﻿148 
  

  

  C. 
  Barus 
  — 
  Methods 
  in 
  Reversed 
  and 
  

  

  do 
  not 
  change 
  their 
  point 
  of 
  impact 
  at 
  the 
  prismatic 
  mirror 
  P\ 
  

   Ifpp 
  represents 
  the 
  principal 
  plane 
  of 
  the 
  objective 
  of 
  the 
  

   telescope 
  and 
  F 
  its 
  principal 
  focns, 
  there 
  should 
  be 
  no 
  acces- 
  

   sory 
  effect 
  for 
  the 
  case 
  y 
  as 
  compared 
  with 
  the 
  case 
  x. 
  

  

  Table 
  I. 
  Reversed 
  Spectra. 
  

   Refracting 
  prism, 
  6 
  = 
  49° 
  45'; 
  B 
  — 
  4*6 
  x 
  10" 
  11 
  (assumed) 
  ; 
  z=E(fi-l) 
  + 
  2BE/M 
  

  

  Remarks 
  

  

  E 
  

  

  e 
  

  

  X 
  

  

  2y 
  

  

  z 
  

  

  P 
  

  

  

  

  

  cm. 
  

  

  cm. 
  

  

  cm. 
  

  

  cm. 
  

  

  cm. 
  

  

  

  Mirror 
  

  

  right, 
  Plate 
  51 
  

  

  : 
  right 
  

  

  •736 
  

  

  •203 
  

  

  •381 
  

  

  •405 
  

  

  •3901 
  

  

  1-526 
  

  

  a 
  

  

  left, 
  

  

  left 
  

  

  a 
  

  

  •208 
  

  

  •391 
  

  

  •408 
  

  

  + 
  •0195 
  

  

  

  a 
  

  

  right, 
  « 
  

  

  right 
  

  

  « 
  

  

  •203 
  

  

  •381 
  

  

  •402 
  

  

  = 
  •4096 
  

  

  

  a 
  

  

  left, 
  

  

  left 
  

  

  a 
  

  

  •207 
  

  

  •389 
  

  

  •401 
  

  

  

  

  a 
  

  

  right, 
  " 
  

  

  right 
  f 
  

  

  •736 
  

  

  •206 
  

  

  •387 
  

  

  •406 
  

  

  

  

  a 
  

  

  left, 
  " 
  

  

  left 
  

  

  a 
  

  

  •207 
  

  

  •389 
  

  

  •406 
  

  

  

  

  a 
  

  

  right, 
  " 
  

  

  right 
  

  

  •434 
  

  

  •124 
  

  

  •234 
  

  

  •249 
  

  

  •2313 
  

  

  1-533 
  

  

  a 
  

  

  left, 
  

  

  left 
  

  

  u 
  

  

  •126 
  

  

  •236 
  

  

  •250 
  

  

  + 
  •0115 
  

  

  

  a 
  

  

  left, 
  

  

  right 
  

  

  u 
  

  

  •126 
  

  

  •237 
  

  

  

  

  = 
  •2428 
  

  

  

  * 
  Plate 
  slightly 
  wedge-shaped. 
  f 
  Different 
  part 
  of 
  micrometer 
  screw. 
  

  

  Results 
  bearing 
  on 
  this 
  subject 
  are 
  given 
  in 
  Table 
  I, 
  in 
  

   which 
  the 
  displacement, 
  <?, 
  observed 
  at 
  M 
  and 
  at 
  JY, 
  as 
  well 
  as 
  

   the 
  displacement 
  y 
  at 
  P\ 
  are 
  recorded, 
  when 
  a 
  plate 
  of 
  glass 
  

   of 
  thickness 
  E 
  is 
  inserted 
  normally 
  to 
  the 
  rays 
  b,b'. 
  The 
  cor- 
  

   responding 
  air 
  path 
  difference 
  computed 
  from 
  A) 
  jjl, 
  B, 
  \, 
  

   should 
  be 
  s, 
  nearly. 
  This 
  is 
  about 
  the 
  value 
  (2y) 
  observed, 
  

   remembering 
  that 
  to 
  set 
  the 
  micrometer, 
  fringes 
  of 
  a 
  particular 
  

   pattern 
  must 
  be 
  selected. 
  The 
  rotation 
  of 
  fringes 
  being 
  but 
  

   90°, 
  or 
  less, 
  there 
  are 
  no 
  fiducial 
  horizontal 
  lines. 
  

  

  The 
  values 
  of 
  x 
  computed 
  from 
  6 
  and 
  <?, 
  however, 
  certainly 
  

   fall 
  below 
  z, 
  being 
  about 
  6 
  per 
  cent 
  and 
  3 
  per 
  cent 
  short 
  of 
  it 
  

   in 
  the 
  two 
  cases, 
  respectively 
  : 
  or 
  again 
  x 
  is 
  - 
  019 
  cm 
  and 
  "014 
  

   (about 
  5 
  per 
  cent) 
  smaller 
  than 
  the 
  mean 
  values 
  observed 
  for 
  

   2y. 
  This 
  extra 
  5 
  per 
  cent 
  of 
  path 
  difference 
  can 
  not 
  be 
  an 
  

   error 
  of 
  observation, 
  or 
  of 
  adjustment, 
  but 
  must 
  be 
  interpreted 
  

   as 
  the 
  path 
  difference 
  added, 
  when 
  the 
  pencil 
  shifts 
  towards 
  

   the 
  edge 
  of 
  the 
  prism 
  (x) 
  instead 
  of 
  being 
  stationary 
  as 
  in 
  y. 
  

   In 
  cases 
  of 
  inverted 
  spectra 
  moreover 
  (next 
  paper) 
  x 
  is 
  usually 
  

   in 
  excess 
  of 
  .3-, 
  and 
  the 
  shift 
  is 
  the 
  other 
  way. 
  The 
  deficiency 
  

   in 
  x, 
  though 
  not 
  equally 
  marked, 
  is 
  present 
  in 
  observations 
  

   both 
  on 
  the 
  right 
  and 
  left 
  side 
  of 
  the 
  prism 
  P' 
  . 
  

  

  13. 
  Breadth 
  of 
  efficient 
  wave-fronts 
  and 
  apparent 
  uni- 
  

   formity 
  of 
  wave 
  trains. 
  Potation 
  of 
  fringes. 
  — 
  It 
  follows 
  from 
  

  

  