﻿Solai* 
  Prominences 
  and 
  their 
  Spectra. 
  165 
  

  

  When 
  the 
  sharp 
  and 
  brilliant 
  reversals 
  of 
  H 
  and 
  K 
  were 
  

   discovered 
  at 
  the 
  beginning 
  of 
  my 
  investigations 
  in 
  prominence 
  

   photography 
  at 
  the 
  Kenwood 
  Physical 
  Observatory, 
  it 
  at 
  once 
  

   became 
  evident 
  that 
  a 
  considerable 
  advance 
  had 
  been 
  made, 
  

   for 
  the 
  substitution 
  of 
  either 
  of 
  these 
  lines 
  for 
  the 
  less 
  re- 
  

   frangible 
  hydrogen 
  lines 
  removed 
  the 
  serious 
  difficulty 
  of 
  

   photographing 
  the 
  longer 
  waves 
  of 
  the 
  C 
  region 
  with 
  short 
  

   exposure. 
  But 
  apart 
  from 
  their 
  position 
  in 
  the 
  spectrum, 
  the 
  

   distinctive 
  peculiarity 
  of 
  H 
  and 
  K 
  specially 
  fits 
  them 
  for 
  

   prominence 
  photography. 
  The 
  narrow 
  bright 
  lines, 
  instead 
  of 
  

   being 
  superposed 
  on 
  a 
  brilliant 
  continuous 
  spectrum, 
  as 
  is 
  the 
  

   case 
  with 
  all 
  of 
  the 
  other 
  prominence 
  lines, 
  lie 
  in 
  the 
  center 
  of 
  

   broad, 
  dark 
  bands, 
  where 
  the 
  troublesome 
  light 
  of 
  the 
  atmo- 
  

   sphere 
  is 
  missing. 
  Thus 
  both 
  slits 
  used 
  in 
  my 
  apparatus 
  for 
  

   photographing 
  the 
  prominences 
  could 
  be 
  much 
  more 
  widely 
  

   opened, 
  without 
  the 
  difficulty 
  of 
  fogging 
  and 
  loss 
  of 
  contrast 
  

   experienced 
  with 
  the 
  other 
  lines. 
  The 
  result 
  was 
  that 
  the 
  

   first 
  photograph 
  made 
  in 
  this 
  way 
  proved 
  a 
  success. 
  The 
  

   prominence 
  drifted 
  slowly 
  across 
  a 
  narrow 
  tangential 
  slit, 
  and 
  

   behind 
  the 
  second 
  slit, 
  at 
  the 
  focus 
  of 
  the 
  observing 
  telescope, 
  

   a 
  small 
  cylinder 
  with 
  its 
  axis 
  parallel 
  to 
  the 
  slit, 
  carried 
  a 
  strip 
  

   of 
  sensitive 
  film 
  at 
  a 
  speed 
  equal 
  to 
  that 
  of 
  the 
  moving 
  solar 
  

   image. 
  A 
  smooth 
  and 
  uniform 
  motion 
  of 
  the 
  cylinder 
  was 
  

   produced 
  by 
  a 
  small 
  clepsydra. 
  The 
  photograph 
  showed 
  the 
  . 
  

   form 
  of 
  the 
  prominence 
  very 
  well, 
  and 
  with 
  considerable 
  con- 
  

   trast. 
  It 
  was 
  then 
  concluded, 
  on 
  account 
  of 
  the 
  great 
  width 
  of 
  

   the 
  dark 
  shades 
  at 
  H 
  and 
  K, 
  that 
  for 
  prominences 
  of 
  not 
  too 
  

   great 
  size 
  (the 
  image 
  of 
  the 
  sun 
  on 
  the 
  slit 
  plate 
  is 
  two 
  inches 
  

   in 
  diameter) 
  it 
  would 
  only 
  be 
  necessary 
  to 
  use 
  a 
  wide 
  slit, 
  and 
  

   give 
  a 
  short 
  exposure. 
  Fig. 
  "6 
  shows 
  the 
  result 
  of 
  such 
  an 
  ex- 
  

   periment. 
  The 
  wide 
  slit 
  was 
  nearly 
  tangent 
  to 
  the 
  sun's 
  limb, 
  

   but 
  did 
  not 
  quite 
  touch 
  it, 
  in 
  order 
  to 
  exclude 
  the 
  direct 
  light. 
  

   The 
  exposure 
  was 
  about 
  2 
  seconds, 
  and 
  the 
  dispersion 
  that 
  of 
  

   the 
  fourth 
  order 
  of 
  a 
  14,438 
  grating. 
  As 
  an 
  object-glass 
  (8J 
  

   inches 
  aperture 
  and 
  42J 
  inches 
  focus) 
  corrected 
  for 
  the 
  visual 
  

   region 
  was 
  used 
  in 
  the 
  observing 
  telescope 
  of 
  the 
  spectroscope, 
  

   the 
  foci 
  for 
  H 
  and 
  K 
  are 
  slightly 
  different. 
  The 
  photograph 
  

   is 
  about 
  twice 
  the 
  size 
  of 
  the 
  original, 
  and 
  was 
  enlarged 
  di- 
  

   rectly 
  from 
  it 
  in 
  the 
  camera. 
  

  

  Although 
  this 
  method 
  will 
  serve 
  to 
  photograph 
  the 
  invisible 
  

   prominences 
  it 
  is 
  evident 
  that 
  there 
  are 
  two 
  objections 
  to 
  it. 
  

   In 
  the 
  first 
  place 
  it 
  would 
  be 
  very 
  troublesome 
  to 
  find 
  invisible 
  

   prominences, 
  and 
  to 
  do 
  so 
  it 
  would 
  be 
  necessary 
  to 
  take 
  a 
  large 
  

   number 
  of 
  photographs 
  with 
  the 
  slit 
  tangent 
  at 
  various 
  points 
  

   on 
  the 
  limb. 
  This 
  could 
  be 
  remedied 
  by 
  using 
  a 
  curved 
  or 
  

   ring 
  slit. 
  Again, 
  prominences 
  surpassing 
  a 
  certain 
  size 
  could 
  

   not 
  be 
  photographed, 
  though 
  for 
  single 
  narrow 
  prominences 
  

  

  