﻿146 
  

  

  G. 
  Barns 
  — 
  Methods 
  in 
  Reversed 
  and 
  

  

  It 
  is 
  preferable 
  to 
  use 
  sunlight 
  directly 
  (without 
  along 
  focus 
  

   condensing 
  lens), 
  as 
  there 
  is 
  a 
  superabundance 
  of 
  light. 
  The 
  

   best 
  results 
  are 
  attained 
  with 
  a 
  long 
  collimator. 
  A 
  spectacle 
  

   lens 
  with 
  a 
  focal 
  distance 
  of 
  one 
  meter 
  is 
  excellent. 
  The 
  range 
  

   of 
  displacement 
  of 
  M 
  is 
  not 
  increased, 
  but 
  the 
  spectra 
  and 
  

   fringes 
  become 
  very 
  sharp. 
  If 
  with 
  the 
  large 
  collimator 
  the 
  

   spectra 
  are 
  just 
  separated 
  in 
  the 
  field 
  of 
  the 
  telescope, 
  by 
  fore 
  

   and 
  aft 
  motion 
  of 
  P, 
  a 
  magnificent 
  display 
  appears 
  resembling 
  

  

  Figs. 
  14-19. 
  

  

  a 
  if 
  a' 
  yf 
  a" 
  s 
  , 
  

  

  Y8a 
  j£ 
  

  

  a 
  thick 
  twisted 
  golden 
  cord. 
  With 
  further 
  separation 
  confocal 
  

   elliptic 
  fringes 
  often 
  cross 
  the 
  gap, 
  as 
  in 
  fig. 
  16. 
  Here 
  a 
  and 
  (3 
  are 
  

   graphs 
  suggesting 
  the 
  wave-lengths 
  of 
  the 
  two 
  spectra, 
  g 
  being 
  

   the 
  gap 
  or 
  deficient 
  overlapping. 
  The 
  appearance 
  in 
  the 
  tele- 
  

   scope 
  is 
  shown 
  at 
  7, 
  £and 
  S' 
  being 
  the 
  spectra. 
  When 
  the 
  

   fringes 
  are 
  erect, 
  huge 
  vertical 
  furrows 
  may 
  lie 
  in 
  the 
  gap. 
  

   When 
  the 
  gap 
  is 
  closed, 
  the 
  linear 
  phenomenon 
  reappears. 
  

   These 
  enlarged 
  fringes 
  vanish, 
  however, 
  within 
  one-fourth 
  mil- 
  

   limeter 
  of 
  displacement 
  at 
  M. 
  

  

  In 
  further 
  experiments, 
  screens 
  «§, 
  s' 
  (fig. 
  15) 
  were 
  placed 
  in 
  

   the 
  paths 
  of 
  the 
  pencils 
  b, 
  b\ 
  so 
  that 
  they 
  were 
  compelled 
  to 
  pass 
  

   through 
  vertical 
  slits, 
  1 
  to 
  3 
  mm 
  wide, 
  in 
  the 
  screens. 
  In 
  this 
  

   way 
  the 
  interfering 
  rays 
  were 
  identified. 
  The 
  first 
  vertical 
  

  

  