﻿Photography 
  of 
  Ripples. 
  193 
  

  

  giving 
  trouble, 
  owing 
  to 
  the 
  damp, 
  was 
  amply 
  compensated 
  

   by 
  the 
  advantage 
  of 
  having 
  a 
  steady 
  roof. 
  This 
  allowed 
  

   of 
  the 
  suspension 
  of 
  the 
  trough 
  from 
  the 
  roof 
  by 
  a 
  string 
  ; 
  

   the 
  annoyance 
  due 
  to 
  the 
  gradual 
  stretching 
  of 
  a 
  rubber 
  cord 
  

   was 
  thus 
  avoided. 
  

  

  Description 
  of 
  Photographs. 
  

  

  The 
  figures 
  are 
  about 
  J 
  natural 
  size, 
  with 
  the 
  exception 
  of 
  

   figs. 
  6 
  and 
  7 
  which 
  are 
  f 
  natural 
  size. 
  

  

  A 
  number 
  of 
  photographs 
  were 
  taken 
  to 
  test 
  whether 
  it 
  

   was 
  possible 
  to 
  avoid 
  suspending 
  the 
  trough. 
  Even 
  when 
  

   the 
  motor 
  and 
  induction-machine 
  were 
  placed 
  upon 
  soft 
  

   rubber 
  bungs, 
  the 
  vibration 
  was 
  still 
  so 
  strong 
  as 
  to 
  cause 
  the 
  

   surface 
  of 
  the 
  mercury 
  to 
  appear 
  furrowed 
  in 
  directions 
  

   parallel 
  to 
  the 
  sides 
  of 
  the 
  trough 
  ; 
  this 
  was 
  not 
  much 
  remedied 
  

   by 
  placing 
  the 
  trough 
  on 
  soft 
  felt 
  pads. 
  When 
  the 
  motor 
  

   was 
  driven 
  slowly, 
  and 
  the 
  table 
  on 
  which 
  the 
  trough 
  stood 
  

   was 
  agitated 
  by 
  the 
  vibration 
  of 
  a 
  fork, 
  the 
  phenomena 
  became 
  

   regular 
  as 
  shown 
  in 
  fig. 
  1 
  . 
  

  

  Fig. 
  1. 
  — 
  Frequency 
  236. 
  

  

  The 
  ripples 
  due 
  to 
  the 
  sides 
  of 
  the 
  trough 
  are 
  of 
  two 
  kinds. 
  

   The 
  small 
  ones 
  are 
  those 
  caused 
  by 
  the 
  fork 
  and 
  the 
  large 
  

   ones 
  are 
  caused 
  by 
  the 
  shaking 
  of 
  the 
  table 
  by 
  the 
  motor 
  and 
  

   induction-machine. 
  

  

  Fig. 
  2. 
  — 
  The 
  disturbance 
  is 
  here 
  caused 
  by 
  stamping 
  with 
  

   the 
  foot 
  on 
  the 
  concrete 
  floor 
  on 
  which 
  the 
  table 
  stands. 
  

  

  Fig. 
  3. 
  — 
  Frequency 
  60. 
  

  

  The 
  trough 
  having 
  been 
  suspended, 
  the 
  mercury 
  surface 
  is 
  

   now 
  free 
  from 
  waves 
  due 
  to 
  adventitious 
  causes. 
  The 
  figure 
  

   shows 
  a 
  point 
  source 
  and 
  its 
  image 
  equidistant 
  from 
  the 
  

   circular 
  mirror. 
  

  

  The 
  production 
  of 
  conjugate 
  foci 
  with 
  ripples 
  is 
  a 
  matter 
  

   of 
  some 
  difficulty. 
  (No 
  example 
  of 
  conjugate 
  foci 
  was 
  given 
  

   in 
  the 
  first 
  paper.) 
  This 
  is 
  owing 
  to 
  a 
  phenomenon 
  which 
  

   often 
  accompanies 
  the 
  reflexion 
  of 
  ripples 
  ; 
  a 
  focus 
  may 
  

   appear 
  in 
  a 
  position 
  not 
  in 
  accordance 
  with 
  the 
  laws 
  of 
  

   reflexion, 
  or 
  a 
  normal 
  focus 
  may 
  be 
  accompanied 
  by 
  a 
  number 
  

   of 
  points 
  having 
  the 
  appearance 
  of 
  foci. 
  This 
  would 
  occur 
  

   if 
  the 
  waves 
  on 
  reflexion 
  were 
  split 
  up 
  into 
  several 
  sets 
  of 
  

   different 
  wave-length, 
  one 
  set 
  having 
  the 
  wave-length 
  of 
  the 
  

   incident 
  waves. 
  

  

  Fig. 
  4. 
  — 
  Frequency 
  60. 
  

  

  This 
  illustrates 
  the 
  abnormal 
  foci 
  referred 
  to 
  above. 
  There 
  

   are 
  two 
  or 
  perhaps 
  three 
  of 
  these 
  to 
  the 
  left 
  of 
  the 
  normal 
  

   focus. 
  That 
  the 
  focus 
  which 
  is 
  the 
  most 
  obvious 
  is 
  really 
  the 
  

  

  