﻿178 
  

  

  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Use 
  of 
  the 
  

  

  spectroscope 
  is 
  unable 
  to 
  resolve 
  them. 
  If 
  now 
  in 
  addition 
  

   to 
  the 
  continuous 
  spectrum 
  we 
  have 
  a 
  bright-line 
  spectrum, 
  

   the 
  lines 
  will 
  cut 
  across 
  the 
  arcs 
  as 
  shown 
  in 
  fig. 
  1, 
  the 
  

   light 
  being 
  absent 
  in 
  the 
  line 
  at 
  each 
  point 
  of 
  intersection. 
  

  

  Fisr. 
  1. 
  

  

  These 
  dark 
  spots 
  in 
  the 
  spectrum 
  lines 
  are 
  the 
  cross-sections 
  

   of 
  the 
  circular 
  interference 
  rings. 
  In 
  fig. 
  1 
  the 
  spectrum 
  

   line 
  A 
  is 
  so 
  placed 
  in 
  the 
  spectrum, 
  that 
  with 
  the 
  given 
  

   separation 
  of 
  the 
  interferometer 
  plates, 
  four 
  dark 
  rings 
  appear, 
  

   the 
  centre 
  of 
  the 
  system 
  being 
  bright. 
  Line 
  C, 
  however, 
  

   shows 
  a 
  dark 
  centre 
  surrounded 
  by 
  three 
  rings. 
  Lines 
  

   A', 
  B', 
  and 
  0' 
  show 
  the 
  state 
  of 
  things 
  when 
  the 
  interfero- 
  

   meter 
  plates 
  are 
  separated 
  by 
  a 
  thicker 
  air-film, 
  seven 
  ring- 
  

   sections 
  appearing 
  in 
  each 
  line. 
  This 
  is 
  merely 
  another 
  way 
  

   of 
  showing 
  that 
  the 
  ring 
  system 
  becomes 
  smaller 
  as 
  the 
  

   plates 
  are 
  separated. 
  (Lines 
  A', 
  B', 
  C 
  are 
  not 
  supposed 
  to 
  

   be 
  the 
  same 
  lines 
  as 
  A, 
  B, 
  C.) 
  

  

  On 
  Plate 
  IV. 
  fig. 
  2 
  we 
  have 
  a 
  photograph 
  of 
  the 
  green 
  and 
  

   blue-green 
  bands 
  of 
  the 
  aluminium 
  arc 
  taken 
  under 
  the 
  

   specified 
  conditions, 
  which 
  shows 
  patterns 
  made 
  up 
  of 
  closed 
  

   curves 
  of 
  elliptical 
  or 
  circular 
  form. 
  These 
  always 
  appear 
  

   when 
  the 
  head 
  of 
  the 
  band 
  is 
  on 
  the 
  side 
  of 
  short 
  wave-length. 
  

   If 
  the 
  band 
  points 
  the 
  other 
  way, 
  the 
  patterns 
  are 
  made 
  up 
  of 
  

   curves 
  of 
  hyperbola 
  form, 
  as 
  is 
  shown 
  by 
  PJate 
  IV. 
  fig. 
  1, 
  

   a 
  photograph 
  of 
  the 
  band 
  of 
  the 
  carbon 
  arc 
  having 
  its 
  head 
  

  

  