﻿108 
  Barus 
  — 
  Trial 
  of 
  Interferential 
  Induction 
  balance. 
  

  

  the 
  train 
  of 
  lenses 
  e, 
  f 
  c. 
  As 
  a 
  rule 
  the 
  flame 
  with 
  a 
  single 
  

   lens 
  would 
  be 
  sufficient 
  and 
  the 
  train 
  is 
  here 
  given 
  to 
  admit 
  of 
  

   the 
  introduction 
  of 
  the 
  stroboscopic 
  disc 
  G 
  when 
  an 
  inter- 
  

   mittent 
  beam 
  is 
  wanted. 
  Other 
  small 
  fixed 
  screens 
  are 
  often 
  

   advantageous. 
  Falling 
  on 
  M 
  (preferably 
  covered 
  with 
  a 
  trans- 
  

   parent 
  coat 
  of 
  silver), 
  the 
  light 
  undergoes 
  interferential 
  refrac- 
  

  

  tion 
  in 
  the 
  now 
  well-known 
  way. 
  The 
  optical 
  parts 
  of 
  the 
  

   apparatus 
  are 
  fully 
  described 
  in 
  Michelson's* 
  great 
  memoir, 
  

   and 
  further 
  reference 
  here 
  is 
  needless. 
  To 
  obtain 
  the 
  inter- 
  

   ference 
  it 
  is 
  usually 
  sufficient 
  (good 
  glass 
  presupposed) 
  to 
  bring 
  

   the 
  images 
  of 
  the 
  same 
  small 
  distant 
  luminous 
  point 
  into 
  coin- 
  

   dence. 
  Removing/* 
  and 
  c, 
  a 
  small 
  hole 
  in 
  G 
  answers 
  the 
  pur- 
  

   poses. 
  If 
  the 
  helix 
  A 
  and 
  its 
  mirror 
  is 
  fixed, 
  the 
  helix 
  B 
  and 
  

   its 
  mirror 
  must 
  be 
  movable 
  in 
  azimuth 
  and 
  altitude. 
  I 
  found 
  

   the 
  large 
  round 
  table 
  of 
  an 
  old 
  Kirchhoff 
  spectrometer 
  excel- 
  

   lent 
  for 
  this 
  purpose. 
  The 
  s 
  helices 
  A 
  and 
  B 
  were 
  mounted 
  in 
  

   place 
  of 
  the 
  telescopes, 
  and 
  the 
  mirror 
  M 
  on 
  a 
  small 
  adjustable 
  

   tripod 
  could 
  then 
  be 
  moved 
  on 
  the 
  flat 
  table 
  into 
  position. 
  If 
  

   the 
  glass 
  is 
  not 
  very 
  good, 
  as 
  is 
  apt 
  to 
  be 
  the 
  case 
  with 
  thin 
  

   mirrors, 
  m 
  and 
  n, 
  and 
  ordinary 
  plate 
  glass, 
  it 
  is 
  necessary 
  to 
  

   find 
  the 
  parts 
  of 
  the 
  plates 
  sufficiently 
  plane, 
  by 
  trial. 
  In 
  

   general 
  it 
  is 
  best 
  to 
  bring 
  the 
  edges 
  of 
  the 
  mirrors 
  to 
  overlap, 
  

   in 
  which 
  case 
  the 
  interference 
  figure 
  is 
  of 
  lune-shaped 
  outline 
  

   and 
  relatively 
  clear. 
  The 
  adjustments 
  may 
  be 
  made 
  with 
  the 
  

   naked 
  eye. 
  For 
  observations, 
  however, 
  the 
  small 
  telescope 
  T, 
  

  

  *Michelson: 
  Yaleur 
  du 
  Metre, 
  Trav. 
  Bureau 
  Int., 
  xi, 
  1894. 
  

  

  