﻿952 
  Dr. 
  Gr. 
  Johnstone 
  Stoney 
  on 
  

  

  interfere 
  with 
  one 
  another, 
  it 
  is 
  an 
  interference 
  effect 
  which 
  

   will 
  result 
  from 
  their 
  being 
  both 
  present 
  ; 
  but 
  if 
  they 
  are 
  

   lights 
  which 
  are 
  incapable 
  of 
  permanent 
  interference, 
  their 
  

   being 
  made 
  to 
  co-operate 
  upon 
  disk 
  ,v 
  f 
  only 
  causes 
  an 
  

   illumination 
  at 
  each 
  point 
  of 
  that 
  disk 
  which 
  is 
  the 
  sum 
  of 
  

   the 
  illuminations 
  which 
  they 
  would 
  have 
  separately 
  produced 
  

   at 
  that 
  point. 
  As 
  we 
  shall 
  presently 
  find, 
  it 
  is 
  with 
  this 
  

   second 
  and 
  simpler 
  case 
  that 
  we 
  have 
  here 
  to 
  deal. 
  And 
  

   accordingly 
  what 
  we 
  have 
  to 
  do 
  is 
  to 
  determine 
  what 
  the 
  

   partial 
  images 
  \\ 
  IV, 
  &c. 
  are, 
  and 
  then 
  to 
  add 
  the 
  illumina- 
  

   tion- 
  which 
  they 
  separately 
  produce 
  at 
  each 
  point 
  of 
  disk 
  ./, 
  

   in 
  order 
  to 
  learn 
  what 
  is 
  the 
  brightness 
  at 
  that 
  situation 
  in 
  

   the 
  complete 
  image 
  at 
  ,r\ 
  i.e. 
  in 
  the 
  image 
  which 
  is 
  identical 
  

   with 
  what 
  the 
  astronomer 
  can 
  under 
  the 
  most 
  favourable 
  

   conditions 
  see 
  in 
  hi- 
  telescope. 
  

  

  CHAPTEE 
  6. 
  

  

  Analysis 
  of 
  diffused 
  light 
  incident 
  upon 
  a 
  .small 
  object. 
  

  

  The 
  task 
  with 
  winch 
  we 
  have 
  to 
  grapple 
  is 
  to 
  find 
  out 
  by 
  

   what 
  process 
  these 
  imago 
  are 
  really 
  formed, 
  for 
  it 
  requires 
  

   but 
  little 
  consideration 
  to 
  satisfy 
  ourselves 
  that 
  the 
  ex- 
  

   planation 
  by 
  Geometrical 
  Optics, 
  and 
  other 
  explanations 
  

   which 
  have 
  been 
  offered 
  are 
  either 
  illusory 
  or 
  insufficient. 
  

  

  57. 
  Geometrical 
  Optics 
  is 
  a 
  body 
  of 
  deductions 
  from 
  the 
  

   supposition 
  that 
  light 
  consists 
  of 
  rays 
  — 
  a 
  supposition 
  which 
  

   is 
  hypothesis 
  not 
  theory. 
  A 
  theory 
  means 
  a 
  supposition 
  

   which 
  we 
  think 
  may 
  be 
  true 
  : 
  a 
  hypothesis 
  is 
  a 
  supposition 
  

   we 
  expect 
  to 
  be 
  useful. 
  The 
  only 
  thing 
  to 
  be 
  seen 
  to 
  when 
  

   examining 
  a 
  theory, 
  is 
  whether 
  it 
  is 
  correct 
  or 
  incorrect 
  ; 
  

   for 
  the 
  merit 
  of 
  a 
  theory 
  is 
  simply 
  to 
  be 
  true, 
  irrespectively 
  

   of 
  whether 
  man 
  can 
  make 
  much 
  or 
  little, 
  or 
  indeed 
  any, 
  use 
  

   of 
  it. 
  On 
  the 
  other 
  hand 
  a 
  hypothesis 
  is 
  a 
  supposition 
  of 
  

   which 
  the 
  express 
  intention 
  is 
  that 
  it 
  shall 
  be 
  helpful, 
  and 
  

   which 
  is 
  legitimate 
  if 
  it 
  enables 
  us 
  to 
  make 
  any 
  real 
  progress 
  

   in 
  our 
  work. 
  No 
  doubt 
  a 
  hypothesis 
  may 
  also 
  be 
  a 
  theory 
  — 
  

   in 
  other 
  words, 
  a 
  supposition 
  which 
  we 
  entertain 
  in 
  the 
  

   expectation 
  that 
  it 
  will 
  assist 
  us 
  in 
  our 
  investigation 
  may 
  

   also 
  be 
  the 
  supposition 
  which 
  we 
  think 
  correctly 
  describes 
  

   what 
  is 
  going 
  on 
  in 
  nature, 
  but 
  it 
  by 
  no 
  means 
  needs 
  to 
  be 
  

   so 
  : 
  and 
  in 
  fact 
  the 
  best, 
  i. 
  e. 
  the 
  most 
  useful, 
  hypotheses 
  are 
  

   often 
  of 
  the 
  kind 
  that 
  make 
  no 
  pretence 
  to 
  being 
  true. 
  The 
  

   correct 
  objective 
  theory 
  of 
  light 
  appears 
  to 
  be 
  that 
  light 
  

   consists 
  objectively 
  of 
  waves 
  of 
  alternating 
  electro-magnetic 
  

   stresses 
  advancing 
  through 
  the 
  sether 
  ; 
  whereas 
  the 
  whole 
  of 
  

  

  