﻿"336 
  

  

  Prof. 
  A. 
  A. 
  Michelson 
  on 
  

  

  of 
  Hertz 
  showed 
  that 
  the 
  electrical 
  impulses 
  (which 
  were 
  

   to 
  be 
  used 
  to 
  establish 
  the 
  required 
  phase 
  relation 
  between 
  

   •the 
  two 
  revolving 
  mirrors) 
  would 
  be 
  affected 
  in 
  the 
  same 
  

   way 
  as 
  would 
  the 
  light-waves 
  themselves. 
  

  

  (It 
  may 
  also 
  be 
  noted 
  in 
  this 
  connexion 
  that 
  the 
  method 
  

   proposed 
  in 
  the 
  same 
  article, 
  and 
  illustrated 
  by 
  fig. 
  1, 
  is 
  also 
  

   not 
  sound.) 
  

  

  It 
  may 
  also 
  be 
  worth 
  mentioning 
  that 
  some 
  preliminary 
  

   experiments, 
  made 
  about 
  two 
  years 
  ago, 
  have 
  shown 
  that 
  it 
  

   is 
  not 
  entirely 
  impossible 
  to 
  employ 
  a 
  mechanical 
  method 
  of 
  

   keeping 
  the 
  two 
  revolving 
  mirrors 
  in 
  a 
  constant 
  phase 
  

   relation. 
  For 
  instance, 
  it 
  was 
  shown 
  that 
  the 
  vibrations 
  of 
  a 
  

   .tuning-fork 
  could 
  be 
  transmitted 
  over 
  a 
  mile 
  of 
  piano-wire 
  

   with 
  a 
  diminution 
  of 
  amplitude 
  of 
  less 
  than 
  one 
  half. 
  

  

  The 
  plan 
  proposed 
  is 
  virtually 
  a 
  combination 
  of 
  the 
  methods 
  

   of 
  Foucault 
  and 
  Fizeau. 
  The 
  essential 
  feature 
  of 
  the 
  observa- 
  

   tion 
  of 
  eclipses 
  corresponding 
  to 
  the 
  latter 
  method, 
  while 
  the 
  

   production 
  of 
  the 
  eclipses 
  is 
  brought 
  about 
  by 
  a 
  revolving 
  

   beam 
  of 
  light 
  as 
  in 
  the 
  former 
  method. 
  

  

  Figures 
  1 
  & 
  2, 
  in 
  which 
  the 
  lettering 
  is 
  the 
  same, 
  will 
  

  

  Fig. 
  1. 
  

  

  Fig. 
  2. 
  

  

  illustrate 
  the 
  essential 
  features, 
  subject 
  to 
  such 
  minor 
  modi- 
  

   fications 
  as 
  experiment 
  may 
  suggest. 
  

  

  The 
  light 
  starts 
  from 
  a 
  slit 
  at 
  a, 
  passes 
  through 
  a 
  lightly 
  

   silvered 
  glass 
  plate 
  P 
  and 
  a 
  lens 
  e, 
  and 
  falls 
  upon 
  the 
  upper 
  

   half 
  of 
  the 
  revolving 
  mirror 
  b. 
  Thence 
  it 
  proceeds 
  to 
  the 
  

   grating 
  c, 
  upon 
  the 
  surface 
  of 
  which 
  it 
  forms 
  an 
  image 
  of 
  the 
  

   slit. 
  It 
  is 
  thence 
  reflected 
  to 
  the 
  lower 
  half 
  d 
  of 
  the 
  re 
  vol 
  vine: 
  

  

  