  

   slide 
  drawn, 
  and 
  the 
  proper 
  exposure 
  given. 
  The 
  exposure 
  of 
  

   course 
  depends 
  upon 
  the 
  aperture 
  arid 
  focal 
  length 
  of 
  the 
  equa- 
  

   torial, 
  the 
  width 
  of 
  the 
  slit, 
  the 
  brilliancy 
  of 
  the 
  grating, 
  the 
  

   sensitiveness 
  of 
  the 
  plate, 
  etc., 
  but 
  with 
  the 
  ordinary 
  dry 
  platu 
  

   of 
  sensitometer 
  No. 
  23 
  furnished 
  by 
  the 
  Seed 
  Company, 
  and 
  a 
  

   slit 
  about 
  0*001 
  inches 
  wide, 
  I 
  usually 
  find 
  that 
  an 
  exposure 
  of 
  

   from 
  20 
  to 
  30 
  seconds 
  gives 
  the 
  best 
  result. 
  

  

  For 
  the 
  first 
  time 
  without 
  an 
  eclipse 
  the 
  prominence 
  spectrum 
  

   was 
  thus 
  photographed 
  early 
  in 
  April 
  of 
  the 
  present 
  year. 
  

   The 
  only 
  bright 
  lines 
  then 
  obtained 
  were 
  found 
  to 
  fall 
  nearly 
  

   at 
  the 
  centers 
  of 
  the 
  dark 
  bands 
  H 
  and 
  K 
  of 
  the 
  solar 
  spec- 
  

   trum, 
  but 
  these 
  were 
  remarkably 
  strong, 
  seeming 
  to 
  fully 
  equal 
  

   C 
  in 
  intensity, 
  and 
  were 
  present 
  in 
  every 
  prominence 
  photo- 
  

   graphed. 
  Work 
  was 
  continued 
  on 
  the 
  violet 
  and 
  ultra-violet 
  for 
  

   some 
  weeks, 
  but, 
  with 
  the 
  exception 
  of 
  some 
  lines 
  which 
  had 
  

   all 
  the 
  appearance 
  of 
  ghosts 
  of 
  the 
  brilliant 
  H 
  and 
  K 
  reversals, 
  

   no 
  new 
  lines 
  were 
  discovered 
  until 
  June 
  '23, 
  when 
  an 
  excep- 
  

   tionally 
  bright 
  prominence 
  was 
  found. 
  This 
  gave 
  four 
  lines 
  

   in 
  the 
  ultra-violet, 
  and 
  the 
  least 
  refrangible 
  of 
  these 
  was 
  found 
  

   to 
  be 
  double. 
  A 
  line 
  slightly 
  less 
  refrangible 
  than 
  H, 
  nearly 
  

   but 
  not 
  quite 
  at 
  the 
  position 
  where 
  the 
  first 
  ghost 
  would 
  be 
  

   expected 
  to 
  fall, 
  was 
  much 
  stronger 
  than 
  any 
  of 
  the 
  other 
  

   ghosts, 
  and 
  it 
  seemed 
  very 
  possible 
  that 
  it 
  was 
  an 
  independent 
  

   line. 
  This 
  prominence 
  remained 
  visible 
  for 
  several 
  days, 
  and 
  

  

  