﻿486 
  On 
  Focal 
  Lines, 
  and 
  Anchor-Ring. 
  Wave-Fronts. 
  

  

  infinity. 
  Both 
  lines 
  when 
  real 
  can 
  be 
  shown 
  on 
  a 
  translucent 
  

   screen. 
  The 
  secondary 
  line, 
  when 
  virtual, 
  can 
  be 
  seen 
  by 
  

   looking 
  into 
  the 
  reflector. 
  The 
  complete 
  circle 
  of 
  the 
  primary 
  

  

  Fig. 
  3. 
  

  

  Fig. 
  4. 
  

  

  can 
  be 
  obtained 
  at 
  once 
  by 
  making 
  the 
  axis 
  of 
  the 
  incident 
  

   beam 
  (of 
  wide 
  angle) 
  coincide 
  with 
  the 
  axis 
  of 
  the 
  reflector, 
  

   so 
  that 
  the 
  area 
  of 
  incidence 
  surrounds 
  the 
  central 
  boss. 
  

  

  An 
  iris 
  diaphragm 
  (as 
  used 
  by 
  microscopists) 
  might 
  with 
  

   advantage 
  be 
  substituted 
  for 
  the 
  pierced 
  disk. 
  

  

  Postscript. 
  — 
  :8ince 
  reading 
  the 
  paper 
  1 
  have. 
  found 
  that 
  

   Maxwell, 
  in 
  a 
  paper 
  entitled 
  " 
  On 
  the 
  Cy 
  elide/' 
  has 
  inves- 
  

   tigated 
  the 
  general 
  form 
  of 
  the 
  wave-surface 
  " 
  when 
  one 
  or 
  

   both 
  of 
  the 
  so-called 
  focal 
  lines 
  is 
  really 
  a 
  line," 
  and 
  has 
  

   indicated 
  the 
  anchor-ring 
  as 
  a 
  particular 
  case. 
  (' 
  Collected 
  

   Papers,' 
  vol. 
  ii. 
  pp. 
  144 
  & 
  151.) 
  

  

  