﻿196 
  On 
  the 
  Photography 
  of 
  Ripples. 
  

  

  Fig. 
  9. 
  — 
  Frequency 
  236. 
  

  

  In 
  this 
  figure 
  we 
  have 
  the 
  analogue 
  of 
  a 
  zone-plate 
  in 
  optics. 
  

   The 
  mercury 
  was 
  agitated 
  by 
  a 
  card 
  cat 
  to 
  represent 
  a 
  median 
  

   section 
  of 
  a 
  circular 
  zone-plate. 
  The 
  central 
  portion 
  was 
  

   2 
  centim. 
  long. 
  This 
  was 
  bounded 
  on 
  each 
  side 
  by 
  a 
  space 
  

   v2 
  — 
  n/L 
  Then 
  followed 
  a 
  length 
  of 
  uncut 
  edge 
  >/3~~ 
  %/2 
  

   centim. 
  long, 
  and 
  so 
  on. 
  It 
  need 
  hardly 
  be 
  remarked 
  that 
  

   the 
  effect 
  at 
  the 
  focus 
  is 
  not 
  twice 
  that 
  of 
  the 
  uncut 
  edge, 
  as 
  

   it 
  would 
  be 
  if 
  we 
  were 
  dealing 
  with 
  areal 
  sources, 
  as 
  in 
  light. 
  

  

  The 
  focal 
  distance 
  as 
  measured 
  on 
  the 
  negative 
  is 
  about 
  

   5*5 
  centim., 
  and 
  \ 
  measures 
  *115 
  centim. 
  The 
  half-periods 
  

   are 
  successively 
  s/\d, 
  s/lXd, 
  &c; 
  whence 
  

  

  •765= 
  s/Xd, 
  

   or 
  d 
  = 
  5'l 
  centim. 
  

  

  Fig. 
  10. 
  — 
  Frequency 
  60. 
  

  

  The 
  focal 
  length 
  of 
  a 
  zone-plate 
  is 
  less 
  for 
  red 
  light 
  than 
  

   for 
  blue. 
  The 
  corresponding 
  effect 
  is 
  shown 
  in 
  this 
  figure. 
  

   (Stop 
  F22.) 
  

  

  Fig. 
  11. 
  — 
  Frequency 
  60. 
  

  

  In 
  attempting 
  to 
  produce 
  refraction 
  of 
  ripples 
  many 
  methods 
  

   naturally 
  suggest 
  themselves. 
  One 
  of 
  the 
  most 
  obvious 
  is 
  

   that 
  of 
  sending 
  ripples 
  from 
  a 
  deep 
  to 
  a 
  shallow 
  portion 
  of 
  

   liquid. 
  But 
  in 
  order 
  to 
  considerably 
  affect 
  the 
  velocity 
  of 
  

   propagation, 
  it 
  is 
  necessary 
  to 
  have 
  the 
  depth 
  of 
  the 
  shallow 
  

   portion 
  only 
  a 
  small 
  fraction 
  of 
  the 
  wave-length. 
  Now 
  

   mercury 
  will 
  not 
  allow 
  itself 
  to 
  be 
  spread 
  out 
  on 
  a 
  surface 
  it 
  

   does 
  not 
  wet 
  to 
  a 
  less 
  depth 
  than 
  about 
  3 
  millim. 
  This 
  diffi- 
  

   culty 
  can 
  be 
  overcome 
  by 
  amalgamating 
  the 
  surface 
  of 
  a 
  piece 
  

   of 
  metal 
  and 
  fixing 
  it 
  in 
  the 
  trough. 
  

  

  Experiments 
  by 
  this 
  method 
  have 
  been 
  unsuccessful. 
  The 
  

   ripples 
  move 
  onwards 
  into 
  the 
  shallow 
  region 
  up 
  to 
  a 
  certain 
  

   depth 
  without 
  change 
  of 
  wave-length 
  ; 
  then 
  they 
  are 
  apparently 
  

   quenched, 
  and 
  the 
  very 
  shallow 
  x 
  parts 
  appear 
  quite 
  unaffected 
  

   by 
  the 
  disturbance 
  in 
  the 
  rest 
  of 
  the 
  liquid. 
  

  

  The 
  figure 
  shows 
  another 
  method 
  of 
  refracting 
  ripples. 
  

   The 
  kinematic 
  surface-tension 
  and 
  the 
  effective 
  value 
  of 
  

   gravity 
  are 
  both 
  lowered 
  when 
  the 
  surface 
  is 
  covered 
  with 
  

   water. 
  A 
  triangle 
  of 
  water 
  was 
  made 
  on 
  the 
  mercury 
  and 
  

   kept 
  in 
  position 
  with 
  glass 
  styles 
  at 
  the 
  corners. 
  The 
  glass 
  

   styles 
  were 
  attached 
  to 
  the 
  sides 
  of 
  the 
  trough. 
  

  

  Rectilinear 
  waves 
  are 
  sent 
  through 
  the 
  triangle 
  and 
  are 
  

   bent 
  towards 
  the 
  base 
  on 
  entrance 
  and 
  emergence. 
  The 
  

   effect 
  is 
  analogous 
  to 
  but 
  very 
  much 
  less 
  marked 
  than 
  in 
  the 
  

   case 
  of 
  light 
  passing 
  through 
  a 
  glass 
  prism. 
  

  

  Fig. 
  12. 
  — 
  Frequency 
  60. 
  

  

  The 
  three 
  black 
  circular 
  patches 
  are 
  drops 
  of 
  water 
  

  

  