﻿Photography 
  of 
  Ripples. 
  195 
  

  

  camera. 
  The 
  phenomena 
  were 
  also 
  so 
  complicated 
  that 
  no 
  

   regularity 
  was 
  observable. 
  By 
  opening 
  the 
  lens-stop 
  we 
  may 
  

   in 
  effect 
  smooth 
  down 
  the 
  less 
  violent 
  ripples, 
  and 
  thus 
  

   obtain 
  a 
  photograph 
  which 
  shows 
  a 
  regular 
  diffraction-pattern. 
  

   (Stop 
  F 
  16 
  was 
  used.) 
  

  

  The 
  bright 
  bands 
  are 
  lines 
  of 
  minimum 
  disturbance 
  pro- 
  

   duced 
  by 
  the 
  interference 
  between 
  the 
  direct 
  waves 
  from 
  the 
  

   centre 
  and 
  the 
  reflected 
  waves 
  from 
  the 
  ends 
  of 
  the 
  ellipse. 
  

  

  Two 
  foci 
  conjugate 
  to 
  the 
  centre 
  are 
  seen 
  near 
  each 
  end 
  of 
  

   the 
  ellipse. 
  Let 
  us 
  assume 
  that 
  the 
  portions 
  of 
  the 
  ellipse 
  

   which 
  are 
  most 
  concerned 
  in 
  deciding 
  the 
  position 
  of 
  these 
  

   foci, 
  are 
  closely 
  adjacent 
  to 
  the 
  extremities 
  of 
  the 
  major 
  axis, 
  

   and 
  calculate 
  the 
  distance 
  of 
  the 
  foci 
  from 
  the 
  centre 
  on 
  this 
  

   assumption. 
  This 
  distance 
  is 
  

  

  ab 
  2 
  

  

  a 
  — 
  

  

  2a 
  2 
  -b 
  2 
  ' 
  

  

  using 
  the 
  ordinary 
  notation. 
  From 
  the 
  known 
  dimensions 
  of 
  

   the 
  ellipse 
  and 
  the 
  scale 
  of 
  reduction 
  of 
  the 
  photograph 
  ('765) 
  

   this 
  distance 
  becomes 
  1*315 
  centim. 
  On 
  measuring 
  the 
  ne- 
  

   o-ative 
  the 
  value 
  1*30 
  centim. 
  was 
  obtained. 
  

  

  CD 
  

  

  Fig. 
  8. 
  — 
  Frequency 
  236. 
  

  

  A 
  comb 
  of 
  ten 
  teeth 
  was 
  used 
  to 
  agitate 
  the 
  surface. 
  The 
  

   points 
  were 
  in 
  the 
  same 
  straight 
  line 
  and 
  equally 
  spaced. 
  The 
  

   distance 
  between 
  the 
  centres 
  of 
  the 
  teeth 
  at 
  each 
  end 
  was 
  

   3'18 
  centim. 
  

  

  The 
  print 
  shows 
  that 
  such 
  a 
  comb 
  behaves 
  like 
  a 
  diffraction- 
  

   grating. 
  The 
  length 
  of 
  a 
  bar 
  and 
  slit 
  on 
  the 
  negative 
  is 
  

  

  3-18 
  x 
  -765 
  _ 
  

  

  - 
  =27 
  centim. 
  

  

  The 
  wave-length 
  (on 
  the 
  negative) 
  is 
  *105 
  centim. 
  Thus 
  the 
  

   direction 
  of 
  normals 
  to 
  the 
  rectilinear 
  waves 
  due 
  to 
  refraction 
  

   is 
  given 
  by 
  

  

  •105/1 
  = 
  '27 
  sin 
  On, 
  

   or 
  

  

  w=2'57 
  sin 
  6 
  n 
  , 
  

  

  where 
  n 
  is 
  the 
  order 
  of 
  the 
  spectrum. 
  Thus 
  the 
  greatest 
  

   value 
  n 
  can 
  have 
  is 
  2 
  ; 
  and 
  we 
  could 
  not 
  expect 
  to 
  find 
  

   evidence 
  of 
  more 
  than 
  five 
  sets 
  of 
  linear 
  waves 
  on 
  the 
  photo- 
  

   graph. 
  At 
  a 
  distance 
  from 
  the 
  comb 
  the 
  central 
  band 
  can 
  be 
  

   made 
  out. 
  This 
  corresponds 
  to 
  the 
  undiffracted 
  image 
  one 
  

   sees 
  when 
  looking 
  through 
  an 
  ordinary 
  grating. 
  On 
  either 
  

   side 
  of 
  this 
  band 
  the 
  first 
  diffracted 
  band 
  is 
  seen. 
  The 
  angle 
  

   between 
  the 
  linear 
  waves 
  in 
  these 
  bands 
  was 
  measured 
  with 
  a 
  

   protractor 
  and 
  found 
  to 
  be 
  49°. 
  Thus 
  1 
  = 
  24'5. 
  This 
  is 
  in 
  

   fair 
  accord 
  with 
  theory, 
  which 
  gives 
  L 
  = 
  23°. 
  

  

  