﻿951 
  Dr. 
  Gr. 
  Johnstone 
  Stoney 
  on 
  

  

  59. 
  This 
  sequence 
  of 
  events 
  naturally 
  divides 
  itself 
  into 
  

   three 
  stages 
  — 
  1°, 
  From 
  cr, 
  the 
  source 
  of 
  light, 
  to 
  the 
  object 
  

   :' 
  : 
  2°, 
  From 
  the 
  object 
  to 
  the 
  pro-telescope 
  ; 
  and 
  3°, 
  Within 
  

   the 
  pro-telescope 
  — 
  in 
  correspondence 
  with 
  whch 
  our 
  study 
  

   of 
  the 
  events 
  will 
  have 
  to 
  consist 
  of 
  three 
  inquiries: 
  1°, 
  How 
  

   the 
  light 
  emitted 
  by 
  a 
  and 
  incident 
  upon 
  .:' 
  is 
  to 
  be 
  analysed 
  ? 
  

   2°, 
  What 
  happens 
  to 
  the 
  portion 
  of 
  this 
  light 
  which 
  gets 
  

   past 
  c', 
  upon 
  its 
  journey 
  between 
  plane 
  71 
  and 
  plane 
  Y\ 
  

   which 
  latter 
  stands 
  in 
  front 
  of 
  the 
  pro-telescope 
  ? 
  And 
  3°,. 
  

   By 
  what 
  further 
  behaviour 
  doi^ 
  this 
  light 
  or 
  some 
  part 
  of 
  it 
  

   afterwards 
  form 
  the 
  image 
  at 
  #'? 
  In 
  the 
  present 
  Chapter 
  

   we 
  shall 
  deal 
  with 
  the 
  first 
  of 
  these 
  inquiries, 
  reserving 
  the 
  

   second 
  and 
  third 
  for 
  consideration 
  afterwards. 
  

  

  60. 
  To 
  fix 
  our 
  ideas 
  we 
  may 
  suppose 
  the 
  source 
  of 
  light 
  a 
  

   to 
  be 
  a 
  bright 
  round 
  disk 
  of 
  paper 
  two 
  inches 
  in 
  diameter 
  

   placed 
  at 
  the 
  situation 
  of 
  / 
  in 
  Plate 
  XIL, 
  i.e. 
  at 
  a 
  distance 
  

   of 
  about 
  a 
  metre 
  and 
  a 
  half 
  to 
  the 
  right 
  of 
  object 
  z' 
  \ 
  and 
  

   we 
  may 
  conceive 
  of 
  this 
  disk 
  as 
  though 
  it 
  were 
  divided 
  into 
  

   patches 
  dai, 
  tftr 
  s 
  , 
  &c., 
  hexagonal 
  like 
  a 
  honeycomb, 
  and 
  of' 
  

   sufficiently 
  small 
  size 
  (which 
  they 
  will 
  be 
  it' 
  about 
  .V 
  mm. 
  

   aero--). 
  Then 
  the 
  light 
  of 
  wave-length 
  \ 
  emitted 
  from 
  

   these 
  small 
  rf<r's 
  and 
  reaching 
  ;' 
  will, 
  under 
  the 
  theorem 
  

   proved 
  in 
  § 
  2'J 
  (see 
  p. 
  335 
  of 
  the 
  Phil. 
  Mag. 
  for 
  last 
  August),. 
  

   he 
  equivalent 
  to 
  u 
  E 
  W's 
  ( 
  undulations 
  of 
  flat 
  waves) 
  travelling 
  

   towards 
  :'. 
  which 
  will 
  he 
  of 
  the 
  same 
  number 
  as 
  the 
  patches. 
  

   inasmuch 
  as 
  one 
  will 
  advance 
  from 
  each 
  da 
  in 
  the 
  direction 
  

   of 
  the 
  line 
  from 
  the 
  middle 
  of 
  that 
  patch 
  to 
  the 
  middle 
  of 
  z' 
  — 
  

   together 
  with 
  an 
  exceedingly 
  small 
  amount 
  of 
  rl 
  (residtial 
  

   light) 
  due 
  to 
  the 
  rulings 
  formed 
  on 
  plane 
  Z' 
  by 
  the 
  P's 
  and 
  

   — 
  Q's 
  described 
  in 
  § 
  29. 
  These 
  rulings 
  produce 
  scarcely 
  

   any 
  illumination 
  at 
  the 
  part 
  of 
  plane 
  Z' 
  where 
  z' 
  is 
  situated. 
  

   In 
  fact, 
  if 
  the 
  da's 
  are 
  as 
  small 
  as 
  we 
  have 
  described 
  them 
  

   and 
  if 
  the 
  object 
  z' 
  is 
  of 
  sufficiently 
  moderate 
  size, 
  this 
  

   residual 
  light 
  will 
  be 
  so 
  faint 
  that 
  in 
  making 
  experiments 
  it 
  

   may 
  be 
  quite 
  left 
  out 
  of 
  account, 
  and 
  the 
  whole 
  of 
  the 
  light 
  

   of 
  wave-length 
  X 
  reaching 
  z 
  1 
  may 
  be 
  regarded 
  as 
  exclusively 
  

   consisting 
  of 
  the 
  same 
  number 
  of 
  u 
  f 
  W's 
  as 
  there 
  are 
  da's 
  in 
  

   a, 
  one 
  arriving 
  at 
  z 
  1 
  from 
  each 
  of 
  the 
  da's. 
  

  

  61. 
  Hitherto 
  we 
  have 
  considered 
  only 
  light 
  of 
  one 
  wave- 
  

   length, 
  whereas 
  the 
  light 
  emitted 
  by 
  a 
  being 
  white 
  light 
  

   contains 
  light 
  of 
  all 
  the 
  wave-lengths 
  included 
  within 
  the 
  

   visible 
  part 
  of 
  the 
  spectrum. 
  Accordingly, 
  the 
  light 
  trans- 
  

   mitted 
  from 
  each 
  one 
  of 
  the 
  da's 
  to 
  z\ 
  is 
  to 
  be 
  regarded 
  as. 
  

   consisting 
  of 
  u 
  f 
  W's 
  of 
  all 
  these 
  wave-lengths, 
  all 
  travelling 
  

   in 
  the 
  same 
  direction, 
  viz., 
  in 
  the 
  direction 
  of 
  the 
  line 
  from, 
  

   the 
  middle 
  of 
  the 
  da 
  to 
  the 
  middle 
  of 
  object 
  z 
  . 
  These 
  from 
  

  

  