﻿82 
  Prof. 
  L. 
  R. 
  Ingersoll 
  on 
  Magnetic 
  

  

  the 
  two 
  thin 
  wedge-shaped 
  shades, 
  which, 
  when 
  moved 
  across 
  

   the 
  slits, 
  darkened 
  one 
  image 
  by 
  cutting 
  off 
  a 
  small 
  part 
  of 
  

   the 
  centre, 
  and 
  at 
  the 
  same 
  time 
  lightened 
  the 
  other 
  until 
  

   the 
  two 
  beams 
  were 
  of 
  equal 
  intensity. 
  Because 
  of 
  slight 
  

   inequalities 
  in 
  the 
  bolometer 
  strips 
  this 
  adjustment 
  had 
  to 
  be 
  

   repeated 
  for 
  each 
  new 
  setting 
  of 
  the 
  spectrometer. 
  

  

  Each 
  of 
  the 
  prisms 
  for 
  the 
  spectrometer 
  was 
  of 
  glass 
  instead 
  

   of 
  rock-salt 
  as 
  formerly 
  used. 
  There 
  was 
  no 
  need 
  of 
  rock- 
  

   salt 
  in 
  this 
  case, 
  since 
  there 
  was 
  at 
  best 
  a 
  considerable 
  thick- 
  

   ness 
  of 
  glass 
  to 
  be 
  traversed 
  by 
  the 
  light, 
  in 
  connexion 
  with 
  

   the 
  double 
  image 
  prisms 
  ; 
  and, 
  moreover, 
  glass 
  has 
  the 
  

   advantage 
  of 
  giving 
  a 
  somewhat 
  more 
  uniform 
  dispersion 
  

   than 
  rock-salt. 
  The 
  prism 
  of 
  lower 
  dispersive 
  power 
  had 
  a 
  

   refracting 
  angle 
  of 
  25°, 
  and 
  was 
  of 
  ordinary 
  flint-glass 
  with 
  

   faces 
  9x9 
  cms. 
  It 
  was 
  used 
  with 
  mirrors 
  of 
  35 
  cms. 
  and 
  

   30 
  cms. 
  focus 
  respectively. 
  The 
  other 
  prism 
  had 
  a 
  45° 
  angle 
  

   and 
  was 
  of 
  extra 
  dense 
  flint 
  with 
  faces 
  9 
  x 
  7*5 
  cms. 
  The 
  

   corresponding 
  mirrors 
  were 
  of 
  50 
  cms. 
  focal 
  length 
  each. 
  

   This 
  arrangement 
  gave 
  a 
  spectrum 
  from 
  three 
  to 
  four 
  times 
  

   longer 
  than 
  the 
  other. 
  

  

  Bolometer 
  and 
  Galvanometer. 
  — 
  The 
  former 
  might 
  be 
  said 
  

   to 
  be 
  of 
  the 
  differential 
  type, 
  as 
  both 
  strips 
  were 
  exposed, 
  

   being 
  vertically 
  above 
  one 
  another. 
  Each 
  strip 
  was 
  '5 
  mm. 
  

   by 
  8 
  mm. 
  The 
  balancing 
  coils 
  and 
  slide 
  wire 
  were 
  all 
  self- 
  

   contained, 
  and 
  the 
  instrument 
  was 
  remarkably 
  steady 
  and 
  

   free 
  from 
  drift. 
  The 
  galvanometer, 
  which 
  was 
  of 
  the 
  Thomson 
  

   astatic 
  type, 
  had 
  a 
  system 
  weighing 
  about 
  2 
  mgs., 
  and 
  was 
  

   worked 
  at 
  a 
  sensibility 
  of 
  from 
  3 
  to 
  5 
  X 
  10~^^ 
  amp. 
  per 
  mm. 
  

   for 
  6 
  ohms 
  resistance. 
  To 
  free 
  it 
  from 
  the 
  influence 
  of 
  the 
  

   magnet 
  three 
  large 
  concentric 
  cylinders 
  of 
  soft 
  steel 
  were 
  

   used 
  as 
  magnetic 
  shields, 
  and 
  in 
  addition 
  the 
  coils 
  themselves 
  

   were 
  embedded 
  in 
  the 
  two 
  halves 
  of 
  a 
  mass 
  of 
  Swedish 
  iron 
  

   3x5 
  cms. 
  This 
  combination 
  gave 
  a 
  shielding 
  effect 
  of 
  over 
  

   '6000 
  times. 
  Proportionality 
  corrections 
  were 
  applied 
  to 
  all 
  

   galvanometer 
  deflexions. 
  

  

  It 
  was 
  necessary, 
  in 
  the 
  process 
  of 
  measurement, 
  to 
  be 
  

   able 
  to 
  reduce 
  the 
  sensitiveness 
  of 
  the 
  bolometer-galvanometer 
  

   combination 
  in 
  some 
  known 
  ratio. 
  This 
  was 
  done 
  by 
  the 
  

   use 
  of 
  four 
  resistances 
  which 
  could 
  be 
  inserted 
  separately 
  in 
  

   series 
  with 
  the 
  galvanometer 
  circuit, 
  and 
  which 
  gave 
  a 
  graded 
  

   reduction 
  of 
  sensibility 
  of 
  from 
  2" 
  9 
  to 
  104*5 
  times. 
  

  

  Control 
  and 
  Operation. 
  — 
  The 
  various 
  parts 
  of 
  the 
  apparatus 
  

   w^ere 
  arranged 
  to 
  be 
  controlled 
  by 
  the 
  observer 
  in 
  front 
  of 
  

   the 
  galvanometer 
  scale. 
  This 
  was 
  done 
  by 
  means 
  of 
  the 
  five 
  

   rods 
  r, 
  r, 
  r, 
  which 
  operated 
  respectively 
  the 
  spectrometer 
  

   tablcj 
  the 
  sliding 
  shade, 
  the 
  bolometer 
  balancing 
  arrangement, 
  

  

  