﻿On 
  Stress 
  and 
  Strain 
  in 
  an 
  Elastic 
  Solid. 
  95 
  

  

  Case 
  2. 
  — 
  In 
  passing 
  from 
  a 
  narrow 
  to 
  a 
  wide 
  ruling 
  in 
  

   going 
  from 
  left 
  to 
  right. 
  

  

  Focus 
  in. 
  Dark 
  band. 
  

  

  Focus 
  out. 
  Light 
  band. 
  

  

  & 
  

  

  Orders 
  on 
  the 
  left 
  of 
  the 
  Central 
  Image. 
  

   The 
  results 
  just 
  given 
  must 
  be 
  reversed. 
  

  

  The 
  theoretical 
  results 
  so 
  obtained 
  were 
  confirmed 
  by 
  

   observations 
  on 
  gratings 
  with 
  prominent 
  errors. 
  It 
  had 
  

   previously 
  been 
  noticed 
  that 
  when 
  the 
  telescope 
  was 
  focussed 
  

   exactly 
  on 
  the 
  surface 
  of 
  the 
  grating 
  the 
  bands 
  disappeared, 
  

   by 
  putting 
  the 
  telescope 
  considerably 
  out 
  of 
  focus 
  the 
  bands- 
  

   were 
  clearly 
  seen. 
  Following 
  the 
  indications 
  of 
  the 
  theory, 
  

   and 
  focussing 
  first 
  in 
  and 
  then 
  out, 
  it 
  was 
  seen 
  that 
  the 
  dark 
  

   bands 
  changed 
  to 
  light 
  and 
  the 
  light 
  to 
  dark. 
  On 
  comparing 
  

   the 
  images 
  on 
  the 
  right 
  and 
  left 
  of 
  the 
  central 
  one, 
  the 
  bands 
  

   were 
  seen 
  to 
  be 
  exactly 
  reversed. 
  The 
  same 
  effect 
  was 
  pro- 
  

   duced 
  by 
  the 
  practically 
  equivalent 
  process 
  of 
  turning 
  the 
  

   p 
  ate 
  upside 
  down*. 
  

  

  In 
  all 
  the 
  gratings 
  that 
  I 
  have 
  examined 
  the 
  principal 
  error 
  

   has 
  been 
  periodic 
  in 
  character, 
  so 
  that 
  the 
  bands 
  are 
  dis- 
  

   tributed 
  over 
  the 
  image 
  with 
  a 
  certain 
  amount 
  of 
  regularity. 
  

  

  In 
  conclusion 
  I 
  beg 
  to 
  express 
  my 
  thanks 
  to 
  Lord 
  Blyths- 
  

   wood, 
  the 
  resources 
  of 
  whose 
  laboratory 
  have 
  been 
  at 
  my 
  

   disposal 
  in 
  examining 
  the 
  gratings. 
  

  

  Blythswood 
  Laboratory, 
  

   Renfrew, 
  N.B, 
  

  

  VIII. 
  A 
  New 
  Specifying 
  Method 
  for 
  Stress 
  and 
  Strain 
  in 
  an 
  

   Elastic 
  Solid. 
  By 
  Lord 
  Kelvin 
  f 
  . 
  

  

  rilHE 
  method 
  for 
  specifying 
  stress 
  and 
  strain 
  hitherto 
  

   JL 
  followed 
  by 
  all 
  writers 
  on 
  elasticity 
  has 
  the 
  great 
  dis- 
  

   advantage 
  that 
  it 
  essentially 
  requires 
  the 
  strain 
  to 
  be 
  infinitely 
  

  

  If 
  we 
  treat 
  F 
  1 
  and 
  F 
  2 
  as 
  two 
  sources 
  of 
  light 
  as 
  in 
  Fresnel's 
  inter- 
  

   ference 
  experiment, 
  the 
  breadth 
  of 
  an 
  interference-band 
  at 
  B" 
  would 
  be 
  

  

  A 
  simple 
  calculation 
  shows 
  that 
  in 
  general 
  this 
  quantity 
  is 
  large 
  com- 
  

   pared 
  with 
  the 
  distance 
  within 
  which 
  there 
  is 
  overlapping", 
  so 
  that 
  the 
  

   interference 
  effect 
  is 
  comparatively 
  insignificant. 
  

  

  * 
  I 
  have 
  devised 
  an 
  arrangement 
  in 
  connexion 
  with 
  Lord 
  Blythswood's 
  

   dividing-engine 
  which 
  makes 
  it 
  possible 
  to 
  cut 
  any 
  number 
  of 
  lines 
  to 
  the 
  

   inch. 
  Since 
  the 
  paper 
  was 
  written 
  I 
  have 
  veiified 
  the 
  results 
  by 
  using 
  a 
  

   plate 
  containing 
  two 
  distinct 
  rulings. 
  

  

  f 
  Communicated 
  by 
  the 
  Author. 
  

  

  