﻿50 
  Mr. 
  F. 
  Twyman 
  on 
  Interferometers 
  j 
  or 
  

  

  a 
  light 
  may 
  be 
  obtained 
  from 
  a 
  Cooper-Hewett 
  Mercury- 
  

   Vapour 
  Lamp, 
  combined 
  with 
  a 
  suitable 
  filter. 
  

  

  The 
  light 
  from 
  the 
  source 
  is 
  reflected 
  by 
  the 
  adjustable 
  

   mirror 
  A 
  through 
  the 
  condensing-lens 
  B, 
  by 
  means 
  of 
  which 
  

   it 
  is 
  condensed 
  on 
  the 
  aperture 
  of 
  the 
  diaphragm 
  C. 
  

  

  The 
  diverging 
  beam 
  of 
  light 
  is 
  collimated 
  by 
  a 
  lens 
  D, 
  and 
  

   falls 
  as 
  a 
  parallel 
  beam 
  on 
  a 
  plane 
  parallel 
  plate 
  K, 
  the 
  second 
  

   surface 
  of 
  which 
  is 
  silvered 
  lightly 
  so 
  that 
  a 
  part 
  of 
  the 
  

  

  Fig. 
  1. 
  

  

  Diagram 
  of 
  Prism 
  Interferometer. 
  

  

  light 
  is 
  transmitted 
  and 
  part 
  reflected. 
  The 
  major 
  part 
  

   should 
  be 
  reflected. 
  One 
  part 
  passes 
  through 
  the 
  prism 
  L 
  

   in 
  the 
  same 
  way 
  as 
  in 
  actual 
  use, 
  and 
  being 
  reflected 
  by 
  the 
  

   mirror 
  F 
  passes 
  back 
  through 
  the 
  prism 
  to 
  the 
  plate 
  K. 
  

   The 
  other 
  part 
  of 
  the 
  light 
  is 
  reflected 
  to 
  the 
  mirror 
  Gr 
  and 
  

   back 
  again 
  to 
  the 
  plate 
  K. 
  Here 
  the 
  separated 
  beams 
  re- 
  

   combine, 
  and 
  passing 
  through 
  the 
  lens 
  E 
  each 
  forms 
  on 
  the 
  

   eye 
  placed 
  somewhat 
  beyond 
  the 
  aperture 
  in 
  the 
  diaphragm 
  P 
  

   an 
  image 
  of 
  the 
  hole 
  in 
  the 
  diaphragm 
  C. 
  One 
  of 
  the 
  re- 
  

   flecting 
  mirrors 
  in 
  its 
  mount 
  is 
  shown 
  in 
  fig. 
  2. 
  

  

  