﻿Rotation 
  in 
  Iron 
  CatJiode. 
  Filmx. 
  79 
  

  

  work, 
  it 
  \yas 
  necessary 
  to 
  resort 
  to 
  a 
  reduction 
  of 
  dispersion. 
  

   But 
  this 
  would 
  mean 
  a 
  sacrifice 
  o£ 
  certainty 
  of 
  wave-length, 
  

   and 
  would 
  mao-ni£y 
  the 
  rather 
  uncertain 
  errors 
  due 
  to 
  the 
  

   width 
  of 
  the 
  slit 
  image 
  and 
  bolometer 
  strip 
  in 
  the 
  spectrum. 
  

  

  To 
  avoid 
  this 
  two 
  distinct 
  sets 
  of 
  prisms 
  and 
  mirrors 
  were 
  

   used 
  in 
  connexion 
  with 
  the 
  same 
  spectrometer 
  table, 
  giving 
  

   in 
  the 
  one 
  case 
  a 
  very 
  intense 
  but 
  correspondingly 
  con- 
  

   tracted 
  spectrum, 
  and 
  in 
  the 
  other 
  a 
  spectrum 
  of 
  several 
  fold 
  

   greater 
  dispersion, 
  but 
  correspondingly 
  weaker 
  in 
  energy. 
  

  

  In 
  practice, 
  then, 
  some 
  film 
  or 
  substance 
  giving 
  a 
  relatively 
  

   large 
  rotation 
  was 
  tested 
  with 
  each 
  of 
  these 
  combinations, 
  

   and 
  the 
  two 
  curves 
  compared 
  with 
  a 
  view 
  of 
  determining 
  

   the 
  efEect 
  of 
  the 
  lower 
  dispersion, 
  if 
  any, 
  in 
  distorting 
  the 
  

   curve. 
  This 
  correction 
  could 
  then 
  be 
  applied 
  with 
  assurance 
  

   to 
  curves 
  obtained 
  with 
  films 
  which 
  gave 
  such 
  small 
  

   rotations 
  that 
  only 
  the 
  lower 
  dispersion 
  arrangement 
  could 
  

   be 
  used. 
  

  

  Tests 
  hy 
  Mechanical 
  Rotation. 
  — 
  While 
  in 
  as 
  simple 
  a 
  form 
  

   as 
  possible, 
  to 
  attain 
  the 
  desired 
  end, 
  the 
  foregoing 
  method 
  

   is 
  necessarily 
  complicated 
  wheu 
  compared 
  with 
  the 
  extremely 
  

   straightforward 
  ways 
  of 
  observing 
  the 
  same 
  phenomena 
  in 
  

   the 
  visible 
  spectrum. 
  Hence 
  it 
  is 
  desirable 
  to 
  have 
  some 
  

   way 
  oF 
  checking 
  the 
  results, 
  or 
  at 
  least 
  of 
  applying 
  a 
  thorough 
  

   test. 
  This 
  was 
  done 
  by 
  mounting 
  the 
  polarizer 
  in 
  a 
  divided 
  

   circle: 
  small 
  and 
  accurately 
  known 
  rotations 
  were 
  then 
  given 
  

   it, 
  causing 
  of 
  course 
  corresponding 
  rotations 
  of 
  the 
  plane 
  of 
  

   polarization. 
  These 
  were 
  measured 
  just 
  as 
  if 
  of 
  magnetic 
  

   origin 
  and 
  checked 
  with 
  the 
  known 
  rotation 
  produced 
  

   mechanically. 
  The 
  results 
  of 
  this 
  test, 
  as 
  well 
  as 
  of 
  the 
  

   measurements 
  spoken 
  of 
  in 
  the 
  last 
  paragraph, 
  will 
  be 
  given 
  

   later. 
  

  

  Apparatus. 
  

  

  In 
  the 
  matter 
  of 
  general 
  arrangement 
  of 
  apparatus 
  the 
  

   former 
  order 
  was 
  preserved, 
  as 
  will 
  be 
  seen 
  from 
  fig. 
  1. 
  

   The 
  w^hole 
  was, 
  however, 
  mounted 
  on 
  massive 
  slate 
  slabs 
  

   which 
  rested 
  on 
  heavy 
  wooden 
  supports. 
  The 
  component 
  

   pieces, 
  with 
  the 
  exception 
  of 
  the 
  optical 
  parts, 
  were 
  con- 
  

   structed 
  in 
  the 
  shops 
  of 
  this 
  Department. 
  

  

  Electromagnet. 
  — 
  This 
  had 
  proved 
  so 
  satisfactory 
  since 
  its 
  

   construction 
  some 
  years 
  ago, 
  that 
  the 
  only 
  change 
  necessary 
  

   was 
  the 
  addition 
  of 
  cooling 
  coils 
  which 
  carried 
  circulating 
  

   tap-water, 
  and 
  kept 
  the 
  ends 
  of 
  the 
  f)ole-pieces 
  at 
  a 
  nearly 
  

   constant 
  temperature. 
  The 
  method 
  of 
  excitation 
  is 
  worth 
  

   brief 
  mention. 
  The 
  winding, 
  which 
  consisted 
  of 
  about 
  6000 
  

   feet 
  of 
  No. 
  10 
  wire, 
  was 
  grouped 
  in 
  eight 
  coils. 
  The 
  four 
  pole 
  

  

  