﻿Transmission 
  Fixed- 
  Arm 
  Spectroscopes. 
  453 
  

  

  III. 
  

  

  A 
  third 
  form 
  of 
  instrument 
  which 
  may 
  be 
  convenient 
  when 
  

   it 
  is 
  desired 
  to 
  have 
  the 
  reversed 
  beam 
  in 
  a 
  separate 
  position 
  

  

  £2^ 
  

  

  OQ 
  

  

  7 
  — 
  T 
  

  

  be 
  I 
  

   S 
  ! 
  

  

  1 
  1 
  ; 
  i 
  

  

  ! 
  i 
  ' 
  ' 
  i 
  i 
  i 
  

  

  !' 
  !' 
  !' 
  : 
  

   I 
  ,; 
  '<' 
  ! 
  

  

  J 
  1 
  L 
  ' 
  

  

  -d 
  

  

  I. 
  

  

  from 
  the 
  direct 
  one, 
  is 
  shown 
  in 
  plan 
  in 
  fig, 
  6 
  and 
  in 
  elevation 
  

   in 
  fig. 
  7 
  (p. 
  454). 
  

  

  A 
  and 
  B 
  are 
  the 
  collimator 
  and 
  telescope 
  as 
  before. 
  The 
  

   refracting 
  prisms 
  are 
  cde 
  and 
  fgh. 
  k 
  is 
  a 
  right-angled 
  

   reflecting 
  prism 
  above 
  the 
  object-glass 
  of 
  the 
  collimator. 
  

   The 
  beam 
  from 
  the 
  collimator 
  inclined 
  slightly 
  upwards 
  as 
  

   before 
  is 
  refracted 
  at 
  the 
  face 
  cd 
  of 
  one 
  prisrn, 
  is 
  totally 
  

   reflected 
  at 
  the 
  face 
  d 
  e, 
  and 
  emerging 
  at 
  the 
  face 
  e 
  c 
  enters 
  

  

  