﻿RED 
  PROMINENCES 
  SEEN 
  DURING 
  TOTAL 
  ECLIPSES 
  OF 
  THE 
  SUN. 
  465 
  

  

  3. 
  On 
  the 
  Diminished 
  Brightness 
  of 
  the 
  Sun's 
  Disc 
  towards 
  its 
  Edges. 
  

  

  That 
  the 
  sun 
  is 
  surrounded 
  by 
  some 
  medium 
  capable 
  of 
  absorbing 
  his 
  light, 
  

   has 
  been 
  considered 
  to 
  be 
  satisfactorily 
  proved 
  by 
  the 
  rapidly 
  diminishing 
  bright- 
  

   ness 
  of 
  his 
  disc 
  towards 
  the 
  edges 
  ; 
  which, 
  Sir 
  John 
  Herschel 
  remarks, 
  " 
  can 
  

   only 
  arise 
  from 
  the 
  circumferential 
  rays 
  having 
  undergone 
  the 
  absorptive 
  action 
  

   of 
  a 
  much 
  greater 
  thickness 
  of 
  some 
  imperfectly 
  transparent 
  medium 
  (due 
  to 
  

   greater 
  obliquity 
  of 
  their 
  passage 
  through 
  it) 
  than 
  the 
  central 
  rays."* 
  He 
  fur- 
  

   ther 
  states, 
  that 
  if 
  the 
  sun 
  had 
  not 
  an 
  atmosphere 
  capable 
  of 
  reflecting 
  light, 
  the 
  

   sky 
  ought 
  to 
  appear 
  completely 
  dark 
  at 
  a 
  total 
  eclipse 
  of 
  the 
  sun. 
  The 
  existence 
  

   of 
  the 
  corona 
  round 
  the 
  moon 
  is 
  therefore 
  a 
  proof, 
  he 
  adds, 
  that 
  the 
  sun 
  has 
  an 
  

   atmosphere 
  capable 
  of 
  reflecting, 
  and 
  therefore 
  of 
  absorbing 
  light. 
  

  

  Now, 
  the 
  absorbing 
  medium 
  indicated 
  by 
  the 
  corona 
  is 
  evidently 
  an 
  extremely 
  

   diffuse 
  atmosphere, 
  extending 
  to 
  a 
  great 
  distance 
  from 
  the 
  sun's 
  surface 
  ; 
  for 
  the 
  

   breadth 
  of 
  the 
  corona 
  is 
  certainly 
  not 
  less 
  than 
  the 
  suns 
  radius. 
  It 
  can, 
  however, 
  

   be 
  easily 
  shewn, 
  that 
  the 
  darkness 
  of 
  the 
  sun's 
  limb 
  compared 
  with 
  his 
  centre, 
  if 
  it 
  

   arises 
  from 
  the 
  absorption 
  of 
  light 
  by 
  the 
  solar 
  atmosphere, 
  must 
  be 
  occasioned, 
  

   almost 
  exclusively, 
  by 
  the 
  absorptive 
  power 
  of 
  those 
  portions 
  of 
  the 
  atmosphere 
  

   which 
  are 
  near 
  the 
  surface 
  of 
  the 
  sun. 
  

  

  It 
  will 
  be 
  sufficiently 
  accurate, 
  for 
  this 
  purpose, 
  to 
  assume 
  that 
  the 
  sun 
  is 
  a 
  

   sphere, 
  of 
  which 
  AB, 
  fig. 
  11, 
  represents 
  the 
  surface, 
  and 
  abb' 
  a' 
  a 
  stratum 
  of 
  

   the 
  solar 
  atmosphere, 
  of 
  so 
  small 
  thickness 
  that 
  its 
  absorptive 
  power 
  may 
  be 
  

   regarded 
  as 
  uniform. 
  As 
  the 
  whole 
  absorption, 
  in 
  passing 
  through 
  such 
  a 
  thin 
  

   stratum, 
  will 
  be 
  very 
  small, 
  we 
  may 
  safely 
  assume 
  that 
  the 
  light, 
  in 
  traversing 
  a 
  

   certain 
  thickness 
  of 
  the 
  absorbent 
  medium, 
  will 
  thereby 
  acquire 
  no 
  additional 
  fa- 
  

   cility 
  for 
  penetrating 
  the 
  remaining 
  portions 
  of 
  it 
  ; 
  equal 
  aliquot 
  parts 
  of 
  the 
  in- 
  

   cident 
  light 
  will 
  therefore 
  be 
  absorbed 
  in 
  passing 
  through 
  successive 
  equal 
  thick- 
  

   nesses 
  of 
  the 
  stratum. 
  

  

  Suppose 
  all 
  the 
  sun's 
  atmosphere 
  excepting 
  abb' 
  a' 
  to 
  be 
  removed. 
  

  

  Let 
  1 
  = 
  the 
  number 
  of 
  intromitted 
  rays, 
  

  

  - 
  = 
  the 
  number 
  of 
  rays 
  lost 
  by 
  absorption 
  and 
  dispersion 
  when 
  the 
  light 
  has 
  

   traversed 
  a 
  unit 
  of 
  thickness, 
  

  

  M 
  = 
  l--' 
  

  

  m 
  

  

  t 
  = 
  the 
  thickness 
  a 
  a 
  of 
  the 
  stratum 
  ; 
  

   Then 
  the 
  number 
  of 
  rays 
  that 
  escape 
  absorption 
  after 
  passing 
  to 
  the 
  eye 
  in 
  the 
  

   direction 
  a 
  a, 
  perpendicularly 
  through 
  the 
  stratum, 
  will 
  be 
  

  

  rt 
  

  

  V 
  m) 
  

  

  W 
  

  

  * 
  Outlines 
  of 
  Astronomy, 
  par. 
  395. 
  

   VOL. 
  XX. 
  PART 
  III. 
  6 
  I 
  

  

  