﻿obtained 
  with 
  Glass 
  Wedges. 
  431 
  

  

  sheet 
  and 
  laid 
  one 
  on 
  the 
  other 
  at 
  random. 
  The 
  obstacle 
  

   was 
  simply 
  a 
  blackened 
  card 
  held 
  in 
  front 
  of 
  the 
  plates, 
  

   which 
  were 
  inclined 
  at 
  45° 
  to 
  the 
  light 
  from 
  a 
  sodium 
  burner 
  

   and 
  photographed 
  direct 
  in 
  the 
  camera. 
  The 
  upper 
  part 
  of 
  

   the 
  plates, 
  being 
  illuminated 
  by 
  the 
  directly 
  transmitted 
  

   light, 
  shows 
  the 
  Newton's 
  rings 
  only, 
  and 
  the 
  fringes 
  under 
  

   consideration 
  are 
  those 
  in 
  the 
  first 
  shadow 
  space, 
  which 
  

   forms 
  a 
  broad 
  band 
  parallel 
  to 
  the 
  edge 
  of 
  the 
  card, 
  and 
  of 
  

   about 
  the 
  same 
  thickness 
  as 
  that 
  of 
  one 
  of 
  the 
  pieces 
  of 
  glass. 
  

   These 
  fringes 
  are 
  quite 
  as 
  marked 
  as 
  the 
  Newton's 
  rings, 
  

   although 
  of 
  course 
  the 
  total 
  intensity 
  of 
  the 
  light 
  is 
  less 
  in 
  

   the 
  shadow 
  space 
  (the 
  upper 
  part 
  of 
  the 
  glass 
  was 
  shaded 
  

   with 
  a 
  card 
  during 
  9/10 
  of 
  the 
  exposure 
  to 
  render 
  the 
  

   intensities 
  of 
  the 
  two 
  parts 
  more 
  equal 
  in 
  the 
  photograph). 
  

  

  I 
  noticed 
  these 
  fringes 
  first 
  when 
  examining 
  some 
  pieces 
  

   of 
  plate-glass 
  by 
  Newton's 
  rings 
  for 
  planeness 
  of 
  surface, 
  

   and 
  was 
  struck 
  with 
  the 
  extremely 
  simple 
  means 
  they 
  give 
  

   of 
  determining 
  at 
  the 
  same 
  time 
  the 
  variations 
  in 
  thickness 
  

   of 
  the 
  plates. 
  In 
  testing 
  a 
  piece 
  of 
  glass 
  for 
  uniformity 
  of 
  

   thickness, 
  when 
  it 
  is 
  plane 
  parallel 
  to 
  within 
  a 
  few 
  wave- 
  

   lengths 
  all 
  over 
  its 
  surface 
  the 
  circular 
  system 
  of 
  fringes 
  

   discovered 
  by 
  Haidinger 
  can 
  be 
  used, 
  but 
  these 
  fringes 
  dis- 
  

   appear 
  whenever 
  the 
  wedge 
  angle 
  is 
  at 
  all 
  appreciable. 
  I 
  

   have 
  seldom 
  been 
  able 
  to 
  see 
  them 
  on 
  ordinary 
  plate-glass, 
  

   which, 
  although 
  it 
  may 
  have 
  an 
  excellent 
  surface, 
  is 
  almost 
  

   invariably 
  wedge-shaped 
  and 
  often 
  with 
  a 
  considerable 
  angle. 
  

   In 
  spite 
  of 
  this, 
  for 
  many 
  purposes 
  ordinary 
  plate-glass 
  w 
  T 
  ill 
  

   serve 
  very 
  well 
  ; 
  for 
  example, 
  in 
  the 
  numerous 
  cases 
  where 
  

   a 
  beam 
  of 
  light 
  has 
  to 
  pass 
  through 
  two 
  pieces 
  of 
  glass 
  in 
  

   succession, 
  slightly 
  wedge-shaped 
  glass 
  w 
  T 
  ill 
  be 
  quite 
  satis- 
  

   factory 
  so 
  long 
  as 
  the 
  two 
  pieces 
  are 
  suitably 
  arranged 
  so 
  

   that 
  their 
  wedge 
  angles 
  annul 
  each 
  other's 
  effect. 
  For 
  such 
  

   purposes 
  the 
  interference-fringes 
  discussed 
  in 
  this 
  paper 
  form 
  

   a 
  very 
  simple 
  method 
  of 
  testing 
  glass. 
  There 
  is 
  no 
  necessity 
  

   to 
  have 
  a 
  standard 
  glass 
  of 
  uniform 
  thickness. 
  If 
  two 
  pieces 
  

   of 
  plate-glass 
  are 
  cut 
  from 
  the 
  same 
  sheet 
  each 
  can 
  be 
  tested 
  

   absolutely 
  by 
  means 
  of 
  the 
  other. 
  The 
  way 
  in 
  which 
  this 
  

   may 
  be 
  done 
  will 
  be 
  rendered 
  more 
  clear 
  by 
  considering- 
  

   some 
  of 
  the 
  properties 
  of 
  these 
  fringes, 
  which 
  are 
  very 
  

   characteristic, 
  and 
  different 
  from 
  those 
  of 
  Newton's 
  rings. 
  

  

  1. 
  As 
  will 
  be 
  seen 
  from 
  fig. 
  1, 
  the 
  fringes 
  represent 
  the 
  

   loci 
  of 
  points 
  at 
  which 
  the 
  thickness 
  of 
  plate 
  i. 
  is 
  a 
  constant 
  

   amount 
  greater 
  or 
  less 
  than 
  that 
  of 
  plate 
  ii. 
  Since 
  each 
  of 
  

   the 
  tw 
  T 
  o 
  interfering 
  rays 
  passes 
  through 
  the 
  air-film 
  the 
  

   positions 
  of 
  the 
  fringes 
  are 
  independent 
  of 
  the 
  thickness 
  of 
  

   the 
  air-film. 
  Thus, 
  on 
  pressing 
  the 
  two 
  plates 
  together 
  or 
  

   separating 
  them 
  the 
  fringes 
  remain 
  apparently 
  fixed 
  to 
  the 
  

  

  