﻿RED 
  PROMINENCES 
  SEEN 
  DURING 
  TOTAL 
  ECLIPSES 
  OF 
  THE 
  SUN. 
  467 
  

  

  4. 
  On 
  the 
  Faculce 
  and 
  Luculi 
  seen 
  on 
  the 
  Sun's 
  Disc. 
  

  

  The 
  supposition 
  that 
  the 
  sun 
  is 
  surrounded 
  by 
  an 
  envelope 
  of 
  cloud, 
  occasion- 
  

   ally 
  penetrated 
  by 
  apertures, 
  may 
  serve 
  to 
  explain 
  the 
  faculce 
  and 
  luculi 
  seen 
  on 
  

   the 
  solar 
  disc. 
  These 
  are 
  portions 
  of 
  the 
  sun's 
  surface 
  brighter 
  than 
  the 
  rest 
  ; 
  

   and 
  they 
  have 
  been 
  supposed 
  to 
  be 
  ridges 
  in 
  his 
  luminous 
  atmosphere, 
  indicating 
  

   violent 
  agitation 
  in 
  their 
  neighbourhood.* 
  May 
  they 
  not, 
  however, 
  be 
  simply 
  

   apertures 
  in 
  the 
  envelope 
  of 
  cloud 
  ? 
  If 
  such 
  apertures 
  exist, 
  the 
  sun's 
  surface 
  

   seen 
  through 
  them 
  will 
  appear 
  more 
  luminous 
  than 
  elsewhere 
  ; 
  for 
  his 
  light 
  pass- 
  

   ing 
  through 
  the 
  apertures 
  will 
  escape, 
  more 
  or 
  less, 
  the 
  absorption 
  suffered 
  by 
  

   the 
  rays 
  which 
  traverse 
  the 
  envelope 
  itself. 
  This 
  explanation 
  of 
  the 
  faculse 
  is 
  quite 
  

   consistent 
  with 
  the 
  well-known 
  fact, 
  that 
  they 
  are 
  best 
  seen 
  when 
  near 
  the 
  sun's 
  

   edge. 
  Thus, 
  if 
  the 
  dotted 
  line 
  hi 
  (fig. 
  11) 
  represent 
  the 
  surface 
  of 
  the 
  envelope, 
  

   and 
  ef 
  e' 
  f 
  two 
  apertures 
  in 
  it, 
  seen 
  from 
  a 
  distant 
  point 
  in 
  the 
  prolongation 
  of 
  

   C 
  A 
  ; 
  the 
  ray 
  g 
  will 
  be 
  contrasted 
  with 
  A 
  h, 
  which 
  has 
  passed 
  perpendicularly 
  

   through 
  the 
  envelope, 
  while 
  the 
  ray 
  g' 
  is 
  contrasted 
  with 
  B 
  i, 
  which 
  has 
  passed 
  

   obliquely 
  through 
  the 
  envelope, 
  and 
  therefore 
  suffered 
  more 
  absorption 
  than 
  A 
  a. 
  

   It 
  is 
  evident, 
  then, 
  that 
  the 
  nearer 
  a 
  facula 
  approaches 
  the 
  sun's 
  limb, 
  the 
  more 
  

   strongly 
  will 
  it 
  contrast 
  with 
  the 
  brightness 
  of 
  the 
  surface 
  in 
  its 
  neighbourhood, 
  

   and 
  the 
  more 
  distinctly 
  will 
  it 
  be 
  seen. 
  

  

  If, 
  however, 
  the 
  faculse 
  be 
  regarded 
  as 
  ridges 
  in 
  the 
  sun's 
  luminous 
  atmosphere, 
  

   their 
  brightness, 
  compared 
  with 
  the 
  rest 
  of 
  the 
  sun's 
  disc, 
  and 
  their 
  increased 
  dis- 
  

   tinctness 
  when 
  near 
  his 
  limb, 
  may 
  also 
  be 
  explained 
  by 
  supposing 
  that 
  a 
  thin 
  en- 
  

   velope 
  of 
  cloud 
  surrounds 
  the 
  sun 
  ; 
  for 
  if 
  A 
  B 
  V 
  a' 
  be 
  the 
  envelope, 
  and 
  A 
  a 
  the 
  

   height 
  of 
  a 
  ridge, 
  the 
  point 
  a 
  will 
  appear 
  brighter 
  than 
  the 
  rest 
  of 
  the 
  surface, 
  be- 
  

   cause 
  its 
  light 
  has 
  only 
  traversed 
  the 
  thickness 
  a 
  a' 
  of 
  the 
  absorbing 
  medium, 
  while 
  

   that 
  from 
  the 
  rest 
  of 
  the 
  sun's 
  surface 
  has 
  passed 
  through 
  the 
  whole 
  thickness 
  

   A 
  a', 
  and 
  has 
  consequently 
  been 
  more 
  absorbed. 
  Again, 
  if 
  a 
  ridge 
  has 
  the 
  posi- 
  

   tion 
  b 
  d, 
  so 
  as 
  to 
  be 
  seen 
  projected 
  on 
  the 
  sun's 
  limb, 
  — 
  supposing 
  all 
  the 
  atmo- 
  

   sphere 
  above 
  a 
  b 
  removed, 
  — 
  we 
  should 
  have 
  the 
  light 
  from 
  a 
  and 
  b, 
  which 
  would 
  

   not 
  then 
  suffer 
  absorption, 
  contrasted 
  in 
  the 
  one 
  case 
  with 
  light 
  which 
  had 
  passed 
  

   through 
  the 
  thickness 
  A 
  a, 
  and 
  in 
  the 
  other 
  through 
  the 
  greater 
  thickness 
  B 
  b 
  of 
  ab- 
  

   sorbent 
  matter. 
  The 
  contrast 
  at 
  b 
  would 
  evidently 
  then 
  be 
  greater 
  than 
  at 
  a. 
  

   If 
  now 
  the 
  rays 
  from 
  A 
  a 
  traverse 
  together 
  the 
  remaining 
  portions 
  of 
  the 
  sun's 
  at- 
  

   mosphere, 
  they 
  will 
  be 
  almost 
  equally 
  absorbed, 
  and 
  the 
  relative 
  brightness 
  of 
  the 
  

   points 
  a 
  A 
  will 
  not 
  be 
  sensibly 
  altered 
  ; 
  the 
  same 
  will 
  be 
  true 
  with 
  regard 
  to 
  B 
  b. 
  

   Hence 
  the 
  contrast 
  of 
  the 
  brightness 
  of 
  the 
  ridge, 
  when 
  compared 
  with 
  that 
  of 
  the 
  

   sun's 
  general 
  surface, 
  will 
  still 
  be 
  greatest 
  at 
  the 
  sun's 
  limb.f 
  

  

  * 
  Sir 
  William 
  Herschel. 
  Philosophical 
  Transactions, 
  1801. 
  

  

  ■f" 
  Sir 
  William 
  Herschel, 
  who 
  regards 
  the 
  faculae 
  as 
  ridges, 
  does 
  not 
  seem 
  to 
  hold 
  that 
  

   opinion 
  very 
  confidently. 
  In 
  his 
  paper, 
  already 
  noticed, 
  he 
  only 
  once 
  states 
  that 
  they 
  " 
  have 
  

   the 
  appearance 
  of 
  elevations." 
  — 
  (Phil. 
  Trans. 
  1801, 
  p. 
  84.) 
  Mr 
  Dawes, 
  however, 
  has 
  recently 
  

  

  