﻿of 
  the 
  Solar 
  Prominences. 
  465 
  

  

  background. 
  This 
  is 
  a 
  well-known 
  contrast 
  effect. 
  The 
  H 
  

   and 
  K 
  lines, 
  for 
  instance, 
  seem 
  to 
  be 
  surrounded 
  by 
  a 
  lighter 
  

   band, 
  which 
  follows 
  the 
  contour 
  not 
  only 
  of 
  the 
  lines, 
  but 
  also 
  

   of 
  the 
  wing 
  by 
  the 
  side 
  of 
  the 
  prominence. 
  If, 
  now, 
  a 
  Fraun- 
  

   hofer 
  line 
  happens 
  to 
  be 
  by 
  the 
  side 
  of 
  a 
  bright 
  line, 
  the 
  con- 
  

   trast 
  is 
  strengthened, 
  and 
  both 
  the 
  bright 
  and 
  the 
  dark 
  lines 
  

   appear 
  more 
  distinctly 
  than 
  they 
  otherwise 
  would. 
  This 
  is 
  the 
  

   only 
  simple 
  way 
  in 
  which 
  I 
  can 
  explain 
  some 
  of 
  the 
  appear- 
  

   ances 
  of 
  the 
  photographs." 
  The 
  first 
  part 
  of 
  the 
  quotation 
  is 
  

   all 
  that 
  concerns 
  us 
  at 
  present, 
  for 
  in 
  the 
  negative 
  which 
  I 
  have 
  

   mentioned 
  as 
  showing 
  this 
  peculiarity 
  particularly 
  well, 
  the 
  

   Fraunhofer 
  lines 
  are 
  hardly 
  visible 
  above 
  the 
  limb, 
  and 
  none 
  

   appear 
  within 
  the 
  dark 
  bands 
  at 
  H 
  and 
  K. 
  As 
  Dr. 
  Schuster 
  

   does 
  not 
  speak 
  of 
  the 
  illumination, 
  I 
  assume 
  that 
  the 
  appear- 
  

   ance 
  was 
  generally 
  seen, 
  and 
  this 
  constitutes 
  another 
  point 
  of 
  

   difference. 
  A 
  penumbra 
  formed 
  by 
  light 
  reflected 
  from 
  the 
  

   back 
  of 
  the 
  plate 
  would 
  probably 
  extend 
  but 
  little 
  higher 
  than 
  

   the 
  central 
  line, 
  but 
  in 
  the 
  future 
  plates 
  backed 
  with 
  a 
  dyed 
  

   collodion 
  film 
  will 
  be 
  employed 
  to 
  obviate 
  any 
  effects 
  of 
  this 
  

   kind. 
  No 
  entirely 
  satisfactory 
  explanation 
  of 
  the 
  peculiar 
  ap- 
  

   pearance 
  of 
  these 
  lines 
  has 
  as 
  yet 
  suggested 
  itself. 
  

  

  But 
  on 
  another 
  point 
  there 
  is 
  little 
  room 
  for 
  doubt.' 
  The 
  

   bright 
  H 
  and 
  K 
  lines 
  certainly 
  extend 
  to 
  a 
  very 
  considerable 
  

   elevation 
  above 
  the 
  sun's 
  limb, 
  and 
  it 
  is 
  extremely 
  probable 
  

   that 
  calcium 
  is 
  carried 
  to 
  the 
  very 
  top 
  of 
  the 
  highest 
  prom- 
  

   inences. 
  With 
  the 
  improved 
  apparatus 
  to 
  be 
  used 
  in 
  a 
  contin- 
  

   uation 
  of 
  this 
  research, 
  I 
  hope 
  to 
  be 
  able 
  to 
  ascertain 
  the 
  

   relative 
  heights 
  of 
  various 
  lines 
  in 
  the 
  prominence 
  spectrum. 
  

   For 
  instance, 
  while 
  a 
  photograph 
  is 
  being 
  made 
  of 
  H 
  and 
  K, 
  

   the 
  height 
  of 
  C 
  in 
  the 
  same 
  prominence 
  can 
  be 
  measured 
  with 
  

   a 
  micrometer. 
  The 
  comparative 
  observations 
  and 
  photographs 
  

   made 
  up 
  to 
  the 
  present 
  time 
  suggest 
  the 
  belief 
  that 
  calcium 
  

   attains 
  the 
  highest 
  elevations 
  reached 
  by 
  hydrogen, 
  and 
  the 
  

   remarkable 
  brilliancy 
  of 
  H 
  and 
  K 
  at 
  the 
  eclipse 
  of 
  1882 
  attest 
  

   the 
  importance 
  of 
  calcium 
  in 
  the 
  prominences. 
  Dr. 
  Schuster 
  

   is 
  of 
  the 
  opinion 
  that 
  the 
  coronal 
  spectrum 
  contains 
  calcium 
  

   injected 
  by 
  the 
  prominences, 
  and 
  this 
  may 
  only 
  very 
  gradually 
  

   descend 
  again 
  to 
  the 
  level 
  of 
  the 
  photosphere.* 
  This 
  supposi- 
  

   tion 
  seems 
  a 
  very 
  plausible 
  one, 
  and 
  if 
  it 
  be 
  at 
  the 
  same 
  time 
  

   considered 
  probable 
  that 
  the 
  H 
  and 
  K 
  bands 
  and 
  their 
  central 
  

   lines 
  are 
  produced 
  by 
  the 
  same 
  substance, 
  the 
  possibility 
  is 
  

   suggested 
  that 
  the 
  broad 
  dark 
  shades 
  may 
  be 
  caused 
  by 
  the 
  

   absorption 
  of 
  the 
  cooler 
  vapor 
  at 
  a 
  considerable 
  elevation, 
  

   while 
  the 
  absorption 
  near 
  the 
  photosphere 
  gives 
  rise 
  to 
  the 
  

   narrow 
  central 
  lines. 
  This 
  view 
  need 
  not 
  necessarily 
  conflict 
  

  

  * 
  Phil. 
  Trans, 
  vol. 
  clxxx, 
  1889, 
  (A.), 
  p. 
  328. 
  

  

  