﻿Solar 
  Prominences 
  and 
  their 
  Spectra. 
  161 
  

  

  collodion 
  plate, 
  and 
  given 
  an 
  exposure 
  of 
  nearly 
  four 
  minutes.* 
  

   Professor 
  Young 
  has 
  very 
  kindly 
  shown 
  me 
  silver 
  prints 
  from 
  

   the 
  best 
  original 
  negatives 
  ; 
  in 
  these 
  only 
  the 
  general 
  outline 
  of 
  

   the 
  prominence 
  can 
  be 
  faintly 
  seen. 
  This 
  is 
  due 
  partly 
  to 
  a 
  

   small 
  displacement 
  of 
  the 
  image 
  during 
  the 
  exposure, 
  as 
  the 
  

   polar 
  axis 
  of 
  the 
  telescope 
  was 
  slightly 
  out 
  of 
  adjustment. 
  

   The 
  nebulous 
  character 
  of 
  G' 
  makes 
  its 
  use 
  objectionable, 
  but 
  

   the 
  serious 
  difficulty 
  with 
  this 
  line 
  lies 
  in 
  the 
  employment 
  of 
  a 
  

   wide 
  slit. 
  The 
  brilliancy 
  of 
  the 
  background 
  of 
  atmospheric 
  

   spectrum 
  increases 
  very 
  rapidly 
  when 
  the 
  slit 
  is 
  opened, 
  while 
  

   the 
  prominence 
  itself 
  grows 
  no 
  brighter. 
  Thus 
  the 
  contrast 
  in 
  

   a 
  photograph 
  is 
  greatly 
  decreased, 
  and 
  the 
  general 
  illumination 
  

   of 
  the 
  field, 
  due 
  to 
  diffused 
  light 
  from 
  the 
  grating, 
  or 
  fluores- 
  

   cence 
  of 
  the 
  prisms 
  or 
  object 
  glasses, 
  conspires 
  to 
  hide 
  all 
  

   details 
  of 
  structure. 
  For 
  these 
  reasons 
  the 
  method 
  has 
  never 
  

   been 
  employed 
  in 
  practice. 
  

  

  It 
  is 
  beyond 
  the 
  scope 
  of 
  the 
  present 
  paper 
  to 
  describe 
  the 
  

   various 
  methods 
  of 
  prominence 
  photography 
  proposed 
  by 
  

   Braun 
  in 
  1872, 
  Lockyer 
  and 
  Seabroke 
  in 
  the 
  same 
  year, 
  Lohse 
  

   in 
  1874 
  and 
  1880, 
  Zenger 
  in 
  1879, 
  and 
  Janssen 
  in 
  1881. 
  Suf- 
  

   fice 
  it 
  to 
  say, 
  that 
  in 
  no 
  instance 
  was 
  any 
  success 
  attained 
  

   sufficient 
  to 
  bring 
  the 
  method 
  into 
  practical 
  use, 
  and 
  in 
  1889 
  it 
  

   was 
  impossible 
  to 
  see 
  where 
  any 
  advance 
  whatever 
  had 
  been 
  

   made 
  beyond 
  the 
  brief 
  experiments 
  of 
  Professor 
  Young 
  with 
  

   a 
  simple 
  open 
  slit. 
  

  

  In 
  undertaking 
  an 
  investigation 
  of 
  the 
  subject 
  in 
  the 
  sum- 
  

   mer 
  of 
  the 
  year 
  last 
  named, 
  the 
  writer 
  devised 
  two 
  methods 
  of 
  

   accomplishing 
  the 
  desired 
  result 
  with 
  a 
  narrow 
  slit, 
  for 
  it 
  was 
  

   evident 
  that 
  with 
  any 
  line 
  in 
  the 
  prominence 
  spectrum 
  as 
  then 
  

   known, 
  the 
  use 
  of 
  a 
  wide 
  slit 
  could 
  not 
  have 
  more 
  than 
  an 
  

   extremely 
  limited 
  application. 
  In 
  the 
  first 
  method 
  the 
  rate 
  of 
  

   the 
  driving 
  clock 
  of 
  the 
  equatorial 
  is 
  so 
  changed 
  that 
  the 
  sun's 
  

   image 
  drifts 
  at 
  right 
  angles 
  across 
  the 
  slit 
  of 
  a 
  spectroscope 
  of 
  

   high 
  dispersion. 
  At 
  the 
  focus 
  of 
  the 
  observing 
  telescope 
  (of 
  

   equal 
  focal 
  length 
  w 
  T 
  ith 
  the 
  collimator) 
  a 
  photographic 
  plate 
  

   moves 
  at 
  the 
  same 
  speed, 
  at 
  right 
  angles 
  to 
  the 
  axis 
  of 
  the 
  tele- 
  

   scope, 
  and 
  in 
  the 
  direction 
  of 
  dispersion. 
  A 
  narrow 
  slit 
  just 
  

   in 
  front 
  of 
  the 
  plate 
  allows 
  only 
  the 
  line 
  in 
  use 
  to 
  fall 
  upon 
  it, 
  

   and 
  thus 
  prevents 
  fogging. 
  It 
  will 
  be 
  easily 
  seen 
  that 
  fresh 
  

   portions 
  of 
  the 
  plate 
  will 
  be 
  uncovered 
  as 
  the 
  prominence 
  

   drifts 
  across 
  the 
  slit, 
  and 
  the 
  result 
  will 
  be 
  a 
  latent 
  image 
  upon 
  

   the 
  photographic 
  plate. 
  

  

  The 
  second 
  method 
  exactly 
  reverses 
  the 
  operations 
  of 
  the 
  

   first. 
  The 
  sun's 
  image 
  is 
  held 
  in 
  a 
  fixed 
  position 
  by 
  the 
  driv- 
  

   ing 
  clock 
  of 
  the 
  equatorial, 
  while 
  the 
  plate 
  at 
  the 
  focus 
  of 
  the 
  

   observing 
  telescope 
  is 
  also 
  stationary. 
  The 
  slit 
  of 
  the 
  spectro- 
  

   scope 
  is 
  caused 
  to 
  move 
  steadily 
  across 
  the 
  end 
  of 
  the 
  collima- 
  

  

  * 
  Journal 
  Franklin 
  Institute, 
  Oct. 
  3, 
  18^0. 
  

  

  