﻿Rotation 
  in 
  Iron 
  Catlwde 
  Films. 
  83 
  

  

  and 
  the 
  two 
  shutters. 
  An 
  auxiliary 
  rod 
  also 
  turned 
  the 
  

   micrometer 
  screw 
  of 
  the 
  polarizer 
  mounting. 
  

  

  Supposing 
  everything 
  in 
  adjustment, 
  the 
  process 
  of 
  making 
  

   observations 
  was 
  fairly 
  simple. 
  The 
  spectrometer 
  having 
  

   been 
  set 
  at 
  the 
  wave-length 
  for 
  which 
  it 
  was 
  desired 
  to 
  

   measure 
  the 
  rotation, 
  the 
  sliding 
  shade 
  was 
  moved 
  by 
  means 
  

   of 
  its 
  screw 
  adjustment 
  till 
  both 
  beams 
  were 
  of 
  equal 
  inten- 
  

   sity, 
  as 
  shown 
  by 
  the 
  galvanometer 
  returning 
  to 
  zero. 
  The 
  

   magnet-switch 
  was 
  then 
  thrown, 
  and 
  in 
  a 
  few 
  seconds 
  the 
  

   i'eading 
  taken. 
  The 
  current 
  was 
  then 
  reversed 
  and 
  the 
  new 
  

   reading 
  noted, 
  and 
  so 
  on 
  until 
  a 
  set 
  of 
  four 
  or 
  five 
  readings 
  

   .-at 
  regular 
  intervals 
  of 
  perhaps 
  15 
  seconds, 
  had 
  been 
  taken 
  

   for 
  each 
  direction 
  of 
  the 
  current. 
  The 
  difference 
  between 
  

   the 
  means 
  of 
  each 
  set 
  of 
  readings 
  would 
  give 
  the 
  quantity 
  <il, 
  

   but 
  it 
  would 
  also 
  contain 
  the 
  small 
  but 
  still 
  noticeable 
  direct 
  

   .action 
  of 
  the 
  magnet 
  on 
  the 
  galvanometer. 
  To 
  eliminate 
  

   this 
  the 
  galvanometer 
  connexions 
  were 
  reversed 
  by 
  a 
  suitable 
  

   switch 
  and 
  the 
  above 
  process 
  repeated, 
  and 
  a 
  sHghtly 
  dif- 
  

   ferent 
  value 
  for 
  dl 
  obtained. 
  The 
  mean 
  of 
  these 
  two 
  values 
  

   gaye 
  the 
  true 
  one. 
  

  

  To 
  measure 
  the 
  intensity 
  I 
  it 
  was 
  first 
  necessary 
  to 
  reduce 
  

   the 
  sensibility 
  by 
  a 
  considerable 
  factor 
  — 
  usually 
  26*6 
  times 
  — 
  

   by 
  pulling 
  one 
  of 
  the 
  switches, 
  Sw^ 
  thus 
  inserting 
  a 
  series 
  

   resistance. 
  Then 
  by 
  closing 
  each 
  shutter 
  separately 
  two 
  

   nearly 
  equal 
  values 
  of 
  I, 
  or 
  rather 
  of 
  1/26*6, 
  would 
  be 
  given, 
  

   the 
  mean 
  of 
  which 
  was 
  used. 
  As 
  explained 
  above, 
  the 
  ratio 
  

   dljl 
  is 
  proportional 
  to 
  the 
  rotation 
  occurring 
  on 
  reversal 
  of 
  

   the 
  magnet. 
  

  

  By 
  repeating 
  this 
  process 
  for 
  a 
  dozen 
  or 
  twenty 
  wave- 
  

   lengths 
  distributed 
  over 
  the 
  spectral 
  region, 
  in 
  which 
  it 
  was 
  

   possible 
  to 
  w^ork, 
  say 
  from 
  \=-6)Lt 
  to 
  2*2 
  /a, 
  a 
  very 
  good 
  

   rotation 
  dispersion-curve 
  could 
  usually 
  be 
  obtained. 
  As 
  in 
  

   the 
  previous 
  work, 
  all 
  the 
  more 
  careful 
  measurements 
  had 
  

   to 
  be 
  made 
  at 
  night. 
  

  

  Large 
  and 
  Small 
  Rotations. 
  — 
  In 
  the 
  determination 
  o£ 
  large 
  

   rotations 
  this 
  method 
  has 
  very 
  evident 
  limitations 
  when 
  com- 
  

   pared 
  with 
  visual 
  schemes, 
  but 
  as 
  the 
  largest 
  measurable 
  

   angle, 
  perhaps 
  30°, 
  is 
  many 
  times 
  what 
  would 
  be 
  met 
  w^ith 
  

   in 
  any 
  work 
  with 
  films, 
  this 
  consideration 
  is 
  of 
  no 
  importance. 
  

   Of 
  more 
  interest 
  is 
  the 
  question 
  of 
  the 
  smallest 
  measurable 
  

   rotation. 
  This 
  is 
  largely 
  a 
  matter 
  of 
  the 
  amount 
  of 
  energy 
  

   incident 
  on 
  the 
  bolometer 
  strips 
  available 
  for 
  measurement, 
  

   and 
  hence 
  depends 
  on 
  the 
  part 
  of 
  the 
  spectrum 
  ; 
  also 
  on 
  the 
  

   general 
  conditions 
  of 
  steadiness 
  of 
  the 
  apparatus. 
  No 
  especial 
  

   pains 
  have 
  ever 
  been 
  taken 
  to 
  actually 
  determine 
  the 
  smallest 
  

   rotation 
  which 
  could 
  be 
  detected, 
  but 
  in 
  practice 
  angles 
  of 
  

  

  G 
  2 
  

  

  