﻿176 
  . 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Use 
  of 
  the 
  

  

  found 
  in 
  graphite 
  to 
  the 
  measurement 
  of 
  magnetic 
  fields, 
  it 
  

   appears 
  that 
  the 
  lack 
  of 
  definition 
  and 
  mechanical 
  strength 
  

   of 
  this 
  material 
  would 
  form 
  serious 
  objections. 
  Also 
  the 
  

   resistance 
  of 
  a 
  graphite 
  piece 
  is 
  much 
  less 
  than 
  that 
  of 
  a 
  

   bismuth 
  spiral. 
  The 
  chief 
  disadvantage 
  in 
  the 
  use 
  of 
  the 
  

   latter 
  lies 
  in 
  the 
  difficulties 
  of 
  determining 
  the 
  temperature 
  

   correction 
  ; 
  the 
  isothermals 
  intersect 
  one 
  another 
  ; 
  this, 
  

   however, 
  holds 
  also 
  in 
  the 
  case 
  of 
  graphite, 
  as 
  is 
  seen 
  by- 
  

   referring 
  to 
  fig. 
  4. 
  

  

  I 
  beg, 
  in 
  conclusion, 
  to 
  express 
  nry 
  gratitude 
  to 
  Prof, 
  

   du 
  Bois, 
  in 
  whose 
  laboratory 
  the 
  experiments 
  were 
  carried 
  

   out, 
  for 
  his 
  constant 
  advice 
  and 
  criticism, 
  and 
  for 
  his 
  kindness 
  

   in 
  placing 
  the 
  powerful 
  du 
  Bois 
  electromagnet 
  and 
  other 
  

   apparatus 
  at 
  my 
  disposal. 
  I 
  am 
  also 
  indebted 
  to 
  Dr. 
  W. 
  J. 
  

   de 
  Haas 
  for 
  many 
  valuable 
  suggestions 
  during 
  the 
  course 
  of 
  

   the 
  investigation. 
  

  

  Bosscha-Laboratorium, 
  

   Berlin. 
  

   January 
  3rd, 
  1913. 
  

  

  VII. 
  On 
  the 
  Use 
  of 
  the 
  Interferometer 
  for 
  the 
  Study 
  of 
  Band 
  

   Spectra. 
  By 
  B. 
  W. 
  Wood, 
  Professor 
  of 
  Experimental 
  

   Physics, 
  Johns 
  Hopkins 
  University*. 
  

   [[Plate 
  IV.] 
  

  

  IN 
  a 
  paper 
  published 
  in 
  1907 
  (Phys. 
  Zeit. 
  viii. 
  p. 
  607) 
  

   I 
  drew 
  attention 
  to 
  peculiar 
  geometrical 
  arrangements 
  

   of 
  Ihe 
  Fabry 
  and 
  Perot 
  interference 
  fringes 
  exhibited 
  in 
  

   photographs 
  of 
  the 
  titanium 
  spectrum 
  made 
  by 
  Dr. 
  Pfund. 
  

   As 
  I 
  showed 
  in 
  this 
  paper, 
  the 
  existence 
  of 
  a 
  symmetrical 
  

   pattern 
  in 
  such 
  a 
  photograph 
  indicated 
  that 
  the 
  lines 
  were 
  

   spaced 
  according 
  to 
  some 
  mathematical 
  law, 
  and 
  I 
  suggested 
  

   that 
  in 
  this 
  way 
  regularities 
  might 
  be 
  found 
  in 
  very 
  com- 
  

   plicated 
  line 
  spectra. 
  

  

  The 
  photograph 
  in 
  question 
  was 
  made 
  by 
  passing 
  the 
  light 
  

   from 
  the 
  titanium 
  arc 
  through 
  the 
  half-silvered 
  plates 
  of 
  a 
  

   Fabry 
  and 
  Perot 
  interferometer, 
  and 
  focussing 
  an 
  image 
  of 
  

   the 
  circular 
  ring 
  system 
  upon 
  the 
  slit 
  of 
  a 
  grating 
  spectro- 
  

   graph, 
  the 
  method 
  being 
  the 
  one 
  universally 
  adopced 
  for 
  the 
  

   determination 
  of 
  wave-lengths 
  by 
  interference 
  methods. 
  

   Owing 
  to 
  the 
  large 
  difference 
  of 
  path 
  between 
  the 
  inter- 
  

   fering 
  streams 
  of 
  light, 
  the 
  order 
  of 
  the 
  fringes 
  changes 
  by 
  

   a 
  considerable 
  number 
  as 
  we 
  pass 
  from 
  one 
  line 
  to 
  the 
  next, 
  

   even 
  in 
  the 
  case 
  of 
  lines 
  lying 
  very 
  close 
  together. 
  If 
  the 
  

   * 
  Communicated 
  by 
  the 
  Author. 
  

  

  