﻿obtained 
  with 
  Glass 
  Wedge 
  

  

  433 
  

  

  to 
  the 
  glass 
  when 
  looked 
  at 
  in 
  a 
  definite 
  direction, 
  but 
  if 
  the 
  

   eye 
  is 
  moved 
  towards 
  the 
  normal 
  to 
  the 
  plates 
  so 
  as 
  to 
  make 
  

   the 
  angle 
  of 
  refraction 
  less, 
  equation 
  (1) 
  shows 
  that 
  the 
  same 
  

   retardation 
  will 
  be 
  produced 
  by 
  a 
  smaller 
  difference 
  in 
  thick- 
  

   ness 
  of 
  the 
  plates, 
  i. 
  e. 
  the 
  fringe 
  under 
  observation 
  will 
  

   move 
  towards 
  the 
  central 
  one 
  of 
  the 
  system. 
  By 
  this 
  means 
  

   the 
  position 
  on 
  the 
  plates 
  of 
  the 
  central 
  fringe 
  can 
  be 
  readily 
  

   located. 
  

  

  3. 
  Efect 
  of 
  Rotation 
  of 
  one 
  Plate. 
  — 
  Over 
  a 
  reasonably 
  

   small 
  area 
  each 
  piece 
  of 
  plate-glass 
  may 
  be 
  looked 
  on 
  as 
  

   forming 
  a 
  wedge 
  of 
  very 
  small 
  angle. 
  Since 
  the 
  fringes 
  

   represent 
  the 
  loci 
  of 
  points 
  of 
  constant 
  difference 
  in 
  thick- 
  

   ness, 
  the 
  direction 
  of 
  the 
  fringes 
  and 
  the 
  fringe 
  width 
  are 
  

   purely 
  questions 
  of 
  the 
  geometry 
  of 
  two 
  superposed 
  wedges, 
  

   and 
  the 
  effects 
  of 
  rotating 
  one 
  wedge 
  on 
  the 
  other 
  may 
  be 
  

   easily 
  followed 
  by 
  making 
  use 
  of 
  fig. 
  5. 
  This 
  is 
  a 
  plan 
  of 
  

  

  the 
  two 
  wedges 
  superposed 
  ; 
  portions 
  only 
  of 
  the 
  wedges 
  are 
  

   drawm, 
  of 
  which 
  the 
  thin 
  ends 
  AB, 
  A'B', 
  and 
  the 
  thick 
  ends 
  

   CD, 
  C 
  1/ 
  D' 
  (marked 
  by 
  a 
  thick 
  line), 
  have 
  exactly 
  equal 
  

   thicknesses. 
  Produce 
  these 
  ends 
  until 
  they 
  intersect 
  at 
  A 
  

   and 
  C. 
  Then 
  the 
  line 
  AC 
  is 
  a 
  line 
  of 
  equal 
  thickness 
  in 
  the 
  

   two 
  plates, 
  and 
  represents 
  the 
  central 
  fringe 
  of 
  the 
  system, 
  

   and 
  the 
  other 
  fringes 
  are 
  lines 
  parallel 
  to 
  this. 
  When 
  the 
  

   wedges 
  are 
  so 
  placed 
  that 
  their 
  contour-lines 
  are 
  parallel 
  to 
  

   each 
  other, 
  the 
  thin 
  ends 
  of 
  each 
  wedge 
  being 
  coincident, 
  

   the 
  fringes 
  will 
  be 
  parallel 
  to 
  the 
  contour-lines 
  and 
  very 
  

   broad. 
  As 
  one 
  wedge 
  is 
  turned 
  round 
  on 
  the 
  other, 
  the 
  

   fringes 
  in 
  general 
  narrow 
  and 
  also 
  turn 
  round, 
  with 
  a 
  different 
  

   angular 
  velocity 
  from 
  that 
  of 
  the 
  wedge, 
  but 
  they 
  again 
  

   become 
  parallel 
  to 
  the 
  contour-lines 
  when 
  the 
  wedge 
  has 
  

   been 
  turned 
  round 
  through 
  two 
  right 
  angles, 
  i. 
  e. 
  when 
  the 
  

   wedges 
  are 
  again 
  parallel 
  to 
  each 
  other, 
  but 
  the 
  thin 
  end 
  

   A'B' 
  of 
  i. 
  is 
  coincident 
  with 
  the 
  thick 
  end 
  CD 
  of 
  ii. 
  The 
  

   Phil 
  Mag. 
  S. 
  6. 
  Vol. 
  16. 
  No. 
  94. 
  Sept. 
  1908. 
  2 
  G 
  

  

  