﻿124 
  Mr. 
  B. 
  Hopkinson 
  on 
  the 
  

  

  who 
  assert 
  the 
  existence 
  of 
  a 
  medium 
  do 
  so 
  in 
  order 
  to 
  

   satisfy 
  a 
  craving 
  for 
  the 
  explanation 
  of 
  gravity 
  in 
  terms 
  of 
  

   other 
  forces 
  more 
  familar 
  though 
  (be 
  it 
  remarked) 
  not 
  

   necessarily 
  more 
  fundamental. 
  For 
  the 
  expression 
  of 
  the 
  

   laws 
  of 
  gravity 
  it 
  is 
  not 
  necessary 
  to 
  introduce 
  any 
  medium. 
  

  

  But 
  now 
  suppose 
  that 
  the 
  force 
  of 
  attraction 
  between 
  the 
  

   two 
  bodies 
  (supposed 
  to 
  be 
  alone) 
  were 
  found 
  not 
  to 
  depend 
  

   simply 
  on 
  their 
  masses 
  and 
  relative 
  position, 
  but 
  to 
  vary 
  in 
  

   an 
  apparently 
  arbitrary 
  manner. 
  To 
  account 
  for 
  such 
  

   variation 
  and 
  to 
  express 
  its 
  laws, 
  it 
  would 
  be 
  necessary 
  to 
  

   imagine 
  a 
  new 
  cause, 
  independent 
  of 
  material 
  bodies. 
  In 
  

   mathematical 
  terminology 
  the 
  force 
  of 
  gravity 
  would 
  be 
  no 
  

   longer 
  a 
  function 
  of 
  the 
  distance 
  between 
  the 
  bodies 
  only, 
  and 
  

   to 
  express 
  ihe 
  facts 
  mathematically 
  we 
  should 
  have 
  to 
  

   introduce 
  new 
  independent 
  variables 
  — 
  for 
  example, 
  the 
  co- 
  

   ordinates 
  of 
  the 
  bodies 
  with 
  reference 
  to 
  a 
  set 
  of 
  axes 
  fixed 
  

   independently 
  of 
  matter. 
  Given 
  these 
  coordinates 
  it 
  might 
  

   be 
  possible, 
  if 
  the 
  facts 
  fitted, 
  to 
  predict 
  the 
  value 
  of 
  gravity 
  — 
  

   a 
  prediction 
  we 
  could 
  not 
  make 
  if 
  the 
  relative 
  positions 
  of 
  the 
  

   two 
  bodies 
  alone 
  were 
  given. 
  There 
  would 
  then 
  be 
  what 
  I 
  

   call 
  a 
  logical 
  basis 
  for 
  postulating 
  a 
  medium 
  having 
  parts 
  and 
  

   serving 
  to 
  define 
  position 
  independently 
  of 
  matter. 
  

  

  No 
  such 
  logical 
  basis 
  can 
  in 
  fact 
  be 
  found 
  in 
  the 
  pheno- 
  

   menon 
  of 
  gravity, 
  whose 
  laws 
  are 
  entirely 
  expressible 
  in 
  

   terms 
  of 
  matter 
  only. 
  Can 
  any 
  be 
  found 
  in 
  the 
  transmission 
  

   of 
  light? 
  I 
  think 
  there 
  is 
  no 
  doubt 
  that 
  Ihe 
  laws 
  of 
  all 
  

   optical 
  phenomena 
  hitherto 
  discovered, 
  except 
  one 
  to 
  be 
  

   presently 
  mentioned, 
  can 
  be 
  adequately 
  expressed, 
  like 
  that 
  

   of 
  gravitation, 
  in 
  terms 
  of 
  the 
  state 
  and 
  positions 
  of 
  material 
  

   bodies 
  now 
  and 
  in 
  the 
  past. 
  We 
  know 
  that 
  at 
  a 
  point 
  P, 
  

   distant 
  from 
  a 
  luminous 
  body 
  A, 
  we 
  shall 
  by 
  the 
  use 
  of 
  

   appropriate 
  instruments 
  experience 
  the 
  disturbance 
  known 
  as 
  

   light. 
  Interference 
  shows 
  the 
  disturbance 
  to 
  be 
  of 
  a 
  periodic 
  

   character, 
  polarization 
  that 
  it 
  is 
  properly 
  represented 
  as 
  a 
  

   directed 
  quantity 
  at 
  right-angles 
  to 
  AP. 
  Its 
  amount 
  is 
  

  

  J 
  — 
  where 
  t 
  is 
  the 
  time, 
  r 
  the 
  distance 
  AP. 
  That 
  is 
  a 
  

  

  statement 
  of 
  some 
  cardinal 
  optical 
  laws 
  without 
  any 
  mention 
  

   of 
  a 
  medium; 
  and 
  there 
  is 
  no 
  doubt 
  that 
  all 
  known 
  terrestrial 
  

   phenomena 
  of 
  the 
  kind 
  can 
  be 
  dealt 
  with 
  in 
  the 
  same 
  way. 
  

   In 
  our 
  mathematical 
  expressions 
  we 
  need 
  to 
  take 
  as 
  inde- 
  

   pendent 
  variables 
  quantities 
  referring 
  to 
  material 
  bodies 
  only 
  ; 
  

   given 
  the 
  values 
  of 
  these 
  quantities, 
  the 
  luminous 
  effects 
  

   follow 
  determinately. 
  There 
  is 
  here 
  no 
  logical 
  basis 
  for 
  an 
  

   aether. 
  Several 
  attempts 
  have 
  been 
  made 
  (the 
  most 
  notable 
  

   being 
  that 
  of 
  Michelson) 
  to 
  experience 
  directly 
  the 
  effects 
  of 
  

  

  