﻿Experimental 
  Study 
  of 
  Optical 
  Systems. 
  

  

  53 
  

  

  from 
  that 
  shown 
  in 
  fig. 
  4 
  to 
  that 
  shown 
  in 
  fig. 
  5. 
  The 
  form 
  

   of 
  surface 
  is 
  in 
  each 
  case 
  the 
  same 
  (see 
  the 
  sectional 
  diagrams 
  

   at 
  the 
  top 
  of 
  the 
  figures), 
  but 
  correction 
  can 
  be 
  carried 
  out 
  

   according 
  to 
  whichever 
  plane 
  of 
  reference 
  is 
  the 
  most 
  favour- 
  

   able 
  from 
  the 
  point 
  of 
  view 
  of 
  the 
  operator. 
  In 
  order 
  to 
  find 
  

   whether 
  Q 
  (fig. 
  3) 
  is 
  a 
  hill 
  or 
  a 
  valley, 
  the 
  cast-iron 
  table 
  M 
  

   (fig. 
  1) 
  can 
  be 
  bent 
  with 
  the 
  ringers 
  so 
  as 
  to 
  tilt 
  the 
  mirror 
  F 
  

   in 
  such 
  a 
  way 
  as 
  to 
  lengthen 
  the 
  ray-path. 
  If 
  the 
  contour 
  line 
  

   at 
  Q 
  expands 
  to 
  enclose 
  a 
  larger 
  area, 
  a 
  hill 
  is 
  indicated, 
  and 
  

   vice 
  versa. 
  Although 
  the 
  words 
  "hill" 
  and 
  "valley" 
  are 
  

   convenient 
  to 
  use, 
  it 
  must 
  not 
  be 
  supposed 
  that 
  the 
  imper- 
  

   fections 
  necessarily 
  result 
  from 
  want 
  of 
  flatness 
  either 
  of 
  one 
  

   or 
  of 
  both 
  surfaces 
  of 
  the 
  prism. 
  The 
  contour 
  map 
  gives 
  the 
  

   total 
  effect 
  on 
  the 
  wave-front 
  produced 
  by 
  double 
  passage 
  

   through 
  the 
  prism, 
  and 
  shows 
  in 
  wave-lengths 
  the 
  departure 
  

   from 
  planeness 
  of 
  the 
  resulting 
  wave-surface. 
  

  

  The 
  Lens 
  Interferometer. 
  

  

  (See 
  figs. 
  6, 
  7, 
  and 
  8.) 
  

  

  Fig. 
  6. 
  

  

  H 
  

  

  Diagram 
  of 
  Lens 
  Interferometer. 
  

  

  In 
  the 
  lens 
  interferometer 
  all 
  parts 
  are 
  left 
  as 
  in 
  the 
  prism 
  

   arrangement 
  except 
  that 
  the 
  mirror 
  F 
  is 
  removed 
  and 
  replaced 
  

  

  