﻿S6 
  Prof. 
  L. 
  R. 
  Ingersoll 
  on 
  Magnetic 
  

  

  these 
  three 
  types 
  it 
  will 
  be, 
  possible 
  to 
  estimate 
  the 
  corrections 
  

   with 
  ample 
  accuracy 
  for 
  any 
  of 
  the 
  curves 
  discussed 
  in 
  the 
  

   present 
  paper. 
  Most 
  of 
  them, 
  as 
  a 
  matter 
  of 
  fact, 
  are 
  of 
  the 
  

   latter 
  two 
  types. 
  

  

  Sources 
  of 
  Error 
  . 
  Accuracy 
  of 
  the 
  Method. 
  

  

  The 
  writer 
  has 
  previously 
  discussed 
  t 
  a 
  number 
  of 
  sources 
  

   of 
  error 
  incidental 
  to 
  this 
  work. 
  In 
  the 
  present 
  case 
  an 
  

   attempt, 
  at 
  least, 
  has 
  been 
  made 
  either 
  to 
  remove 
  these 
  or 
  

   make 
  them 
  subjects 
  of 
  special 
  experimentation, 
  with 
  a 
  view 
  

   of 
  determining 
  the 
  magnitude 
  of 
  the 
  errors 
  they 
  introduce. 
  

   It 
  would 
  seem 
  then 
  that 
  the 
  accuracy 
  with 
  which 
  magnetic 
  

   rotations 
  can 
  be 
  measured 
  in 
  the 
  infra-red 
  is 
  largely 
  dependent 
  

   on 
  the 
  care 
  with 
  which 
  the 
  preliminary 
  experiments 
  have 
  been 
  

   carried 
  out. 
  

  

  Fortunately 
  it 
  is 
  possible, 
  as 
  previously 
  stated, 
  to 
  get 
  a 
  

   check 
  on 
  this 
  accuracy 
  by 
  making 
  known 
  mechanical 
  rotations 
  

   of 
  the 
  plane 
  of 
  polarization 
  and 
  measuring 
  them 
  just 
  as 
  if 
  

   of 
  electromagnetic 
  origin. 
  Table 
  II. 
  shows 
  the 
  results 
  of 
  

   these 
  tests. 
  The 
  rotation 
  of 
  the 
  polarizer 
  of 
  1°*178 
  — 
  pro- 
  

   duced 
  by 
  six 
  complete 
  turns 
  of 
  the 
  micrometer 
  tangent-screw 
  

   — 
  was 
  measured 
  with 
  the 
  apparatus 
  for 
  a 
  number 
  of 
  wave- 
  

   lengths 
  and 
  with 
  each 
  of 
  the 
  two 
  sets 
  of 
  prisms 
  and 
  mirrors. 
  

   Lest 
  it 
  be 
  very 
  reasonably 
  objected 
  that 
  the 
  mechanical 
  

   rotations 
  have 
  been 
  already 
  used 
  to 
  determine 
  a 
  series 
  of 
  

   values 
  of 
  the 
  reduction 
  factors 
  A, 
  B, 
  C, 
  and 
  D, 
  — 
  and 
  that 
  if 
  

   these 
  values 
  are 
  used 
  the 
  table 
  would 
  necessarily 
  show 
  a 
  

   correspondence, 
  — 
  the 
  series 
  (2), 
  determined 
  by 
  the 
  preferable 
  

   or 
  rotating 
  sector 
  process, 
  has 
  been 
  used 
  in 
  this 
  computation. 
  

  

  Table 
  II. 
  

  

  Test 
  of 
  Accuracy 
  of 
  Method 
  by 
  Measurement 
  of 
  a 
  known 
  

  

  Rotation 
  of 
  1°-178. 
  

  

  Wave- 
  length. 
  

  

  With 
  45° 
  prism. 
  

  

  1 
  WaTe-length. 
  

  

  ■ 
  

  

  With 
  25° 
  prism. 
  

  

  •69 
  u 
  

  

  *ri9i° 
  

  

  ! 
  -59 
  /« 
  

  

  r]66° 
  

  

  •7,5 
  

  

  n-J58 
  

  

  -64 
  

  

  1-160 
  

  

  •81 
  

  

  1-170 
  

  

  -72 
  

  

  1-170 
  

  

  •88 
  

  

  1.163 
  

  

  •85 
  

  

  M72 
  

  

  •98 
  

  

  1-174 
  

  

  104 
  

  

  1-168 
  

  

  1-23 
  

  

  ]-169 
  

  

  1-31 
  

  

  1-170 
  

  

  1-61 
  

  

  1-170 
  

  

  i 
  164 
  

  

  1-171 
  

  

  1-84 
  

  

  1-169 
  

  

  ! 
  1-97 
  

  

  1-173 
  

  

  2-01 
  

  

  1-189 
  

  

  ! 
  

  

  

  * 
  The 
  smaller 
  amount 
  of 
  energy 
  makes 
  measurements 
  in 
  the 
  shorter 
  

   •wave-leno-ths 
  less 
  reliable. 
  

  

  i- 
  Phil. 
  Mag. 
  he. 
  cit. 
  p. 
  51. 
  

  

  